SSR - solid state relay

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The SSR its uses and perculiarities.

An SSR--or to give its correct name 'Solid-State Relay' is a development and improvement over the mechanical coil and contact relay or contactor used for switching high powers using a low controlling supply voltage. The SSR contains no moving parts, so is not subject to mechanical wear.

It has many advantages and a couple of disadvantages over the mechanical counterpart. Principle advantage is the expected life-span is improved as there are no mechanical parts or contacts to arc or burn out. They are more immune to wear caused by frequent on/off cycles than the standard mechanical Relay type of device, and can give a finer temperature control due to the speed at which they can cycle.

As there are no open contacts in an SSR, they do not make any sparking or other issues that could cause an explosion--should an explosive vapour of Methanol be present in the vicinity.

Principle Disadvantages are the fact being a 'Semiconductor' device it can fail Short-Circuit, meaning that the supply will be permanently connected to the load no matter what the control voltage is doing. Why they fail can be many- Incorrect use, wrong type for application, or too high an operating temperature ie no heatsink attached.

Another disadvantage is that the SSR during normal use generates some heat as there is a small voltage-drop across the internal Triac semiconductor devices. The voltage-drop is usually around 3-5V at say, 10A so a heat of up to 50W can be developed--This is quite a lot of heat energy. --This is quite normal, but must be dealt with if the device is to give long service and no unexpected failures.

SSR's Must be fitted to a Heatsink that is recommended by its makers. Often the SSR can be purchased with the correct type heatsink for its capacity and it should be remembered that this heatsink should be mounted in such a way as to allow clear unobstructed airflow at all operating times.

Testing circuits using SSR's can give some unexpected results--even though the device is operating normally. A cold test (without any power connected, just using a meter for continuity) will give incorrect readings. This is due to the way the device is made. A simple meter test would show up a short-circuit SSR but not confirm whether the device is good in other ways.

Due to the internal construction and Snubber-Circuit inside the SSR it can be confusing to test under fault conditions. If there is Mains supply to the SSR, and the load (heater, motor etc) is disconnected or faulty then there will be a Voltage at the Output connection Even WITHOUT any Control Voltage supply as measured with a meter. --This does not mean the item is faulty. If a load, such as 100W Incandecant Lamp is connected in place of the usual switched device, then this voltage will disappear and the readings will be as expected providing the SSR is functioning normally.

It should be remembered, that Any Supply from an SSR can be LIVE AT ALL TIMES--even when working properly and without a Load Connected, and there is a potential for a severe shock if the switched side is inadvertently touched if the Mains Supply is still connected and turned On.

Many home-constructors of BioDiesel plant control-panels use a combination of both SSR type and Mechanical Relays for their advantages and disadvantges. Often a Standard 30A relay is used in a Safety Circuit that supplies the power to the SSR part that runs the heaters. This first standard relay would be connected to a number of safety devices such as Pressure Level Switches, Float Level Sensors and Over Temperature cut-out switches. As the normal operation of this safety circuit is infrequent the wear and life of the mechanical relay is good, and the SSR does the harder job of frequent on/off cycles for temperature control.

(Thats yer lot for now!)

Heatsink sizing

For reliable operation the heatsink should be sized to ensure that the silicon junctions inside the SSR remain below 125 degrees C. You can find more information of the calculations here: http://en.wikipedia.org/wiki/Thermal_resistance_in_electronics

Essentially, you should use a heatsink with a thermal transfer coefficient of about 0.8 K/W. It's not necessary to purchase a dedicated heatsink, a fairly large lump of metal will suffice such as a metal control cabinet. However, do monitor the temperature. If your heatsink is uncomfortably hot to touch, you should think about increasing the size of it.