An HBControls Power Controller is a solid-state relay pre-assembled onto either a DIN or panel-mount heat sink. Each Power Controller is ready-to-use, eliminating the need for thermal calculations or heat sink selection. The maximum allowable load current for the assembled solid-state relay in a 40°C ambient temperature is provided in the datasheet for each controller. Solid-state relays are electronic relays that, unlike electromechanical relays and contactors, have no moving parts. As a result, their lifespan is significantly longer than comparable electromechanical relays. The typical MTBF (mean time between failures) for a solid-state relay is >7 million hours. Solid-state relays can also operate significantly faster, with on/off times measured in cycles rather than seconds. An instantaneous (also called a "random turn-on) solid-state relay can switch power to a load is less than 100µS. Solid state relay power controllers are similar to electromechanical relays and mercury contactors in terms of functionality. All of these devices switch power to or from an electrical load upon the application of an input or control signal. As previously mentioned, solid-state relays contain no moving parts, making them faster, quiet and highly reliable. Some of the benefits of using a solid-state relay power controller are: * Longer life expectancy and number of operations * Resistant to shock and vibration * Silent operation - no acoustical noise * Fast response time * No mechanical or moving parts

In terms of function, all three switching technologies are very similar to each other. That is, each switches power to/from an electrical load when voltage is applied to their input. However, electromechanical relays and mercury contactors utilize mechanical contacts to perform the switching function. HBC Power Controllers utilize solid state relays, which have no moving parts and perform the switching function exceptionally fast without any acoustical noise. The lack of moving parts also means that the life expectancy of a Power Controller is significantly longer than a comparable mechanical relay.

The most common AC output Power Controllers utilize solid state relays with two back-to-back (inverse-parallel) SCRs in the output. These SCRs conduct load current when the AC sine wave is positive with respect to its anode. In other words, during a normal conduction cycle, one SCR will carry the full load current during ½ of the AC sine wave and the other will carry the load current once the polarity of the sine wave reverses. Most DC output Power Controllers utilize MOSFET solid state relays, which typically have a very low on-state impedance (Rds) and dissipate a minimal amount of power compared to AC output SSRs in applications below 100A of load current. Unlike AC output SSRs, DC controllers utilizing MOSFET SSRs can be wired in parallel to reduce power dissipation and effectively increase the maximum load-current rating. AC output controllers cannot be used to switch DC loads since the holding current of the SCRs will prevent them from turning off when the input signal is removed. The only exception is in applications where the load current is interrupted by other means. MOSFET controllers can be wired in inverse-series with the load to switch AC load current. This is an effective method for significantly reducing conducted emissions or minimizing power dissipation. However, since two relays are required the cost of doing so is often prohibitive. Consideration must also be given to the maximum off-state voltage rating of the controller, which can be damaged if exceeded by the peak AC voltage.

AC output solid state relays are not perfect switches, having a forward voltage drop (Vf) of approximately 1Vrms when in the on state. Therefore, they dissipate power in the form of heat at a rate of about 1W per ampere of load current. An SSR switching 50 amps of load current will dissipate approximately 50W of power. For the sake of simplicity, if we assume that the thermal conductivity of air is 20°C/W, then the base plate of a solid state relay would reach roughly 1,000°C when carrying 50 amps while suspended in free-air. It would, of course, actually melt long before then, but the calculation makes the point. HBControls Power Controllers utilize highly efficient heat sinks to prevent the solid state relay from overheating during normal use. Datasheets for each controller provide maximum allowable current ratings in a given ambient temperature, eliminating the need for thermal calculations or heat sink selection.

Because solid state relays dissipate heat in direct proportion to load current, and the fact that SSR life expectancy is dependent upon how well that heat is dissipated, the maximum allowable load current rating will almost always change whenever it is installed in an application. For example, a 90 amp solid state relay can carry 90 amps of load current only if it is mounted to a heat sink that can effectively dissipate 90W of power in the given ambient. If the heat sink is too small for 90W of dissipation, then the load current must be reduced to prevent the relay from overheating. Generally, it’s advised to keep the base plate temperature of an SSR below 80°C, although in many applications they can operate safely and reliably with a base plate temperature up to 100°C. If we wanted to switch 90 amps of load current in a 40°C ambient environment and keep the base plate temperature below 80°C, then we must prevent the base plate temperature from increasing by more than 40°C (80°C - 40°C ambient) while dissipating 90W of power. That means we would need at least a 0.44°C/W heat sink (+40°C allowable rise / 90W). If the same relay were mounted to a 2°C/W heat sink, for example, then the maximum allowable load current would be 20A (20A x 2°C/W = +40°C temperature rise). Without any type of external heat sink, most AC output solid state relays cannot carry more than 5 to 7 amps of load current in a 40°C ambient. The base plate of the relay helps dissipate some of the heat, but its thermal efficiency is limited by size. HBControls Power Controllers utilize highly efficient heat sinks to prevent the solid state relay from overheating during normal use. Datasheets for each controller provide maximum allowable current ratings in a given ambient temperature, eliminating the need for thermal calculations or heat sink selection.

Thermal impedance, or thermal resistance, is the measure of how resistant an object is to heat flow. The lower the impedance, the more efficiently the object will transfer heat. Thermal impedance is a critical aspect of Power Controllers as it determines how much power it can dissipate within a specified ambient temperature, which in turn determines how much load current it can effectively switch in an application. In most cases, the thermal impedance of a Power Controller is an irrelevant specification as full load-current ratings at different ambient temperatures are provided in the controller’s datasheet. However, understanding the importance of thermal impedance does help in understanding why heat sinks are required and vary in size for different ratings at different ambient temperatures.

HBControls Power Controllers utilize semiconductor-based relays to switch power to and from AC or DC loads. These relays typically utilize SCRs or MOSFETs for AC or DC loads, respectively. However, in some cases, diode or IGBT modules may be utilized when required within the application.

Electromechanical relays and contactors utilize mechanical contacts that are subjected to arcing every time they make (when closing) or break (when opening) power to / from the load. Over time, the arcing that occurs with each operation damages the contacts and eventually results in relay failure. The ‘heavier’ the load, the more significant the arc and the lower the life expectancy of the relay. Therefore, the life expectancy of an EMR typically ranges from 100,000 to 500,000 operations, which can be exceeded in just a few months in many industrial applic