halogen dimmer

Not quite sure what you mean? Simply bypassing the dimmer and running the bulb straight off the transformer should be ok. Or if you know it's the DIAC, replace it.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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I think he means replace the diac with the two diodes. Sure, that would work but as noted above, just bypassing the whole thing should will work also.

But why does he think the diac is faulty?

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There's a tendency to assume it's the bit you don't understand or can't test.

The obvious answer would be to just get rid of the dimmer entirely.

One assumes this: 'mother and child' uplighter (240v). Is something that already has a transformer stepping the mains down to 12 and not just some scheme of firing the triac to maintain 12 volts???????

If this is one of the schemes that uses the firing angle to keep the voltage from the 240 mains at 12 or lower - you don't want to mess with it. Two diodes will replace the diac - but diacs usually fire at ~30 volts and putting diodes in will have them fire at .6 volts that would really screw up the thing if there's no transformer . . .

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I think it's highly unlikely that would work- a diac isn't simply two standard diodes bunged back to back, it is IIRC more like two 30v zeners back to back.

Morse

Zeners aren't right either.

Why not replace the diac?

Diac? Are you sure it's a diac you mean? Can you describe the circuit and the component in question, and how you tested it? The answer is no, if the diac is faulty it should be replaced with a diac, it's not as simple as two standard diodes back to back.

I've repaired stacks of dimmers and rarely had to change a diac. They tend to fail only when the triac they drive shorts out, which often results in other burnt out components.

Where is the 12V derived from? Does it come from an isolating 240V-12V transformer then go through the dimmer? If so then *theoretically* you'd just wire the 12V straight to the bulb bypassing the dimmer circuit, no faffing with diodes is necessary. If the dimmer is on the mains side then bypass the dimmer and take mains straight to the transformer primary.

However, without knowing exactly what you have I couldn't recommend the above procedures as they may raise important safety issues.

If the 12V is derived by dropping 240V to 12V by a triac chopper (not likely legal in the EU) then it's strongly recommended you get it professionally repaired or bin it.

I've seen halogen lights with 'electronic transformers', which are switchmode power supplies designed to be dimmed. I wouldn't recommend delving inside these as they provide isolation from the mains as well as

240v-12v conversion and can't just be bypassed.

Morse

I didn't say they were, I mentioned them as a closer approximation electrically than standard diodes.

Why not indeed? I certainly didn't imply any different course of action.

Morse

Thinking about it a bit more- the closest approximation I can come up with would be four 30v zeners in a serial-parallel combo. Two zeners in series with anodes together in parallel with two zeners in series with cathodes together. This would drop 30.6 ish volts in either direction if I'm not mistaken.

Still, a proper diac is the right way to go, messing with Frankenstein jobs is pointless other than to satisfy one's curiousity ;-)

Morse

I've never known one blow on its own - it's usually the triac failing that takes it.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

IIRC, a diac is like a triac with no gate - when it fires the dropped voltage is a couple of volts only. This is unlike a zener.

Just two zeners in series with the anodes together would do this.

Or cathodes together, either way.

You're right :-) Too late at night and too much beer for thinking!

Morse

Ah right, that buggers up that theory then!

Morse

Pretty sure you need a real diac. For higher voltages a neon lamp can do it too.

Yeah a diac is the way to go for what the OP is looking for, I was just responding to another part of the thread.

Homer J Simpson schrieb:

[two zeners as replacement for a diac]

You're right: but would there be a difference in function between a triac triggered (at 30 V) by a diac and a triac triggered (at 30 V) by two back-to-back zeners? Wouldn't both of them conduct current until its next zero crossing?

Wondering

Reinhard

Thanks to all of you who replied. The diac is split and measures 0.640 ohms in either direction. marking on it: BTB04 600SAP. Another similar diac on the board, for the 300w halogen circuit, is high resistance both ways. I'll find a replacement diac for the faulty one rather than experiment with diodes. There is so little space for diodes anyway. A low value fusible resistor in the 12v circuit is o/c. I

The point of the diac is to dump enough current into the gate to turn on the triac. A zener doesn't do that - it just passes the current once the voltage is exceeded.