Frequently asked questions
What is a Carlo Gavazzi solid state relay?
A solid state relay is similar to an electromechanical relay or contactor but without any moving parts. Unlike mechanical relays, Carlo Gavazzi solid state relays utilize back-to-back (anti-parallel) SCRs to perform the switching function. This enhances their ability to withstand surge currents, and eliminates arcing, mechanical wear and acoustical-switching noise. This makes them an ideal solution in harsh industrial environments and in many commercial applications, especially where reliability, long life and shock & vibration resistance are desired.
Do Carlo Gavazzi solid state relays require a heat sink?
HB Controls primarily offers two types of Carlo Gavazzi solid state relays; panel mount solid state relays, and DIN mount solid state relays with integral heat sinks.
A solid state relay will dissipate energy in the form of heat at a rate of approximately 1W per amp of load current. Carlo Gavazzi panel mount solid state relays have a thermally efficient power circuit that transfers heat through the relay's base plate and into the panel to which it is mounted. In many cases, this is sufficient enough to dissipate some of the heat generated, but the amount dissipated is dependent upon the size of the panel, type of material, coating, etc.. When load current exceeds roughly 7 amps, the best solution for dissipating heat and ensuring reliable operation is to use an external heat sink. Our team at HBControls specializes in thermal dissipation and can assist you in selecting the right heat sink for your application - 800.879.7918 / email@example.com.
Carlo Gavazzi DIN mount solid state relays have an integral heat sink and are already derated for full load current in a 40°C ambient temperature. They are essentially "plug & play", eliminating the need for thermal calculations, heat sink selection and assembly.
Does the load current rating of a solid state relay change when mounted to a heat sink?
The "actual" load-current rating of a solid state relay is dependent upon the efficiency of the heat sink to which it’s mounted. Solid state relays generate heat at a rate of about 1 Watt per ampere of load current. This heat must be transferred away from the internal SCRs, through the relay’s base plate and into the surrounding ambient air to prevent it from overheating. Mounting a solid state relay to a heat sink increases its ability to transfer this heat, and how efficiently the heat sink transfers the heat determines how much current the solid state relay can continuously carry.
A common mistake made by many engineers is to use solid-state relays with higher current ratings whenever there’s an issue with the temperature of a solid-state relay with a lower current rating. For example, a Carlo Gavazzi 50 amp solid state relay generates heat at a rate of 1 Watt per amp and is rated to 50 amps when mounted on a 1°C/W heat sink. A Carlo Gavazzi 100 amp solid state relay also generates power at a rate of 1 Watt per amp, which means that its maximum allowable current rating when mounted to the same 1°C/W heat sink is also approximately 50 amps. The internal temperature of the 100 amp solid state relay will be slightly cooler due to the efficiency of it’s design, but not enough to provide a significant increase in the actual current rating. In order to carry more than 50 amps, a larger, more efficient heat sink (or forced airflow) is required.
What are the advantages of using a solid state relay instead of an electromechanical relay, contactor or mercury displacement relay?
· Life expectancy – Solid-state relays can operate reliably for decades in most applications. Theoretically they can operate reliably for centuries but, since the first solid-state relay was invented in 1972, that theory has yet to be proven. However, there are Carlo Gavazzi solid state relays that have been in the field for decades that are still operating normally and reliably.
· Silent operation – Solid-state relays do not generate acoustical noise when switching power to/from the load since they contain no moving parts. Therefore, they are the preferred solution in many commercial or residential applications where acoustical noise can quickly become an annoyance.
· Shock & vibration resistance – Since solid-state relays have no moving parts, they are not prone to false triggering under load in harsh environments subject to shock and vibration.
· Fast operation – Instantaneous turn-on solid state relays can switch power to a load in less than 100 microseconds of receiving an input signal. This makes them ideal for phase-angle control applications where tight control over power to the load is required.
· PLC compatibility – As opposed to large contactors or other mechanical relays, solid-state relays can switch heavy loads with only a few milliamps of input current.
· Environmentally friendly – Carlo Gavazzi solid-state relays contain no mercury. Also, the abbreviated life expectancy of electromechanical relays means that hundreds, if not thousands of EMRs will fail and have to be disposed before a single end-of-life solid-state relay failure.
What is a "zero-crossing" solid state relay?
“Zero crossing” refers to the point in the AC sine wave at which the solid-state relay begins to conduct load current after the input signal applied. If an input signal is applied when the AC sine wave is close to its peak (outside of its “zero-crossing window”), the solid-state relay will not turn-on until it reaches the zero-crossing point. The zero-crossing function gives solid state relays a significant advantage over electromechanical relays and contactors in applications with loads that have a low initial resistance. Since the output of the SSR will not turn on until the sine wave is close to zero, inrush currents are kept to a minimum.
How do you determine the heat sink requirement for a solid state relay?
At HBControls, this is our specialty. We have a wide range of thermally-efficient heat sinks perfecty suited for Carlo Gavazzi single-phase and three-phase solid state relays. Feel free to contact our technical team with your application's specifications and requirements - 800.879.7918 / firstname.lastname@example.org. We can help you find the right heat sink and can probably save you money, time and headache by doing the assembly for you.
However, for those that like math, the formula is, Tbp = Tamb + (P x (Rhs + 0.2°C/W)), where Tbp is the desired maximum temperature of the solid-state relay’s base plate (typically <100C°), Tamb is the ambient temperature inside the panel, P is the power dissipated (your load current) and Rhs is the thermal impedance of the heat sink. The formula includes a 0.2°C/W "buffer" to account for the impedance of a thermal pad or grease.