Fig. 1

Silicon Controlled Rectifiers Connected as a Power Triac

by Lewis Loflin

Silicon-controlled rectifier (SCRs) are used for high power AC switching. When used in pairs to simulate their Triac cousins they can switch higher power levels. This is because only one SCR is one at one time thus a lower duty cycle.

Fig. 1 shows the circuit I built and it works. I also found out what doesn't work.

They are also called Thyristors. For more on the properties of SCRs related to these circuits:

Right wrong way to connect SCRs as a Triac.

Fig. 2

Fig. 2 illustrates the right and wrong way to connect silicon-controlled rectifiers as Triacs. I built the circuit on the right that is presented across the web. It does not work with both SCRs on all the time.

The better way to view this is the left side where anode is tied to cathode. The two gates must be separate to be properly triggered as individual devices.

SCRs can be triggered with photo triacs or photo SCR optocouplers.

Fig. 3

The best option to trigger SCRs or Triacs is with opto-coupler as shown in Fig. 3.

The H11CX photo SCR opto-couplers are designed to the express purpose of triggering SCRs. The problem is many of these parts are hard to find today.

One can use the MOC30XX photo Triac opto-couplers with the addition of a diode in the gate circuit. My test show no practical difference in operation.

Single MOC3011 optocoupler triggers two SCRs.

Fig. 4

Fig. 4 is the exact circuit diagram for the setup I built in Fig. 1.

Two separate SCR modules using H11C6 optocouplers connected as Triac.

Fig. 5

I also built two separate SCR modules based on Fig. 3 with a single opto-coupler for each SCR. Here I used H11C6 photo SCR opto-couplers with input LEDs connected in series.

Two separate SCR modules using MOC3011 optocouplers connected as Triac.

Fig. 6

Fig. 6 does the same thing as Fig. 5 but I use MOC3011 type opto-couplers with the additional gate diode.