Question 1

What is a solid state relay power controller?

Accepted Answer

HBControls Power Controllers are solid-state relays pre-assembled onto either a DIN-rail or panel-mount heat sink. Each Power Controller is fully assembled and ready to use, eliminating the need for thermal calculations or heat sink selection. The maximum allowable load current for the assembled solid-state relay, based on a 40 °C ambient temperature, is specified in the datasheet for each controller.

Solid-state relays are electronic switching devices that, unlike electromechanical relays and contactors, contain no moving parts. As a result, their operational lifespan is significantly longer than comparable electromechanical devices. Typical solid-state relays have an MTBF (mean time between failures) in excess of 7 million hours under rated operating conditions. Solid-state relays also operate much faster, with switching times measured in milliseconds or microseconds rather than seconds. An instantaneous (also called “random turn-on”) solid-state relay can switch power to a load in less than 100 µs.

Solid-state relay power controllers perform the same basic function as electromechanical relays and mercury contactors: switching power to or from an electrical load in response to an input or control signal. However, because solid-state relays have no moving parts, they offer faster response times, silent operation, and significantly higher reliability.
 
Benefits of using a solid-state relay power controller include:
Long operational life and high number of switching cycles
Resistance to shock and vibration
Silent operation with no acoustical noise
Fast response time
No mechanical or moving parts

Question 2

Why solid-state power controllers outlast mechanical relays

Accepted Answer

In terms of basic function, electromechanical relays, mercury contactors, and solid-state power controllers are very similar. Each device switches electrical power to or from a load in response to an applied control signal.

Electromechanical relays and mercury contactors perform this switching function using mechanical contacts. In contrast, HBC Power Controllers utilize solid-state relays, which contain no moving parts. As a result, solid-state power controllers switch significantly faster and operate without any acoustical noise.

The absence of moving parts also eliminates contact wear, allowing solid-state power controllers to achieve a much longer operational life compared to comparable mechanical relays or contactors.

Question 3

What is the difference between AC and DC output solid-state relays? Can they be used interchangeably?

Accepted Answer

The most common AC output solid-state relays utilize two back-to-back (inverse-parallel) SCRs. Each SCR conducts load current when it is forward-biased (anode positive with respect to cathode). During normal operation, one SCR carries the full load current during one half-cycle of the AC sine wave, and the second SCR conducts once the polarity of the sine wave reverses.

Most DC output solid-state relays utilize MOSFETs, which typically have a very low on-state resistance (Rds(on)) and therefore dissipate significantly less power than AC output SSRs in applications below 100 A of load current. Unlike AC output SSRs, DC controllers using MOSFET-based SSRs can be wired in parallel to reduce power dissipation and effectively increase the maximum load-current rating, provided proper thermal and electrical design practices are followed.

AC output solid-state relays cannot be used to switch DC loads because SCRs rely on the natural current zero of an AC waveform to turn off. In DC applications, the load current does not naturally fall below the holding current, preventing the SCRs from turning off when the control signal is removed. The only exception is applications where the load current is interrupted by other external means.

MOSFET solid-state relays can be wired in inverse-series (back-to-back) with the load to switch AC load current. This method can significantly reduce conducted emissions and, in some cases, minimize power dissipation. However, because two relays are required, the cost is often prohibitive. Care must also be taken to ensure the MOSFET off-state voltage rating exceeds the peak AC voltage, as exceeding this rating can permanently damage the output device.

Question 4

Why do solid state relays require heat sinks?

Accepted Answer

AC output solid-state relays are not ideal switches and exhibit a forward voltage drop (Vf) of approximately 1 Vrms when conducting load current. As a result, they dissipate power in the form of heat at a rate of roughly 1 watt per ampere of load current. For example, an SSR switching a 50-amp load will dissipate approximately 50 watts of heat during normal operation.

If this heat is not efficiently removed, the temperature of the relay will rise rapidly. For illustration purposes, if a solid-state relay dissipating 50 W were suspended in free air with an effective thermal resistance of approximately 20 °C/W, the baseplate temperature would theoretically rise by 1,000 °C above ambient. In practice, the device would fail long before reaching this temperature, but the calculation highlights the necessity of proper heat sinking.

HBControls Power Controllers utilize highly efficient heat sinks to safely dissipate this heat and prevent the solid-state relay from overheating during normal operation. Datasheets for each controller specify the maximum allowable load current at a given ambient temperature, eliminating the need for additional thermal calculations or heat sink selection.

Question 5

Why does the load current rating of a solid state relay change when mounted to a heat sink?

Accepted Answer

Solid-state relays dissipate heat in direct proportion to load current, and their long-term reliability depends on how effectively that heat is removed. As a result, the maximum allowable load current of an SSR is directly tied to the thermal performance of the heat sink used in the application.

For example, a 90 A solid-state relay can carry 90 A of load current only if it is mounted to a heat sink capable of dissipating approximately 90 W of heat in the given ambient environment. If the heat sink cannot safely dissipate that amount of power, the load current must be reduced to prevent the relay from overheating.

In most applications, it is recommended to keep the SSR baseplate temperature below 80 °C, although many devices can operate safely at baseplate temperatures up to 100 °C. If a relay is installed in a 40 °C ambient environment and the baseplate temperature is limited to 80 °C, the allowable temperature rise is 40 °C. To dissipate 90 W of power while staying within this limit, a heat sink with a thermal resistance of 0.44 °C/W or lower is required (40 °C ÷ 90 W).

If the same relay were instead mounted to a 2 °C/W heat sink, the allowable power dissipation would be limited to 20 W (40 °C ÷ 2 °C/W). Assuming a dissipation rate of approximately 1 W per ampere, the maximum allowable load current would be reduced to 20 A.

Without any external heat sink, most AC output solid-state relays can carry only 5 to 7 amps of load current in a 40 °C ambient environment. While the relay baseplate provides some heat dissipation, its effectiveness is limited by size and surface area.

HBControls Power Controllers utilize highly efficient, application-matched heat sinks to safely dissipate this heat during normal operation. Datasheets for each controller specify the maximum allowable load current at a given ambient temperature, eliminating the need for thermal calculations or heat sink selection.

Question 6

What is the thermal resistance specification (Rjb or Rth) for a solid-state relay?

Accepted Answer

Thermal resistance (sometimes referred to as thermal impedance) is a measure of how resistant a device is to heat flow. It is typically expressed in degrees Celsius per watt (°C/W). A lower thermal resistance indicates more efficient heat transfer. In solid-state relays, thermal resistance describes how effectively heat is conducted from the semiconductor junction through the relay baseplate and into an external heat sink.

Common specifications such as Rjb (junction-to-baseplate) or Rth define this internal thermal path. These values are important because they determine how much power the relay can safely dissipate for a given baseplate or ambient temperature, which ultimately limits the maximum allowable load current.

In practice, however, the thermal resistance of a solid-state power controller is often not a critical specification for the end user. HBControls Power Controllers are supplied with datasheets that specify maximum allowable load current at various ambient temperatures. These ratings already account for the internal thermal resistance of the relay and the performance of the integrated heat sink.

That said, understanding thermal resistance helps explain why solid-state relays require heat sinks and why heat sink size varies with current rating and ambient temperature. It also clarifies why the same relay may have different current ratings when mounted to different heat sinks or used in different environments.

Question 7

What switching technology do solid-state relays utilize?

Accepted Answer

HBControls Power Controllers use semiconductor-based solid-state relays to switch power to or from AC and DC electrical loads. These relays most commonly employ SCRs for AC load switching and MOSFETs for DC load switching. In certain applications, other semiconductor devices—such as diodes or IGBT modules—may be used when specific electrical or control requirements dictate.

Question 8

Why is the life expectancy of a solid-state power controller higher than that of a mechanical relay?

Accepted Answer

Electromechanical relays and contactors use mechanical contacts to switch electrical power to or from a load. Each time these contacts open or close under load, electrical arcing occurs. Over time, this arcing erodes the contact surfaces, leading to increased resistance, unreliable operation, and eventual failure. Heavier loads produce more severe arcing, further reducing relay life. As a result, the typical life expectancy of an electromechanical relay is often limited to 100,000 to 500,000 operations, a threshold that can be reached in just a few months in high-cycle industrial applications.

Solid-state power controllers are not subject to this failure mechanism because they contain no moving or mechanical contacts. Instead, they switch power electronically using semiconductor devices. Consequently, their life expectancy is typically specified in terms of mean time between failures (MTBF) rather than number of operations. Depending on operating conditions and thermal management, the MTBF of a solid-state power controller commonly ranges from one million to ten million hours, reflecting significantly higher long-term reliability compared to mechanical switching devices.

Question 9

What are the primary advantages and disadvantages of solid-state relays compared to electromechanical and mercury relays?

Accepted Answer

Advantages

Long life expectancy
Solid-state relays contain no moving parts and are therefore not subject to mechanical wear or contact erosion. As a result, they can operate reliably for decades in most applications. While life expectancy is typically specified as MTBF rather than number of operations, many solid-state relays installed in the 1970s remain in service today.

Silent operation
Because solid-state relays switch electronically rather than mechanically, they produce no acoustical noise. This makes them ideal for commercial, residential, and noise-sensitive environments.

Resistance to shock and vibration
The absence of moving parts makes solid-state relays highly resistant to shock and vibration, eliminating false triggering or contact bounce in harsh industrial environments.

Fast switching speed
Instantaneous (random turn-on) solid-state relays can switch power to a load in less than 100 microseconds after receiving an input signal. This fast response makes them well suited for applications such as phase-angle control where precise power regulation is required.

PLC compatibility
Solid-state relays can switch high load currents using only a few milliamps of control current, making them easy to interface directly with PLCs and low-power control circuits.

Environmentally friendly
Solid-state relays contain no mercury and typically have a much longer service life than electromechanical relays. Over the lifetime of a system, far fewer devices require replacement and disposal compared to mechanical switching technologies.

Disadvantages

Higher initial cost
The upfront cost of a solid-state relay is often higher than that of a comparable electromechanical relay. However, when factoring in maintenance, replacement, and downtime, the total cost of ownership is frequently lower over the life of the application.

Heat dissipation and size
Solid-state relays dissipate power as heat during normal operation and therefore require an appropriately sized heat sink. This can increase the overall size of the solution compared to a standalone mechanical relay.

Limited off-state galvanic isolation
Some applications require complete physical disconnection from the AC mains when the output is off. In such cases, a solid-state relay alone may not provide sufficient galvanic isolation. A common solution is to use a solid-state relay in series with an electromechanical relay—allowing the SSR to handle load current switching while the mechanical relay provides physical isolation. This approach significantly reduces contact arcing and extends the life of the mechanical relay.

Question 10

What are zero-crossing and instantaneous (random or asynchronous) turn-on solid-state relays?

Accepted Answer

Zero-crossing solid-state relays are the most common type used in AC applications. As the name implies, these relays wait until the AC mains waveform passes through its zero-voltage crossing before switching from a non-conducting to a conducting state. This behavior minimizes electrical noise and conducted emissions by avoiding turn-on at higher voltage levels.

Instantaneous (also called random or asynchronous turn-on) solid-state relays switch from a non-conducting to a conducting state immediately upon receiving an input signal, regardless of the instantaneous phase of the AC waveform. Typical turn-on times are less than 100 microseconds, making these relays suitable for applications requiring precise timing or phase-angle control.

Both zero-crossing and instantaneous solid-state relays will continue to conduct load current after the input signal is removed until the AC waveform naturally crosses zero. As a result, turn-off time depends on the point in the AC cycle at which the control signal is removed, but will rarely exceed 8.33 milliseconds on a 60 Hz mains or 10 milliseconds on a 50 Hz mains.

Question 11

When should zero-crossing or instantaneous turn-on solid-state relays be used?

Accepted Answer

Zero-crossing solid-state relays are the most common type used in AC applications. These relays wait until the AC mains waveform passes through its zero-voltage crossing before switching from a non-conducting to a conducting state. Because the instantaneous voltage across the load is near zero at turn-on, inrush current is minimized. Turning on near the zero-crossing point also significantly reduces conducted electrical emissions, making zero-crossing SSRs well suited for applications with EMC compliance requirements.

As a general rule of thumb, zero-crossing SSRs are best suited for loads with a power factor between 0.7 and 1.0, although some devices can operate reliably with power factors as low as 0.5. Typical applications include resistive heating elements, certain lighting systems, and lightly inductive loads. Zero-crossing SSRs are not suitable for phase-angle control applications, as they will not turn on once the AC waveform has passed their zero-cross detection window, which is typically limited to voltages below approximately 30 Vpk.

Instantaneous (also called random turn-on) solid-state relays switch to a conducting state within approximately 100 microseconds of the control signal being applied, regardless of the instantaneous phase of the AC waveform. These relays are commonly used for more heavily inductive loads such as motors, transformers, and low power-factor solenoid or contactor coils. Because they can be triggered at any point in the AC cycle, instantaneous SSRs are well suited for phase-angle control applications, including lighting dimming systems and heating applications that require tight, dynamic control of power delivered to the load.

Question 12

Phase-angle vs burst-fire power controllers: what’s the difference and when should each be used?

Accepted Answer

Phase-angle power controllers regulate output power by turning on at a controlled point within each AC sine wave. The later the controller turns on during the cycle, the less power is delivered to the load. Output power is proportional to the control signal; for example, applying 5 V to a 0–10 V input controller results in approximately 50% power to the load. Phase-angle control provides very fast, fine-grained power adjustment and is commonly used in lighting dimming applications and heating systems that require tight, dynamic temperature control.

Because phase-angle controllers often turn on closer to the peak of the AC sine wave, they can generate a significant amount of conducted electrical noise and harmonic distortion on the AC mains. As a result, additional filtering is often required in applications that must comply with EMC standards.

Burst-fire power controllers also provide proportional power control, but do so by applying full AC cycles to the load in timed groups rather than modulating each individual sine wave. The number of complete cycles delivered is determined by the control signal and the controller’s internal time-base period. For example, if a controller has a time base of 10 AC cycles and the control signal requests 50% output, the controller will apply power for 5 consecutive cycles and then remain off for the next 5 cycles.

Burst-fire control is most commonly used in heating applications where precise temperature control is required but rapid, millisecond-level power modulation is not necessary. Because burst-fire controllers switch only at zero-crossings and apply full cycles, they generate significantly lower conducted emissions compared to phase-angle controllers. However, they are generally not suitable for lighting applications, as the on/off cycling can produce visible flicker.

Typical control input options for both phase-angle and burst-fire power controllers include 0–10 V, 0–5 V, 4–20 mA, and potentiometer input.

Question 13

How quickly can a solid-state relay switch power to or from a load?

Accepted Answer

Instantaneous (also called random turn-on) solid-state relays can switch power to a load in less than 100 microseconds after the control signal is applied, regardless of the phase of the AC waveform. This fast response makes them suitable for applications requiring precise timing or phase-angle control.

Zero-crossing solid-state relays wait until the AC waveform passes through its zero-voltage crossing before turning on. As a result, turn-on delay depends on when the control signal is applied relative to the AC cycle, but will not exceed 8.33 milliseconds on a 60 Hz mains or 10 milliseconds on a 50 Hz mains.

Question 14

What type of isolation does a solid-state relay provide between the control system and the load circuit?

Accepted Answer

Solid-state power controllers use optocouplers (also called optical isolators or photocouplers) to provide electrical isolation between the low-voltage control input and the high-voltage output power circuit. This isolation prevents dangerous voltages on the load side from propagating back into the control system.

Several types of optocouplers are used in solid-state relays. In AC power controllers, the most common devices are triac-driver optocouplers (often called photo-triacs). In addition to providing input-to-output isolation, the specific triac driver selected determines whether the power controller operates as a zero-crossing or instantaneous (random turn-on) device.

DC power controllers typically use photovoltaic optocouplers. These devices generate sufficient gate-drive voltage to quickly drive the output MOSFETs into full enhancement, preventing linear operation and minimizing power dissipation in the output devices.

Typical optocouplers used in power controllers provide 4 kV peak isolation between the input and output circuits, along with approximately 12 mm of creepage (surface distance across the PCB) and 6.4 mm of clearance (direct air spacing between conductors), helping ensure safe and reliable operation in industrial environments.

Question 15

What is off-state leakage current?

Accepted Answer

Off-state leakage current is the small amount of current that flows through a solid-state relay even when no control signal is applied. When connected to the AC mains, semiconductor power devices are not perfect open circuits, and a small leakage current will be present in the off state.

This leakage current is typically very low—generally less than 1 mA for AC output solid-state relays and only a few hundred microamps for DC output SSRs—and is normally not hazardous to personnel. However, even small leakage currents can produce a measurable voltage across the load when the controller is off. The resulting voltage depends on the impedance of the load and is often negligible, but it may become problematic in applications where safety or compliance standards require the load to be physically disconnected from the AC mains when de-energized.

Off-state leakage current can increase significantly in solid-state relays that incorporate RC snubber networks (a resistor and capacitor in series) across the output. Because the RC network is connected in parallel with the output semiconductors, current continues to flow through the snubber when the relay is off. In AC applications, the impedance of the snubber capacitor decreases as line frequency increases, which can cause leakage current to rise—often by an order of magnitude or more—at standard mains frequencies such as 60 Hz in North America.

Question 16

What is dv/dt and how does it impact the performance of a Power Controller?

Accepted Answer

The term dv/dt describes the rate of change of voltage with respect to time across the output of a solid-state relay. Excessive dv/dt can cause unintended turn-on of the output devices, affecting the behavior of a power controller even when no control signal is present.

Static dv/dt, sometimes referred to as turn-on dv/dt, occurs when a rapid rise in voltage appears across the output of a solid-state relay while it is in the off state. If the dv/dt exceeds the device’s rating, the relay may momentarily turn on without an input signal.

Commutating dv/dt, also known as turn-off dv/dt, occurs when the relay is conducting load current and attempts to turn off after the control signal is removed. This condition is most commonly associated with inductive loads. As load current approaches zero, the phase shift between voltage and current can result in a rapid voltage rise across the output devices, potentially causing the relay to re-trigger.

Static dv/dt events are typically brief and benign. In such cases, the controller may conduct load current for the remainder of the AC half-cycle and then turn off naturally at the next zero-crossing (less than 8.33 ms on a 60 Hz mains). Because only a small amount of energy is transferred to the load, the effect is usually inconsequential. Commutating dv/dt events, however, can be more problematic. In severe cases, the controller may remain conducting when it is expected to be off, potentially allowing full-load current to flow.

The typical maximum dv/dt rating for solid-state relays is approximately 500 V/µs. This rating can be increased through the use of an RC snubber network, either as an integral part of the controller design or installed externally across the output terminals. While snubber networks are effective at limiting dv/dt, they also increase off-state leakage current and must be applied judiciously.

Applications where dv/dt performance is particularly important include motor-driven equipment operating near personnel, motor-reversing circuits, and systems where off-state semiconductors provide the only separation between different phases of a three-phase power system.

Question 17

What is di/dt?

Accepted Answer

The term di/dt refers to the rate of change of current through the output of a power controller with respect to time. Unlike dv/dt events, which typically result in temporary or nuisance behavior, exceeding the maximum di/dt rating of a solid-state power controller can cause permanent damage to the output devices.

The most common failure mode resulting from excessive di/dt is a shorted output circuit, usually involving only one of the two output SCRs. Because only one SCR conducts during each half-cycle of the AC waveform, this type of failure often results in the load being half-wave powered when the controller is expected to be fully off.

Di/dt failures most commonly occur under short-circuit load conditions, where current rises extremely rapidly. They can also occur following a dv/dt mis-trigger when the load exhibits high inrush-current characteristics, such as certain types of lamps or highly capacitive loads.

The typical maximum di/dt rating for most power controllers is approximately 10 A/µs. Unlike dv/dt protection, RC snubber networks do not increase the di/dt capability of a power controller. However, they can reduce the likelihood of dv/dt-induced mis-triggering that could lead to a di/dt event. The most effective method of di/dt protection is the use of an air-core series choke, as the reactance of an inductor increases with the rate of current change, limiting current rise during transient conditions.

Question 18

What is an RC snubber network and when should one be used?

Accepted Answer

An RC snubber network consists of a resistor and capacitor connected in series and placed in parallel with the output of a solid-state relay. The snubber may be an integral part of the relay's design or installed externally across the output terminals.

The primary function of an RC snubber is to limit the rate of voltage rise (dv/dt) across the output semiconductors when the controller is in the off state. Excessive dv/dt can cause unintended turn-on of the output devices, resulting in a dv/dt failure. Most solid-state relays have a maximum dv/dt rating of approximately 500 V/µs, which can be exceeded in certain applications.

RC snubber networks are most commonly used in applications involving inductive loads, such as motors, solenoids, contactor coils, or transformers, where rapid voltage transients can occur when current is interrupted. They may also be required in systems with long cable runs, motor-reversing circuits, or installations where solid-state devices provide the only separation between different phases of a power system.

While RC snubbers are effective at reducing dv/dt-related mis-triggering, they also increase off-state leakage current and should be applied only when necessary. Their use represents a trade-off between improved dv/dt immunity and increased leakage in the off state.

Question 19

What is I²T?

Accepted Answer

I²t is a measure of the energy withstand capability of the output semiconductor in a solid-state relay. It is derived from the manufacturer’s published maximum single-cycle surge current rating and represents the amount of energy the device can absorb before the silicon junction is damaged or fuses open.

The I²t value is calculated using the peak single-cycle surge current rating—typically around 10 times the continuous forward-current rating of the device. This peak value is divided by √2 to obtain an RMS current, squared, and then multiplied by the duration of one AC half-cycle (8.33 ms for 60 Hz or 10 ms for 50 Hz):

I²t=Irms²​×t

This calculation provides a useful metric for evaluating whether a solid-state relay can survive short-duration overcurrent events such as inrush currents or brief fault conditions.

The key distinction between di/dt and I²t is that di/dt describes the rate of change of current, whereas I²t describes the total energy delivered during an overcurrent event. Di/dt failures are typically associated with rapid current rise and often result in a shorted output device. In contrast, I²t failures are caused by excessive energy dissipation and usually result in catastrophic damage to the output semiconductor, leaving the device in an open-circuit state.

Question 20

What is the maximum allowable operating temperature for a solid-state relay assembly?

Accepted Answer

Most HBControls solid-state power controllers are rated to carry their maximum specified load current in a 40 °C (104 °F) ambient environment with a 100% duty cycle (continuous operation). As ambient temperature increases above this level, the maximum allowable load current must be reduced to prevent excessive internal temperatures.

In general, the allowable load current derates approximately linearly as ambient temperature rises, reaching 0 amps at 80 °C. These ratings include additional design margin to ensure safe and reliable operation across a wide range of real-world applications.

Because thermal performance is influenced by factors such as mounting orientation, airflow, enclosure design, and load characteristics, actual operating limits may vary. Please contact us directly if you would like to discuss solid-state relay performance or current derating for your specific application.

Question 21

What is thermal fatigue and how does it impact the life expectancy of a solid-state relay?

Accepted Answer

A common misconception is that high operating temperature alone significantly reduces the life expectancy of a solid-state relay. While excessive temperature can contribute to failure, long-term reliability is more strongly affected by thermal cycling, also known as thermal fatigue, rather than steady-state temperature itself.

Thermal fatigue results from repeated thermal excursions—changes in temperature that occur as the relay heats up and cools down during normal operation. Solid-state relays dissipate power in proportion to the load current they conduct. If heat is not removed efficiently, the internal temperature of the relay can rise rapidly. When operating conditions change—such as load cycling, duty-cycle variation, or ambient temperature shifts—the relay experiences repeated heating and cooling cycles.

These temperature fluctuations place mechanical stress on internal materials because different components expand and contract at different rates. For example, the copper conductors that carry load current expand at a different rate than the solder joints and semiconductor materials used within the relay. Over time, repeated thermal cycling can fatigue these interfaces, leading to cracked solder joints, degraded connections, and ultimately premature device failure.

HBControls solid-state power controllers are designed to minimize thermal fatigue by using appropriately sized, high-efficiency heat sinks that reduce both peak operating temperature and the magnitude of thermal excursions. For high-current applications or installations operating in elevated ambient temperatures, active cooling solutions—such as forced-air cooling—are also available to further improve thermal stability and extend service life.

Question 22

What are the best methods for protecting a solid-state relay from excessive voltage transients?

Accepted Answer

Two primary methods are used to protect HBC solid-state power controllers from transient voltage spikes: metal-oxide varistors (MOVs) and transient voltage suppression (TVS) diodes.

An MOV is connected externally across the output terminals of the power controller, in parallel with the output power semiconductors. MOVs behave similarly to back-to-back diodes and enter an avalanche conduction state when their specified voltage threshold is exceeded. When a transient occurs, most of the surge energy is diverted through the MOV and into the load, while the remaining energy is absorbed by the MOV itself and dissipated as heat. MOVs are effective at clamping high-energy transients, but their performance degrades over time. The service life of an MOV is determined by both the number of transient events and the energy content of those events.

TVS diodes are incorporated internally within the SSR portion of the power controller and are connected in parallel with the optocoupler output circuitry, which is in series with the gates of the output SCRs. Like MOVs, TVS diodes respond very quickly to voltage transients, but they are not designed to absorb large amounts of energy. Instead, current through the TVS is limited to approximately 150 mA. Once this threshold is reached, the SCR gate is triggered, forcing the output SCR into conduction. This action shunts the transient energy away from the TVS device and into the load, preventing damage to the diode. Because the TVS itself is not required to absorb the transient energy, this protection method is highly repeatable and does not degrade with repeated events.

After the transient is suppressed, both MOV and TVS devices stop conducting. In the case of TVS-based protection, because an output SCR has been triggered, load current will continue to flow through that SCR until the AC waveform reaches the next zero-crossing point (less than 8.33 ms on a 60 Hz mains). At that point, the controller naturally turns off and returns to its non-conducting state.

Question 23

What are conducted emissions in solid-state relay applications?

Accepted Answer

In the context of solid-state relays, conducted emissions refer to electromagnetic energy generated by the switching action of the output semiconductors and coupled back onto the AC power network through the supply conductors.

In zero-crossing solid-state relays, the output does not conduct immediately when the AC waveform reaches zero volts. A small amount of voltage from the AC supply is required to trigger the SCRs into conduction. As a result, depending on the relay design, the output typically does not switch into full conduction until the AC waveform reaches approximately 6 Vpk. This rapid transition from a non-conducting to a conducting state produces electrical noise that is conducted onto the AC mains.

In addition, SCR-based solid-state relays inherently turn on and off during each half-cycle of the AC waveform, even when a continuous control signal is applied. As the AC voltage approaches zero, the load current falls below the SCR holding current and conduction ceases. Once sufficient voltage is again present to trigger the SCR, conduction resumes. This repetitive on–off–on behavior occurs every half-cycle and results in the continuous generation of conducted electrical noise on the AC supply. The resulting voltage harmonics are typically most significant in the 150 kHz to 1 MHz frequency range.

Many regulatory standards limit the amount of conducted emissions that equipment may place on the AC mains. This is particularly important for equipment intended for use in the European Union. Most zero-crossing solid-state relays are suitable for Class A equipment, which is intended for industrial or commercial environments. Class B equipment—such as medical, residential, or consumer devices—is subject to much stricter limits on conducted emissions, and additional filtering is almost always required when solid-state relays are used in these applications.

Instantaneous (random turn-on) and phase-angle solid-state relays generate significantly higher levels of conducted emissions due to switching at higher voltages within the AC waveform. As a result, additional line filtering is typically required for these controllers in applications where Class A or Class B compliance is necessary.

Question 24

Can a solid-state relay switch power to and from three-phase loads?

Accepted Answer

Yes. Solid-state relays can be used to switch three-phase loads in several different ways, depending on the application, load configuration, and thermal constraints.

One approach is to use three single-phase solid-state relays, one for each phase, with their control inputs connected in parallel. Each phase of the three-phase supply is wired through the output of its respective relay and then to the load. When the control signal is applied, all three relays turn on simultaneously and conduct the load current for their associated phases. While effective, this method requires additional panel space and heat sinking and is generally less efficient than other options.

A more common solution is to use a three-phase solid-state power controller, which integrates three independent power switching sections into a single device with one common control input. This approach simplifies wiring, reduces assembly time, and often saves panel space. However, overall size is still largely dictated by the amount of heat that must be dissipated and the heat sink required for the application.

In many applications, the preferred method is to switch only two of the three phases using either a two-pole solid-state contactor or two single-phase solid-state relays, while wiring the third phase directly to the load. This configuration reduces power dissipation by approximately 33%, since only two sets of output semiconductors are conducting load current. As a result, it can significantly reduce heat sink size, panel space requirements, and system cost.

This two-pole switching method works well for both Delta and Wye (Star) configured loads. The primary exception is a Wye-connected load with a neutral connection. In that case, all three phases must be switched; otherwise, current will continue to flow through the phase that remains directly connected to the load.

Question 25

What is meant by “active” and “passive” cooling?

Accepted Answer

Passive cooling refers to the natural convection process by which heat generated by a solid-state relay is transferred to a heat sink and then dissipated into the surrounding ambient air. As the heat sink warms, air in contact with its surface rises and is replaced by cooler ambient air, allowing heat to be carried away without the use of moving components. When properly oriented, air flows upward through the fins of the heat sink, gradually reducing the temperature of the power semiconductors. Lower ambient temperatures improve the effectiveness of passive cooling.

Proper heat sink orientation and adequate panel ventilation are critical for passive cooling to function effectively. If airflow into and out of the enclosure is restricted, heat can accumulate around the controller, raising the local ambient temperature. This increased ambient temperature further reduces cooling efficiency and causes the solid-state relay to operate at higher temperatures. While the system will eventually reach thermal equilibrium, uncontrolled temperature rise can significantly reduce reliability or result in thermal failure.

Active cooling uses fans or blowers to forcibly move air through the heat sink fins, significantly increasing heat transfer efficiency. Airflow is typically specified in linear feet per minute (LFM) or cubic feet per minute (CFM). By actively pushing air through the heat sink, the thermal resistance of the assembly is reduced, allowing more heat to be dissipated at a given ambient temperature.

For example, consider a power controller switching 50 amps of load current using a 90 A solid-state relay mounted to a 1.0 °C/W heat sink in a 40 °C ambient environment. Under passive cooling, the relay will dissipate approximately 50 W, resulting in an estimated baseplate temperature of:

50 W × 1.0 °C/W = +50 °C rise
+40 °C ambient
≈ 90 °C baseplate temperature

If forced air is applied—such as 500 LFM of airflow across the same heat sink—its effective thermal resistance can improve by approximately 65%. Under the same operating conditions, the estimated temperature rise becomes:

50 W × 1.0 °C/W × 0.35 = +17.5 °C rise
+40 °C ambient
≈ 57.5 °C baseplate temperature

In this example, the addition of a 120 mm fan rated at approximately 75 CFM (500 LFM) reduces the baseplate temperature by roughly 32.5 °C (≈90 °F).

The effectiveness of active cooling depends on several variables, including enclosure airflow, inlet and exhaust venting, and the ability of the fan or blower to overcome static pressure within the panel. Because these factors vary widely between installations, the most reliable way to evaluate cooling performance is to measure the actual baseplate temperature during normal operation.

Question 26

What options are available for purchasing products from HBControls?

Accepted Answer

HBControls products can be purchased directly through several convenient channels. Orders may be placed by phone, email, or— for many products—directly through our website. Payment terms are also available for repeat or ongoing business needs, subject to approval by our accounting team.

To place an order or discuss purchasing options, you may contact us at 800.879.7918 or through the Contact Us page on our website.

HBControls products are also available through our network of authorized distributors, who can provide competitive pricing and local support. A complete list of authorized distributors can be found at:
https://www.hbcontrols.com/distributors

Question 27

Is discounted pricing available for larger solid-state relay or power controller orders?

Accepted Answer

Yes. Volume-based pricing is available for larger orders. The easiest way to obtain discounted pricing is through one of our authorized distributors, who can provide competitive quotes and local support. A list of authorized distributors is available at:
https://www.hbcontrols.com/distributors

HBControls can also provide customized quotations based on your specific requirements. To discuss pricing options, please contact us directly at 800.879.7918 or submit a quote request through our website at:
https://www.hbcontrols.com/quote

Question 28

Does HBControls ship overseas?

Accepted Answer

Yes. HBControls serves a global customer base and ships solid-state relays and power controllers to most non-embargoed, non-sanctioned countries worldwide. We currently ship to more than 100 international destinations, including Canada, the United Kingdom, Germany, Denmark, Austria, Australia, Japan, and many others.

If you are unsure whether we ship to your country or require additional information regarding international orders, please contact our sales or support team for assistance.

When placing an international order, customers are responsible for understanding and complying with local customs, duties, and tax regulations applicable in their country. We recommend contacting your local customs authority prior to ordering to avoid delays or unexpected charges.

Question 29

Will HBControls provide ongoing technical support after purchase?

Accepted Answer

Yes. Providing ongoing technical support is a core part of how we work with our customers. One of the things that sets HBControls apart is the quality and reliability of our after-sale support.

Whether you need assistance with an existing installation, guidance on a new project, or help deciding whether solid-state relays are the right solution for your application, our experienced support staff is available to help.

You can contact us at 800.879.7918 or through the Contact Us page on our website. Our ticketing system ensures that all calls and inquiries are promptly routed to a knowledgeable customer support specialist, so you get timely and accurate assistance.

Question 30

Are there solid-state relays or power controllers available that are not published on the website?

Accepted Answer

Yes. The products shown on our website represent only a portion of the solid-state relay and power controller configurations available. Many additional options and variations can be selected directly within the online product pages, and further custom configurations are available upon request.

If you don’t see a product that fits your specific application, please contact us and let our team help identify the right solution. You can reach us at 800.879.7918 or through the Contact Us page on our website.

Question 31

Can HBControls customize a product for a specific application?

Accepted Answer

Yes. HBControls offers custom solid-state power controller solutions designed to meet specific application requirements. Our support team can work directly with you to evaluate your needs and develop a solution tailored to your system.

Custom products are most commonly developed for higher-volume applications; however, depending on design complexity, we can also support one-off or prototype projects. If you would like to discuss a custom solution or explore available options, please contact our support team at 800.879.7918 or through the Contact Us page on our website.

Question 32

Does HBControls warranty their work?

Accepted Answer

Yes. HBControls solid-state relays and power controllers are covered by a standard one-year warranty against defects in materials and workmanship.

If a manufacturing defect results in product failure, the device may be eligible for repair or replacement, at HBControls’ discretion, during its operational life. If you have questions regarding product quality, warranty coverage, or the return process, please contact our Quality Control department for assistance.

HBControls Quality Control
 HBControls, Inc.
 18 Rosary Lane
 Hyannis, MA 02601
 Phone: 800.879.7918
 Fax: 508.676.7346
 Website: www.hbcontrols.com

HBControls’ liability under this warranty is limited to the repair or replacement of the defective product. HBControls is not responsible for incidental or consequential damages, including costs associated with additional parts, labor, downtime, or equipment damage beyond the supplied product.

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