Solid State Relay

How about connecting digital logic to the nasty world of 115 to 230 volts or more? There is always the solution of using an electromechanical relay. However, solid state is the current trend. A solid state relay device can make it possible for a battery-operated supply to turn on light bulbs, electric motors, radios, or almost anything else you can imagine.

Solid state relays can provide isolation from high voltage and can drive tens of amps. They are commercially offered in plastic cases, with a heat sink on the bottom and screws for attaching wires. Usually, they cost a lot and if damaged they can not be repaired. Most SSR's (solid state relays) are made in large quantities and usually rated at about 10 Amps or more. Usually, it does not worth to pay for a ten-amp relay when all you need is a two-amp relay. Besides, the larger the SSR, the larger the leakage currents.

However, using a few parts, you can build yourself a solid state relay at a fraction of the price of a commercially made one. And the best thing is that this specific relay can be repaired if anything goes wrong.

How it works

A solid state relay is much like a switch that is controlled by an input voltage or current. This switch can only be used for AC voltages. Trying to switch a DC line will result in a relay that closes but never opens. This is because, it uses a Triac which can be turned-off only if the current drops to zero. 

Our SSR circuit is shown below. D1 diode is used for reverse voltage protection. R1 limits the input current . Q1 is used as current sink in order to keep the current through the LED (the LED inside U1) at an almost stable value. When the voltage across R2 reaches about 0.65V, Q1 begins to conduct, shunting current from the LED. The result is that, although the current of R1 rises as the input voltage rises, the current through the LED stops increasing at a specific value; The minimum LED current that will operate the TRIAC. This value is set by R2.          

Choosing the TRIAC

Make sure that you TRIAC will be able to handle the voltage required. Switching a 115-volt AC line, requires a 250V TRIAC. A 220V line requires a 400-V TRIAC. Next to be considered is the maximum current.  Any TRIAC will handle its nominal current  if it  is properly heat sinked. Remember, that many loads (such as motors) draw a lot more current on start-up than they do during normal operation.

There is also another requirement; The gate current. Using an opto-isolator which will provide about 100mA should be adequate for any TRIAC you can find in a T-220 package. Remember also, to choose an isolated TRIAC for safety precaution. Isolated TRIACs provide electrical isolation from the electrical connections to the case.  That eliminates the need of using mica washers for isolating the heat-sink from the case. However, if you don't know whether or not your TRIAC is isolated, simply measure the resistance from each lead to the case. An isolated TRIAC will measure open on all three leads.

Choosing the opto-isolator

Many companies make opto-isolators. Make sure you are using one which has TRIAC output and a compatible pinout for your design. For instance, a MOC3010 will be sufficient. Table 1 shows some typical TRIAC-output optoisolators, compatible with our design.

Instead of using a simple opto-isolator, you can also use a zero-crossing one (such us MOC3031). A zero-crossing SSR accepts triggering at any time but delays turning on the AC load until the next time the AC voltage passes through zero volts. This is useful for eliminating RFI  (Radio Frequency Interference) and for preventing a large current for flowing into the load almost instantly. 

 Construction and safety

Although the SSR can certainly be built without a PCB (Printed Circuit Board), using the PCB pattern we provide, will make things easier. Some PCB lines would have 110 or 220 Volts. From an electrical point of view, that is perfectly safe. However, it's probably a good idea to cover all printed circuit lines with a silicone sealer. Also, prefer to use isolate TRIACS and always ground their heat sink to the AC safety wire (green or yellow, or earth ground). The SSR can be triggered from 4 to 10V (the input voltage). Exceeding 10V may harm the optoisolator's LED.     

Parts List

R1 =100 ohms 1W
R2=39 ohms, see table 1
R3=180 ohms
R4=2K2
R5=10K
C1=10nF, 450V
U1=see table 1
Q1=2N3904
D1=1N4002
TR1=Q4006L4 or equivalent

All resistors are 1/4W, 5%, unless otherwise noted.

Attachments

Solid state relay project - PCB details