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I have a triac DIAC cirtcuit. I don't have any DIACs on hand, but I do have plenty of TN1215s. Is it possible to generate the required pulses using an SCR to drive the TRIAC's gate?

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  • \$\begingroup\$ Maybe- with a bridge rectifier and a zener to the gate but it will be very high trigger current with that particular SCR and will require changing other component values and ratings. People are throwing out that kind of dimmer now because they don't handle LED lamps well- you could salvage. \$\endgroup\$ – Spehro Pefhany Sep 27 '16 at 16:36
  • \$\begingroup\$ @SpehroPefhany Or get a $0.1 DIAC, of course. \$\endgroup\$ – Asmyldof Sep 27 '16 at 17:03
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The circuit will quite likely work without the DIAC - i.e., short out where the diac should be until you can get one - but may be a bit "twitchy".

Without the DIAC the TRIAC will turn on when the trigger voltage gets high enough. The problem is that this is not very well defined and will vary depending which quadrant the triac is operating in and, probably, with temperature.

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Figure 1. Trigger angle and resultant waveform.

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Figure 2. Triac triggering modes and quadrant numbers. Source Wikipedia (with addition of numbers).

Note that your circuit works on quadrants 1 (both mains and trigger positive) and quadrant 3 (both negative) only.

If the triac triggers levels in the first and third quadrant then there may be a noticable flicker between positive and negative half-cycles particularly on the romantic end of the pot.

The DIAC solves this by holding back the trigger until the capacitor voltage rises to about 22 V (if my memory is right) and then allows the capacitor to discharge through the gate pin. The point is that a fraction of a volt difference on 22 V will result in less phase angle difference between positive and negative half-cycles than directly triggering. The downside with the triac is that you can never get 100% power.

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  • \$\begingroup\$ Hmmm. So an SCR circuit cannot replace a DIAC? \$\endgroup\$ – user148298 Sep 27 '16 at 19:17
  • \$\begingroup\$ Not directly, anyway. The DIAC is bidirectional and in your application both directions are used. An SCR isn't and won't work on its own at least. \$\endgroup\$ – Transistor Sep 27 '16 at 19:20

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