Solid State Relay and Contactor

1. Which relay should I use, solid state relay or mechanical relay?

For high current power switching, there are three commonly used devices: contactor, solid state relay (SSR), and mercury relay.

A) Contactor
Contactor is an electromechanical switch. When the current rating is more than 15A, it is normally called contactor instead of relay. 

The advantages of using contactor to switch the power are the followings: (1) It does not generate heat as the SSR does. So there is no need for heat sink to dissipate the heat. (2) It normally comes with two poles or three poles. So when it is off, both power lines are open. It makes the system safe. (3) When it fails, it stays open for most of cases. This is relatively safer than the SSR. (4) The cost is low.

There are also some disadvantages of using contactors: (1) It has a limited life time. The average life of a contactor is about 100,000 times open and close at rated current load. So, the switch needs to be switched at low frequency, typically 20-40 seconds for each action, to keep the contactor last longer. (2) Because of the low switching rate, the control result will not be as good as the fast switching devices when controlling a fast response system. (3) The electromagnetic interference (EMI) is high. When switching off the contactor, the coil can generate a high voltage spike that can cause damages to the controller or make it unstable. (4) A high current contactor produces a loud noise when it opens and closes. They noise might be okay in an industrial environment, but it can be annoying in a quiet environment such as a lab or a house.

When selecting a contactor, you need to decide the number of poles, the coil voltage, and the maximum current. In North America, 2 poles contactor is needed for applications using 240V AC. The coil voltage should match the output voltage from the controller. But it does not have to be the same as the voltage which the contactor is going to switch. The current rating should be equal or higher than the maximum current drawn by the load.

B) Solid-state relay (SSR) 
SSR is a semiconductor switch.  It provides a higher precision control than contactors. It is used in most of high end ovens. There are many different types of SSRs on the market. For the ease of discussion, we concentrated on DC triggered AC SSR with zero-crossing trigger function. This is the most common type of SSRs our customer need.

The advantages of SSR are the following: (1) There is no mechanical component to wear out. So it can have infinite life time when used properly. And it can be switched at very high frequency. This high frequency switching feature provides higher precision control. (2) It produces almost no electronic noise (e.g., EMI) because of the power is switched on and off when AC voltage is at the zero level. 3) It does not generate any acoustic noise either.

The disadvantages of the SSR are the following: (1) It produces heat when passing the current. Typically, each amperage of current produces about 1.3 to 1.5 watts of heat. The heat needs to be removed by heat sink or forced air. Otherwise, the SSR will be burned quickly. For details about how to cool the SSR, please see Section 3 blow. (2) SSR is fragile. High voltage spike or a short to the ground will damage it because SSR blows faster than a fuse or circuit breaker does. (3) When a SSR fails, there is higher probability for it to fail in short condition than the contactor will. When that happens, the power can pass through the SSR continuously. If the system does not have additional protections such as over temperature protection, it can be unsafe. (4) Because of the heat generating issue, many people only put one SSR on one hot line in a 240VAC system in North American. This is like to put a single pole switch for 240 VAC. When powered off, the load is still at high voltage. It can be a safety issue if live electric wire is not protected. For safety reason, a double pole high current switch or contactor is need to cut the power completely off. (5) The cost of SSRs, the cooling devices, and extra switches make the total cost of the system higher than a contactor system.

C) Mercury relay
This type of relay overcomes disadvantages of both contactors and SSRs. However, owing to environmental considerations about the toxicity of mercury, mercury relays are mostly obsolete although modern encapsulated units still have some applications. They are generally being replaced by solid state relays.


2. Which SSR should I choose?

There are many different types of SSR. When choosing a SSR, here are two most basic considerations:
  • Load voltage (output voltage). There are AC SSR and DC SSR. The AC SSR is for switching AC current. The DC SSR is for switching DC current.  For the AC SSR, the most common one is single phase SSR, but we also sell three phase SSR. It is like 3 pole contactor.
  • Control voltage (input voltage). The most commonly used SSRs are the DC triggered (typically 4-30 VDC). There are also AC triggered SSRs. But this type of SSRs is not very common. AC triggered SSRs are mostly used to replace electromechanical relays with AC coil. For DC triggered AC SSRs, there are zero-crossing triggered type and random triggered type.

3. Do I need a heat sink for the SSR?

SSRs are made of semiconductors that have limited conductance. When passing a current, heat will be produced. Each ampere of current will produce about 1.3 to 1.5 watts of heat. When a 12 Amps current passes through a SSR, approximately 16 watts of heat will be produced in the SSR. As heat is produced, temperature inside the SSR will rise. The Maximum temperature on the metal surface of the SSR should not pass 70°C (158°F). If it rises above 70°C, the SSR may not shut off and eventually gets damaged. High temperature can also shorten the life or damage other components in the same box.

The temperature of the SSR depends on the amplitude of the current, duty cycle, and the ambient temperature. The rule of thumb is that if the current is more than 8 Amps, you will need a heat sink or mount it on a thick aluminum sheet. If it is more than 15A, you need either to use a 25A external heat sink or a 40A SSR heat sink or forced air (with fan). Increasing the capacity of the SSR (using a 40A SSR instead of 25A SSR for a 15A load) will not significantly reduce the heat; a heat sink is still needed. However, higher capacity SSR is structurally more durable. When more than 30A is passed, the box that holds the internal SSR needs to have good ventilation with forced air.

In most cases, the heat generating on SSRs is only an issue during the beginning stage of a heating process when the heater is running at full power. Once the temperature is close to the set point, it will probably take less than 50% of the power to maintain the temperature. Since the generated heat is directly related to the current passing through the SSR, the heat produced at steady state might be insignificant and can be ignored. For 12 Amps load, if you system takes less than 3 minutes to heat up to the steady state, you might not need a heat sink. A typical case is for home espresso machine.  

4. Can I test a SSR with a multimeter?

4.1) Q: When there is no input for the SSR, I can still measure a live voltage on output of the SSR. Does that mean the SSR is defective?
A: No. The SSR is a semiconductor device that has a leak current. The leakage is less than the range of 0-30 mA. Large capacity SSR has higher leakage than the small capacity SSR. If you measure the output voltage at an open circuit condition, you will get a live voltage that is the same as the AC input. (This is the typical case when user tests the output with a voltage tester).That does not necessarily mean the SSR is burned through. However, if there is a load (more than 10 Watt, such as a heater) on the output, you should not detect any significant voltage because it is drained by the load.

4.2) Q: When I measured the resistance of the output of SSR, it changed from mega ohm to 1-20 kilo ohm range when an input trigger is applied. Should it be zero ohm?
A: No, it should not. This is because almost all AC SSR on the market has a zero voltage crossing detection circuit. It will not turn on (change resistant to zero) unless it detects an AC voltage that is changing the flow direction (across zero voltage). The zero voltage detection function is for protecting inductive loads. Without an AC load, the output resistance will never change to zero. It should be noted that changing the resistance from mega ohm to kilo ohm does not mean this SSR is good. But It tells you it has not been burned open and is responding to input.

4.3) Q. How to test an AC SSR correctly?
A. The most reliable way is connect the output with a 40W light bulb. If the light bulb responds to the input trigger, the SSR is fine. You can also refer to this guide ( 

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