This is the circuit diagram of solid state relay. In reality, it is not a relay after all. There’s no ‘relay’ found, just the electronics circuit which does the switching. It operates a similar way as a relay; it is possible to use a low voltage to switch a higher voltage or better than ordinary relay. This ‘relay’ is placed in between one of the 115/220V AC wires even though it’s typical practice to leave the neutral wire the way it is and switch the phase or hot wire. See diagram for ‘LOAD’.
On condition that there is no dc voltage present (left side of circuit diagram), the phototransistor inside the TIL111 blocks and as a result no electric current is present. To ensure of that the base of the TIL111 is fed towards the emitter (e) through an 1M resistor. This method avoids the base of transistor BC547B going low and therefore remains biased ‘on’. The collector is thus also low, and consequently the gate (g) of the TIC106M thyristor, which is still in the ‘off’ state. Through the 4-diode bridge rectifier circuit there’s no electric current, except for the miniscule basis and collector current of the BC547B that is not enough over the 330 ohm resistor to switch on the TIC226M Triac. The current via the ‘Load’ is thus really extremely small.
Having a particular input voltage, say 5 volt, the diode within the TIL111 lights up and activates the phototransistor. The voltage drop over the 1Meg ohm resistor in series with the 22K resistor increases to a particular set point that it’ll block the BC547B transistor. The collector current immediately will follow that of the AC voltage to a particular value that will initialize the Thyristor. This creates a adequate large voltage drop over the 330 ohm resistor to switch the Triac ‘on’. The voltage over the Triac at that time is just a few volts so that the practically the entire 115/220 AC voltage is over the ‘Load’.
The Triac is secured via the 100nF capacitor and also the 47 ohm resistor, the 100nF capacitor over the 330 ohm resistor is to avoid undesirable biasing of the Triac generated by small spikes. For making the posibility to switch this circuit with various voltages, a FET BF256A has been added. This FET acts like a current-source by means of connecting the source (s) together with the gate (g). What this means is that this FET determines the current via the TIL111, no matter what voltage is set on the input (within particular tolerances of course). The diode 1N4148 is to secure the circuit in case of polarity reversal.(The TIL111 is also known as ‘optoisolator’ with an NPN output and may be replaced using a NTE3042)
A great point of a circuit like this, is the seperation of the DC and AC voltages, so this solid state relay circuit might be applied in many kind of applications, up to around 1.5 KiloWatt, in case you mount the Triac on a huge cooling-rib.The ‘M’ indicator noted on the Triac means it’s a 600volt design, a ‘D’ stands for 400volt. So be sure you go for the M model.(Tony: The NTE replacements for this circuit are 600volt models which is more than sufficient for our 110/115VAC.) Also, when you make a decision to make a PCB for this circuit, to design sufficient space between the AC lines and do not make these AC tracks far too narrow.
Solid state relay circuit was designed by Jan Hamer, translated and republished by Tony van Roon