SCR Phase Control Speed Control/Dimmer

The DIAC is a 28V bidirectional (bilateral) trigger device that is used on virtually all inexpensive phase controls. The trigger voltage is somewhat high for phase control of 115VAC. Years ago, there was a similar low voltage (6 to 8V) trigger device called a Shockley diode. Unfortunately, these never caught on and today are EXTINCT. The 2N6027 Pr
SCR Phase Control Speed Control/Dimmer - schematic

ogrammable Unijunction Transistor (PUT) can perform a similar function, but is polarity sensitive so it does not lend itself to TRIAC control. However, if two such PUT trigger circuits are employed for anti-parallel SCRs, some interesting things are apparent. Most important is the ability to control both trigger circuits with a single potentiometer so that both half-cycles are controlled identically. To obtain best balance, the zeners and capacitors must be matched. Both zeners and capacitors are specified for a tolerance of 5%, but I selected mine for better than 1% with my DMM. Purchase a few additional components so that you may obtain a good match. PUT operation is simple. Its threshold voltage is programmable so that it can trigger at low voltages. In this circuit it is set via the 12V zeners. When the PUT anode voltage exceeds the gate voltage by one junction drop, the PUT fires and dumps the timing capacitor into the SCR gate circuit. It resets when the AC line voltage reverses. I made a PUT relaxation oscillator flasher circuit that you may wish to experiment with: The typical DIAC controlled TRIAC dimmer tends to snap-on  at the minimum voltage when the adjustment is slowly increased. After snap-on, the voltage may be reduced if desired. The snap-on  phenomenon is caused by the lagging phase relationship of the AC voltage signal that appears across the timing capacitor before the DIAC threshold is reached. After...

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