7/27/2019 SCR Phase Control Speed Control

1/3

SCR Phase Control Dimmer Circuit

This SCR phase control works much like the common TRIAC dimmer, but has numerous advantages including increased

current capability, robustness and absence of minimum voltage snap-on. A complementary, symmetrical trigger circuit

consisting of two PUTs (programmable unijunction transistors) enables firing of two anti-parallel THYRISTORs (SCRs). The

circuit makes up a two terminal power device that is simply inserted between the AC power source and load. Besides

controlling the intensity of incandescent lighting, it is useful in controlling the speed of universal (commutator brush type) ACmotors.

http://www.electroschematics.com/8411/scr-phase-control-dimmer/

SCR Phase Control Dimmer Schematic

http://www.electroschematics.com/8411/scr-phase-control-dimmer/http://www.electroschematics.com/8411/scr-phase-control-dimmer/http://www.snumam.com.br/http://www.dolphins-software.com/http://www.autotest.com/

7/27/2019 SCR Phase Control Speed Control

2/3

Bill of Materials

Bill of Materials File

SCR Phase Control Dimmer BOM .xls

The ideal triggering device

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 Programmable 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: http://www.electroschematics.com

/6904/programmable-unijunction-transistor-put-flasher-circuit/

Snap-on

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 it

http://www.electroschematics.com/8411/scr-phase-control-dimmer/http://www.electroschematics.com/

7/27/2019 SCR Phase Control Speed Control

3/3

initially fires, the timing starts at line voltage zero crossing.

The timing circuit in this control completely avoids this problem by resetting the capacitor voltage each half-cycle via reverse

diodes across the two timing capacitors.

Advantages of SCRs over TRIACs

Thermally, there are numerous advantages. Splitting the output current in half reduces the device current significantly and

provides a much lower thermal resistance to the ambient. Also, SCRs are rated for a maximum Tj of 125, while TRIACs

may be limited to only 110C. Then, of course, SCRs are available in current ratings extending to hundreds of amps well

over ten times that of the largest TRIACs. SCRs are simply more rugged.

Its only disadvantage is obvious: it simply takes more hardware and circuitry.

The snubber

R1 & C3 form a snubber that is connected across the power devices. It performs two important functions as follows:

First it provides a circuit to absorb reactive energy when either SCR recovers (stops conducting current). As you may know,

a rectifier conducts current in the reverse direction for a short period of time (e.g. 5S) when it becomes reversed biased. The

flow of this current stops abruptly and any series circuit inductance then causes the generation of a transient voltage (spike).

The snubber is a place for this current to go so it cannot develop a high voltage.

Second, it provides a resistive load for high frequency line noise and transients. Such noise or transients can potentially cause

an SCR to turn on for a half-cycle. One common source of transients is simply the power switch when it closes.

Film resistors are not robust in handling voltage transients so I specified a ceramic composition resistor. Polyester capacitors

have poor reliability at 230VAC, so I specified a polypropylene capacitor that is AC rated. In my circuit, I used a Quencharc

device that contains both resistor and capacitor it is AC rated for 125VAC.

Oscillographs

The load voltage is easy to see here as it increases in phase. The base line is rather indistinct due to the low power load used

(7.5W lamp) the snubber reactance causes significant voltage drop across the lamp.

The gate current indicates a peak current of about 200mA and a time constant of about 32S essentially 47 * 0.68f.

The leading edge spike is caused by the inductance of the 1 shunt resistor used. This can easily drive a much larger SCR

the first one I made used a 90A thyristor doubler. The minimum gate current pulse width rule of thumb is 5S in which the

gate current exceeds Igt (gate current threshold) this allows sufficient time for the load current to ramp up to the SCR

holding current.

The gate voltage has a different appearance than the output of the programmable unijunction for two reasons: First, the gate

is a non-linear resistance that tends to act like 2 or 3 series diodes. Secondly, when the SCR is conducting, junction voltage

appears on this terminal as well.

SCR Phase Control Project Photos

Note the absence of the isolation transformer it is located under the bench.

For the future

AC Switch Circuit turn this into an AC switch via a photomod

SCR Phase Control Speed Control/Dimmer http://www.electroschematics.com/8411/scr-phase-control-dimmer/

3 of 6 12-Aug-13 0:01

http://www.electroschematics.com/8411/scr-phase-control-dimmer/http://www.electroschematics.com/8411/scr-phase-control-dimmer/