How to use SCR to control the power of a heater

Question

I would like to point out that I am not an expert (as it is perceived by the question) in the use of scr, diac, and triac and that this question is more for academic purposes than use in real life. Basically, I need to better understand how the scr is commonly used.

I wish to understand how to use SCR in a dimmer with a given heater resistance value and an input voltage frequency value. My goal should be to fully understand how I should pick the other components and if the schematics that I provided below are right or wrong.

The first schematic is very simple. The RC part should activate the SCR during the positive halfwave of the input. For R very small the SCR should be almost always active during the positive halfwave while for R very large the SCR should be almost never active during the positive halfwave. It is correct what I wrote? How should I pick the values for R and C? Supposing I want to keep the SCR active for a given percentage of the positive half-wave, how should I act?

^{simulate this circuit – Schematic created using CircuitLab}

I saw on on schematics that I found on internet that almost all add a diac before the gate of the SCR. What is it's purpouse? How does it change things? I think that is to not stress the gate of the SCR but I'm not sure.

^{simulate this circuit}

Furthermore, in some cases there is a diode from mass to gate. I think that is because in the input negative half-wave the SCR isn't used but, again, I'm not sure that this is the real and only reason. If a triac is used instead of the SCR the diode is obviously absent.

^{simulate this circuit}

I know that a real schematic should be much more complex but, for now, I'm interested in keeping it as simple as possible in order to fully understand.

Answer 1

To answer the DIAC part of your question:

The gate trigger voltage will vary from device to device and will be temperature dependent. It's typically around 0.5 V or so. All of these factors mean that it is difficult to get good repeatability.

Another factor is that for AC circuits where full-wave control is required a TRIAC is used. These can have different trigger values on the positive and negative half-cycles so that the waveform is not symmetrical.

The DIAC solves some of this. It has a breakdown voltage - typically around 20 V - and this dwarfs the variations mentioned above. In addition, once triggered it stays on so that the full charge of the capacitor can run through the SCR gate giving a reliable turn-on.

Links

• Phase Control Using Thyristors by Littlefuse.
• Thyristor Theory and Design Considerations Handbook by ON Semiconductor.

You might also find this question / answer useful: A question on zero crossing versus random-fire SSRs.

Answer 2

Your first circuit shown does not work. Using just an RC circuit means that the SCR must be a 'sensitive gate SCR' with very low trigger current. The RC charge is complicated in that the capacitor will charge to the gate break over voltage on the negative half cycle (typically 6-8 V) and only charge to the forward gate (typically 0.7 V). This makes it very difficult to calculate the RC required for any given positive turn on point, and when the reverse gate voltage is exceeded it depends on the SCR construction what will happen (it may clamp or avalanche).

Your second circuit shown does not work either. While using the DIAC improves things, in this version the negative half cycle is almost guaranteed to avalanche the SCR gate. In the positive half cycle, the RC network can be calculated for the DIAC trigger point (typical ranges are from 25 - 40 V). Once the DIAC triggers then the capacitor is discharged into the SCR gate. Pulse currents may be in the 500 mA plus region.
You also would typically place a resistor ( 1- 2k Ohm) across the SCR gate.

Have a look at some typical SCR's on Digikey, you will see for the range I chose the gate current ranges from 200 uA (sensitive gate) to 60 mA.

Your third circuit will work, but it should have a resistor of 1-2k Ohm across the 1N4148 Diode.
Here the SCR is triggered by dumping the capacitor charge into the gate for the positive half cycle.
For the negative half cycle the capacitor charge is clamped by the 1N4148.
This means for both half cycles the capacitor is charged from the discharge level (typically about 1.5 V) achieved at the last trigger. Now you can work out the RC network reasonably accurately.

It's worth noting that you can use a single SCR to achieve dimming on both half cycles of the mains voltage by using a bridge rectifier.

^{simulate this circuit – Schematic created using CircuitLab}

This type of circuit works well in the 240 V AC world where it provides a good range of adjustment. It's not quite as good in the 120 V AC world since the adjustment range is not as good. For example if you used a 40 V DIAC and had a bridge rectifier with 1.8 V drop, and another 0.7 for the SCR gate the total is a 42.5 V drop from the AC input signal. That means that you can never set early turn on less than 42.5 V which is about 25% of the peak value for 120 V AC but only 12.5% of a 240 V AC waveform.