How to use SCR to control the power of a heater
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.