H11C6 SCR output opto-coupler.
Fig. 1

Light Activated Silicon Controlled Rectifier Based Optocouplers

by Lewis Loflin

My purpose here is to use SCR output Optocouplers to control power SCRs. This requires knowledge of basic AC rectification. This is because in simple terms a silicon-controlled rectifier is a diode with a gate.

See Basic Power Supply Rectification Tutorial.

The H11C series consists of a gallium-arsenide infrared emitting diode optically coupled with a light activated silicon controlled rectifier in a 6-pin dual-in-line package. They come in voltage ratings of 200 and 400 volts.

The data sheet call this "symmetrical transistor coupler".

25 W logic indicator lamp driver
Detector 400mW RMS on state current 300mA
200 V symmetrical transistor coupler (H11C1, H11C2, H11C3)
400 V symmetrical transistor coupler (H11C4, H11C5, H11C6)

Also see What is a Light Activated Silicon Controlled Rectifier? (LASCR)

SCR and photo SCR transistor theory.
Fig. 2

Symmetrical Transistor Coupler

What is meant by "symmetrical transistor coupler"? A silicon controlled rectifier is a four layer semiconductor device. It acts as two back-to-back transistors as shown on the left in Fig. 2.

When SW1 is pressed a small current flow through the gate resistor turning on Q2. Q2 then turns on Q1 that keeps Q2 turned on. The combination can only be turned off disconnecting +Vcc from the anode.

With the photo SCR on the right light enters the Q4 base circuit creating a small current turning on Q4. Q4 will then turn on Q3. As before the device stays on until +Vcc is turned off.

As the H11CX data notes we must have a resistor between the gate and cathode for correct operation. Otherwise the unit is unstable and will turn itself on.

Bridge rectifier.
Fig. 3

For the rest of these circuits I'll be using pulsating, unfiltered DC from a bridge rectifier as shown in Fig. 3. The load is a 24-volt panel light bulb. One can use any resistive load.

The actual output voltage is RMS * 0.9 = 21.6 volts.

SCR used as a halfwave rectifier.
Fig. 4

Fig. 4 illustrates the use of a SCR as a halfwave rectifier. When SW4 is pressed current flows to the gate during the positive half-cycle turning on the SCR. The SCR stays off during the negative half-cycle.

The output voltage is RMS * 0.9 / 2 = 10.8 volts. Switch SW3 is not needed.

H11C4 directly driving a light bulb.
Fig. 5

In Fig. 5 we have a H11C4 used to turn on a light bulb. When the input LED is turned on the photo SCR is switched on. No mechanical switches are involved. This is designed for microcontroller control. This is still limited to 300mA.

H11C4 turning on a high power SCR.
Fig. 6

Fig. 6 illustrates the main use of these photo SCR opto-couplers: controlling higher power SCRs.

Arduino phase control of SCR with H11C4 opto-coupler.
Fig. 7

In Fig. 7 we use an Arduino to phase control pulsating DC. This is often used in 3-phase power control circuits in industry.

This uses the same program as my basic Arduino AC power control.

See In Depth Look at AC Power Control with Arduino.

Example three-phase power supply control for a welder. Full wave DC output.
Fig. 8

Fig. 8 illustrates fullwave phase control rectification for a welder. It uses three separate opto-couplers one for each phase. If using an Arduino it would require three complete circuits from Fig. 7 with a single control potentiometer tied to each ADC0 together.

To see an actual welder diagram see Miller CP250-TS 3 Phase Welder.

Opto-coupler gate resistors not shown.

As of 7/10/2019:

H11C3 is a NTE3046 ~ $4.40.

4N39 much the same as NTE3046 https://www.aliexpress.com $0.42 each Qty. 100

Or $1.70 Qty 10 + $1.05 shipping to US.