Basic SCR circuit as two bipolar transistors.
Fig. 1

Silicon Controlled Rectifier Review and Circuits

by Lewis Loflin

A silicon controlled rectifier sometimes called a Thyristor is simply a diode with a gate. They are used in a number of circuits usually AC-DC power control. They are related to Triacs that I won't cover here.

See Basic Triacs and Silicon Control Rectifiers.

In theory they can be represented as two transistors connected as shown in Fig. 1. When a small current flows between the gate and cathode the device will conduct and keep conducting until anode to cathode current flow is halted.

Referring to Fig. 1 when SW1 is pressed the small gate current, limited by a 270 Ohm resistor, turns on Q2. Then Q2 turns on Q1 and they keep each other turned on.

Basic SCR test circuit with DC input.
Fig. 2

In Fig 2 I have redrawn the circuit. Press SW2 the gate current turns on SCR1. Press SW1 (normally closed) the current path is broken the lamp turns of.

Basic SCR test circuit with AC input.
Fig. 3

In Fig. we input an AC voltage. Press SW4 the lamp comes on at half power. SCR1 acts merely as a halfwave rectifier. Release SW4 and the lamp turns off. When the AC cycles to zero SCR1 is turned off.

The diode is used to block the negative half-cycle from the SCR gate.

This property is useful when dealing with AC power control circuits. See Basic Power Supply Rectification Tutorial.

H11C6 SCR output opto-coupler.
Fig. 4

In Fig. 4 is an SCR based opto-coupler used to control higher power SCR circuits. This provides an interface between a PLC or low voltage microcontroller.

Dual SCRs acting as a Triac
Fig. 5

For more on this see the following:

SCR case styles.
Fig. 6

Fig. 6 illustrates a few of the many case styles available. Some can carry hundreds of amps of current and are very expensive.

Another issue is gate sensitivity. Many high power SCRs require higher gate drive currents. Check you data sheet. The unit in the lower left corner I've seen used in industrial welding equipment.

BT137-600E SCR case.
Fig. 7

Fig. 7 is the BT137-600 I used in many of my projects on these webpages. While rated at 8 amps be aware of heat sinking issues. I would not run the device above 6 amps. The gate is fairly sensitive and I built several SCR modules with these used with H11C4 opto-couplers.

Note this dual SCR circuit made from those separate opto-coupler SCR modules.

Fig. 8

Fig. 8 illustrates the MCR100-6 SCR rated at 800mA at 400 volts. I used these to build lower-power test circuits at 24 volts before moving over to the BT137-600 at 120 volts.

They can also be used to drive less sensitive, high power SCRs.

Both the BT137-600 and MCR100-6 I bought off of Ebay. Be careful when reading the Ebay pages because some vendors throw the term Triac around for SCRs and Thyristor around for Triacs. Check the part number data sheet before you buy!

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