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Top tips for the safe installation of SSRs

  • Heatsinks and SSRs
  • Protective Measures / Electrical Noise
  • Surge Ratings
  • Fusing
  • SSR Applications


Adequate heatsinking, including consideration of air temperature and flow, is essential to the proper operation of a solid state relay (SSR). It is necessary that the user provide an effective means of removing heat from the SSR package. The importance of using a proper heat sink cannot be overstressed, since it directly affects the maximum usable load current and/or maximum allowable ambient temperature. Lack of attention to this detail can result in improper switching (lockup) or even total destruction of the SSR. Up to 90% of the problems with SSRs are directly related to heat.

All solid state relays develop heat as a result of a forward voltage drop through the junction of the output device. Beyond a point, heat will cause a lowering (or derating) of the load current that can be handled by the SSR. “Heatsinks” are used to create a method of removing heat away from the relay, thus allowing higher current operation.

With loads of less than 4 amperes, cooling by free flowing convection or forced air currents around the unit is usually sufficient. Loads greater than 4 Amps will require heat sinks.

We recommend mounting our units on the heatsinks listed on the heatsinks and accessories web page. However, when this is not possible, and units are to be mounted to some other heatsinking object, material heat conductivity should be kept in mind. Our heatsinks are approximately equivalent, in heat dissipation, to a sheet of aluminum 1/8″ thick by the dimensions shown:

S505-HEATSK-2.1 12″ X 12″

S505-HEATSK-1.5 15″ X 15″

S505-HEATSK-1.0 18″ X 18″

(Given proper ventilation and ambient temperature).

In comparison, twice the amount of steel and four times the amount of stainless steel would be needed to achieve the same effect.

Units should not be mounted in an enclosed area without proper air flow. Units should also never be mounted to a plastic base or to painted surfaces.

The heat sink should be positioned with the fins in a vertical position with an unimpeded air flow. The vertical mounting will aid in heat dissipation in that heat may rise from the heat sink unobstructed.

Any panel mount Solid State Relay must be mounted to a clean, bare (non-painted) surface that is free of oxidation.

Silicone (thermal) grease should be placed on the metal base of the relay before mounting to a metal surface. Heat Transfer is affected by the thickness of the thermal compund, uniformity of application and how firmly the relay is attached to the heatsink. We suggest an evenly applied 0.002″ thick layer of Dow Corning 340 or equivalent and torque of 10 inch-pounds on both of the SSR mounting screws. Note that a thicker layer of thermal compound actually decreases heat transmission.

Care must be taken when mounting multiple SSRs in a confined area. SSRs should be mounted on individual heatsinks whenever possible. Panel mount SSRs should never be operated without proper Heat Sinking or in Free Air as they will THERMALLY SELF DESTRUCT UNDER LOAD.

A simple Rule-Of-Thumb for monitoring temperature is to slip a thermocouple under a mounting screw. If the base temperature does not exceed 45 degrees Celcius under normal operating conditions, the SSR is operating in an optimal thermal environment. If this temperature is exceeded the relay’s current handling ability must either be thermally improved by the use of a heatsink, or greater air flow must be provided over the device through the use of a fan. Some cases may require the selection of a higher current output SSR and thermally derating the device accordingly.

Remember that the heatsink removes the heat from the Solid State Relay and transfers that heat to the air in the electrical enclosure. In turn, this air must circulate and transfer its heat to the outside ambient. Providing vents and/or forced ventilation is a good way to accomplish this. Heatsinks should always have at least one inch below them, so air can enter the finned heat sink area. Heatsinks should always have empty space above them so the warm air can exit the heat sink area. If horizontal, plastic, wiring trays are used above the heat sink, then the empty space should be greater than the depth of the plastic tray. For example, if you use 4 inch deep wiring trays, leave >4 inches of empty space above the relay.

All Solid State Relays are capable of running at full rated power (with proper heatsink), however it is strongly suggested that they be used at no more than 80% power, to provide a safety margin in case of higher than expected Volage, Temperature, dirt on the heatsink,etc.

**A Special Note to DIN Rail Mounted Single Phase Relays**:

These devices are provided with an integral heatsink and should be mounted so as to provide 1″ (25mm) of space between the units, for best air flow (the 80% of power rule still applies). They can be mounted against each other if the end units in a row are derated an additional 10% and the middle units are derated 10% more than the end ones. However, when using the new RV design din rail relays, the recommended space betwen units is decreased to 0.18 inches, due to the advanced thermal design. Therefore, you can install a bank of RV relays with 0.18 inch spacing and they will meet their specifications without further de-rating.


SSRs generally do not fail due to electrical noise, unless they happen to mistrigger during a point in the line cycle when an excessivly high current surge might occur. Usually, a malfunction due to noise is only temporary, such as turning on when the SSR should be off, and vice-versa.

By its very nature, noise is difficult to define, being generated by the randomness of contact bounce and arcing motor commutators, etc. Noise, more properly defined as ElectroMagnetic Interference (EMI), affects the SSR by feeding signals into the sensitive parts of the circuit, such as the SCR.

A built-in snubber RC network across the output is effective in reducing sensitivity to noise, especially at lower frequencies. This is standard with CII’s AC relays.


The metal oxide varistor was developed about the same time as the SSR and has subsequently become a trustworthy companion of the SSR, providing much needed protection in some of its more hostile environments.

An MOV can be used as follows: across the incoming line to supress external transients before they enter the system; across the load to supress load generated transients; or more frequently, across the SSR to protect it from all transient sources. In the latter case, the MOV can be conveniently mounted to the same SSR output terminals as the load wiring.

An MOV can be used effectively across such loads as transformers and switching power supplies where spikes too fast to be absorbed by the transformer itself may be fed back into the primary (SSR load) winding. Used within its ratings, the MOV will most likely outlive its associated equipment and provide low cost protective insurance for the SSR.


There are very few completely surgeless SSR loads. Next to improper heatsinking, surge current is one of the most common causes of SSR failure. Overstress of this type can also seriously impair the life of the SSR. Therefore, in a new application, it would be wise to carefully examine the surge characteristic of the load and then select a device that can adequately handle the inrush as well as the steady state condition, while also meeting the lifetime requirements.

In addition to the actual surge ratings given for SSR’s, the rate of rise of surge current (di/dt) is also a factor in AC thyristor types. Exceeding its value may result in destruction of the device. As a guide, the amperes-per-microsecond (di/dt) withstand capabilities are typically in the order of their single cycle surge ratings.

The highest surge current rating of an SSR is typically 10 times the steady-state RMS value, and it is given as the maximum peak current for one line cycle. It should be noted that a surge of this magnitude is allowable only 100 times during the SSR’s lifetime. Furthermore, control of conduction may be momentarily lost due to a surge. This means that it may not be possible to turn off the SSR by removal of control power both during and immediately after the surge. The output thyristor must regain its blocking capability and the junction temperature allowed to return to its steady state value before reapplication of the surge current, which may take several seconds.

It should be noted that the preceding cautionary notes apply only to the extreme limits where the SSR should not be designed to operate anyway.

Generally, DC SSRs do not have an overcurrent surge capability, since the output transistors are usually rated for continuous operation at their maximum capacity. The tendency is for the DC SSR to cut off (current limit), thus impeding the flow of excessive current. However, the resultant overdissipation may destroy the relay if the surge is prolonged.


Fast, “Semiconductor Fuses” are the only reliable way to protect SSR’s. They are also referred to as current-limiting fuses, providing extremely fast opening while restricting let-through current far below the fault current that could destroy the semiconductor. This type of fuse tends to be expensive, but it does provide a means of fully protecting SSRs against high current overloads where survival of the SSR is of prime importance.

An I2T fuse rating (ampere-squared seconds) is useful in aiding in the proper design of SSR fusing. This rating is the bench mark for an SSRs ability to handle a shorted output condition.

We advocate circuit protection through the use of a properly selected I2T (semiconductor fuse).

Devices such as electromechanical circuit breakers and slow blow fuses cannot react quickly enough to protect the SSR in a shorted condition and are not recommended!! Fast blow type fuses may be appropriate for some applications.

Every SSR has an I2T rating (see SSR specifications on this Web site). The procedure is to select a fuse with an I2t let-through rating that is less than the I2T capability of the solid state relay for the same duration.

I2T relates directly to the published fuse characteristics. System designers who are considering using I2T fuses should consult a good technical manual dealing with the application of these fuses when designing their systems (i.e. Littlefuse’s semiconductor fuse catalog).


Solid State Relays (SSRs) cannot always be applied in exactly the same way as Electromechanical (EMRs) and when such is the case, caution should be taken.

Inductive Loads

While most SSR loads, even lamps, include some inductance, its effect with resistive loads is usually negligible. Only those loads that utilize magnetics to perform their function, such as transformers and chokes (windings), are likely to have any significant influence on SSR operation. These loads can create large current surges and the SSR should be derated accordingly. ***Please call Technical Support for additional Suggestions.

Transformer Switching

Extremely high current surges are commonly associated with transformers, especially those with a penchant for saturation. The zero voltage turn on feature of standard SSRs can increase this possibility and might require that special precautions be taken. The zero current turn off characteristic of SSRs, while minimizing the problem, will not prevent it.

From the practical and economic standpoint, the best choice may still be a standard SSR, overrated to withstand huge surges. ***Please call Technical Support for additional Suggestions.

Motor Switching

Dynamic loads such as motors and solenoids, etc., can create special problems for SSRs. High initial surge current is drawn because their stationary impedance is usually very low. As a motor rotates, it develops a back EMF that reduces the flow of current. This same back EMF can also add to the applied line voltage and create ‘overvoltage’ conditions during turn off. Most of the surge reducing techniques discussed earlier can also be applied to motors. It should be noted that overvoltage caused by capacitive voltage doubling or back EMF from the motor cannot be effectively dealt with by adding voltage-transient suppressors. Suppressors such as Metal Oxide Varistors (MOVs) are typically designed for brief high voltage spikes and may be destroyed by sustained high energy conduction. It is therefore important that SSRs are chosen to withstand the highest expected sustained voltage excursion. ***Please call Technical Support for additional Suggestions.

Lamp Switching

The inrush current characteristic of incandescent (tungsten filament) lamps is somewhat similar to the surge characteristic of the thyristors used in AC SSR outputs, making them a good match. The typical ten times steady state ratings which apply to both from a cold start allow many SSRs to switch lamps with current ratings close to their own steady state ratings. CAUTION: Using SSRs for driving mercury, fluorescent, or HID lamps should be avoided. If they must be used, the SSR must be severly derated and thoroughly tested in the specific application.

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