Comments on Is it possible to use two zener diodes in series back to back to replace a diac?
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Is it possible to use two zener diodes in series back to back to replace a diac?
I've accidentally burnt my dimmer with a short. It is a common dimmer with a triac/diac/pot of the simplest configuration (the triac is a BTA16). After replacing the triac, this still does not work, and I'm almost sure the problem is the DB3 diac (breakdown voltage at 32 V). I don't have this beast in my lab, so, I would like to know if it is possible to use two 32V zener diodes in series, back-to-back, to obtain approximately the same effect, a question that may be interesting for its own.
Here is a somewhat simplified schematic (snubber and some filtering cap not included)
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No, two zener diodes are not equivalent to a DIAC.
While they both have a specific breakdown voltage, the DIAC is a special 3-layer device that exhibits a negative resistance characteristic once it breaks down:
In contrast, the two zener diodes simply stop conducting once the voltage is reduced below the breakdown level.
In a typical phase-control dimmer, it is the charge on the timing capacitor that is used to trigger the TRIAC. The negative resistance characteristic of the DIAC is what allows this to happen. The back-to-back zener diodes would not pass enough current to trigger the TRIAC.
You can use a couple of SCR-connected transistors (or an actual SCR) to simulate the negative-resistance characteristic of a DIAC, but only in one direction. Each of these circuits only works correctly when the upper terminal is positive with respect to the lower. But you can work around that limitation by "wrapping" either circuit in a bridge rectifier.
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