I've found this method of triggering of the SCR through DIAC and I'm trying to understand it. I was able to figure out the whole trigger circuit and how to compute the values of RC in order to have the desired delay time to trigger the DAC. What I don't understand is how should I syncronize the trigger part with the wave from the line.


simulate this circuit – Schematic created using CircuitLab

After reading the comment of @user287001, isn't something like that almost correct? (suppose that is enough for me to use only an half-wave)


simulate this circuit

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    \$\begingroup\$ It's useless to try to understand this. It's nonsense. V1 must be synchronous AC with V2, a wire is missing at the bottom between C1 and SCR's cathode, SCR must be replaced with a TRIAC if you want to use both half cycles of the V2. A resistor should be added in series with R1.. Now, if one turns R1 to zero, the smoke is a very probable result (after adding the missing wire). Consider DIAC as a dam that suddenly opens when the voltage in C1 gets to plus or minus 32V. The dam must get closed for a new triggering pulse. I's not possible ifV1 doesnt goto negative or at least to zero. \$\endgroup\$ – user287001 Oct 9 '17 at 10:53
  • \$\begingroup\$ the phase timing network R1 C1 needs to be connected to the AC supply not a DC supply. \$\endgroup\$ – Jasen Oct 9 '17 at 11:10
  • \$\begingroup\$ I'm not expected that the circuit above works. But I'm almost sure that one can control the trigger of the SCR with a DC source. I've found too much schematics in papers with such a thing. However I know that I miss something, I wrote the question for this reason. \$\endgroup\$ – gvgramazio Oct 9 '17 at 12:13
  • \$\begingroup\$ DC source is possible, but you must take some signal from the V2 to control the timing of the triggering. Diac is not ok, you must use something that can be forced ON and OFF with a control signal, That control signal must be generated in a proper timing circuit. \$\endgroup\$ – user287001 Oct 9 '17 at 12:37
  • \$\begingroup\$ @user287001 Do you mean that I should use some external timer if I want to use DC source? \$\endgroup\$ – gvgramazio Oct 9 '17 at 12:58

You are almost there.

The only missing thing is synchronization with ac votage. This is how you can achieve it:

triac dimmer

Now the capacitor charging will be in sync with applied ac voltage. This will give you triac firing in 1st and 3rd quadrant.

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  • \$\begingroup\$ In this way how can I compute the RC constant? Do I have problems of hysteresis? I mean that in order to have a low power transmission to the lamp I first need to regulate the potentiometer to have an intermediate power transmission and then regulate it to have the desired low power transmission. Or, at least I think so. \$\endgroup\$ – gvgramazio Oct 9 '17 at 12:09

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