Important Information on Using Solid State Contactors and Relays

It is important to us that your solid state relays and contactors(SSC) last their incredible life-time! To ensure safe and reliable applications when using any of our SSCs, there are 8 important facts to be mindful of:

Keep in mind different manufacturers may specify their device ratings differently, the following generally applies to our Mach-1 and MAX-Qseries solid-state contactors only. To download a PDF version of this file, click here


  1. Switching Constant Current Loads:

DO NOT use solid state contactors (SSCs) to switch constant current without ensuring open-circuit voltage is within limits of SSC in operation.


  1. Placement in Circuits:

MAXQ™/Mach1™ SSCs can switch either the high side or the low side. However, the switching of the high side is preferred as it is deemed safer.


  1. Rated Current, Rated Voltage Selection:

Note some manufacturers may specify current ratings based on transistors’ performance at 25°C. Other manufactures may specify ratings without any de-rating considerations; in both cases the rated performance cannot be practically achieved. MAXQ™/Mach1™ SSC specifications are based on a more realistic approach, which is when the device is operating continuously with a baseplate temperature of 90°C or 100°C.

Tolerances need to be considered in the design of all electrical applications. Most semiconductor failures are the result of misuse, improper selection, or the lack of transient suppression. While selecting SSC ratings, you must consider parameters such as inductance, inrush current, transients, and environmental temperatures.

3.1 Rated Current Selection:

For all applications, the SSC’s rated current must be higher than the circuit’s worst-case peak current. It is important to recognize, for example, the peak current of a DC motor circuit may be the initial inrush current, or the stall current that’s likely 5x or higher. Similarly, the peak current of a capacitive circuit may be the initial short circuit current.

              Helpful Tips

  • Choose SSCs with higher rated current, enhances both reliability and thermal performances.
  • For most of our models, multiple units can be connected in parallel to share current and/or to reduce power loss. However, to ensure that peak current is also shared, both units require special customizations. Contact us for more details.
  • For general overcurrent protection, use a quick fuse or air circuit breaker.

3.2 Rated Voltage Selection:

Consider choosing a rated voltage 2x higher than the nominal voltage for resistive loads and even higher for inductive loads. For some of our SSC models, you may connect multiple units in series to increase rated voltage. Contact us for more details.

*Regarding di/dt and Inductive Loads

MAXQ™/M1™ SSC can open a circuit within 10s of µS. As a result, a significant back EMF may be generated when opening high currents. SSC’s opening time may be customized to improve EMI behaviour during inductive switching; however, this is not always ideal. For high current, high frequency, or inductive switching, it is essential to choose ratings and make external clamping decisions based on the potential energy stored in the inductor. Naturally, BEMF voltage should not exceed the SSC’s rated voltage; however, for applications where the overall energy released by the inductor is low when compared to SSC's switching power, external inductive clamping may not be required.        

Helpful Tips

For applications with high inductance or high di/dt, consider the following:

  • Choose SSCs with higher overall ratings
  • Choose SSCs with transient protection option (e.g. RC snubbers and MOVs)
  • Reduce SSR's operating temperature
  • Clamp the circuit externally with one of the methods described in section 4 below


    1. Managing Inductive Kicks:

    When switching inductive loads, unclamped high-energy inductive kicks may damage the SSC. Even if damages do not occur immediately, it may affect SSC's long-term reliability. For switching inductive loads, clamp the load with one of the two methods described below. MOV or RC snubbers are not recommended for high inductance load, because MOV is not intrinsically reliable, and the RC snubber not be sufficient.

    4.1) Freewheel Diode/Catch Diode Method

    This is the most common and effective method. The reverse-biased rectifier diode would exponentially decay the flyback energy by looping it back to the load, dissipating energy through the resistance of the wires and the load. The duration of the decay, depending on the load characteristics, may only last for 10’s of milliseconds.

    Freewheel Diode Selection:

    • “DC Blocking Voltage” should be ≥2x the operating voltage.
    • “Repetitive Peak Current” should be much larger than the operating current; “Peak Forward Current” should be ≥ the operating current if possible.
    • Choose diodes with faster recovery and higher ratings for high-frequency applications.

    4.2) Freewheel Diode + TVS/Zener Diode Method

    For high-frequency inductive applications requiring faster dissipation of the magnetic field, adding a TVS/zener diode in conjunction with a freewheel diode is recommended. This shortens the duration of the flyback kick, typically by a factor of 5 at the cost of slightly higher peak transient voltage.

    Freewheel Diode Selection:

    Same as “Freewheel Diode Selection” above.

    TVS Diode Selection:

    • “Vrwm” or “working voltage” ≥ operating voltage.
    • “Ipp” ≥ expected transient peak.
    • “Vc” ≤ maximum voltage rating of the protected component.    

    Flyback Protection Comparison (Load: 12V Electromagnet)

    a)b) c) 

    a) without external clamping

    b) freewheel diode only

    c) freewheel diode + zener diode


    1. Applications Requiring Extra Reliability:

    Should the SSC operate more frequently than usual, or if reliability is of greater concern over cost, consider the following:

    • Clamp the circuit against any transients
    • Choose SSCs with higher overall ratings
    • Improve cooling to ensure SSC stays at a much lower temperature
    • Switch with two SSCs in series/parallel to add redundancy
          1. PWM/Switching Frequency:
          • Do not exceed the rated max switching frequency of the contactor. This may cause SSC’s duty cycle to increase until SSC no longer turns OFF.
          • Rapid switching will also generate additional heat. Therefore, at higher PWM frequencies (typically >3kHz), user may need to de-rate the operating current, improve the heatsink, or clamp the load against BEMF.
          • For applications operating at higher frequencies, the SSC’s output duty cycle may be marginally different from the programmed input. Measure and compensate for this change if necessary.


              1. Heatsink/Cooling Requirement:

              Cooling must be carefully considered for all solid state switches. A minimum thermal derating table is provided on the side label of each SSC.  It should also be noted that SSC’s long-term reliability falls with an increase in operating temperature. For most of our solid state contactor models, nominal current can be shared. You may even reduce the total heat dissipation by wiring multiple units in parallel.

              Cooling requirements are determined by both the “maximum allowable baseplate temperature”, as well as the “maximum allowable junction temperature”. If a maximum allowable baseplate temperature is not provided, consider using the value of 100°C for MAXQ- Series and 90°C for Mach1- Series, respectively. The following calculation can be used to estimate the minimum thermal resistance required of the heatsink.

              Heatsink RSA = (TMax allowable baseplate°C  - TAmbient°C) / Power Loss – Rjc case-heatsink thermal resistance

              However, for MAXQ- Series solid state contactors operating at less 40% of rated current, you may use the following method of calculation.

              Heatsink RSA = (TMax allowable junction°C  - TAmbient°C) / Power Loss – Rjc junction-case thermal resistance – 0.1°C/W  

                            To maximize heat dissipation through the heatsinks, always apply a thin layer of thermal paste (1W/mK or greater), and tighten all panel mount screws according to the recommended torque.


              1. Storage/Handling:
              • The SSC’s long term storage condition should be at an ambient temperature of 0 to 40°C with a relative humidity of 45 – 85%.
              • Do not drop the relay or subject it to a hard impact.
              • Do not subject the relay to excess vibration.
              • Do not store or use the relay in environments exposed to salt, dust, or metallic dust.
              • Do not store or use the relay in environments directly exposed to oil and chemicals.
              • Always ensure proper working knowledge and safety precautions, and handle all electrical components with care.

              Do not touch SSC’s output terminations when power is ON or immediately after power is switched OFF.

              Conduct wiring only when power and input control signals are completely CUT OFF.

              SSC and heatsink may likely be hot and cause burns. Do not touch them until power is OFF and surfaces are cooled.


              To download a PDF version of this file, click here