halogen dimmer

[Sammy]

I have a 12v halogen dimmer light - a 'mother and child'
uplighter (240v). The diac is faulty. I don't need to have
the dimmer function working.
It would be ok if I could just have it at full brightness
all the time. Could I substitute 2 diodes in parallel, one
in either direction, to achieve full
brightness?. Any particular diodes?. == Sammy ==

 

[Dave Plowman (News)]

have a 12v halogen dimmer light - a 'mother and child'
uplighter (240v). The diac is faulty. I don't need to have
the dimmer function working.
It would be ok if I could just have it at full brightness
all the time. Could I substitute 2 diodes in parallel, one
in either direction, to achieve full
brightness?. Any particular diodes?. == Sammy ==

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.

 

[Sam Goldwasser]

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.

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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[Homer J Simpson]

But why does he think the diac is faulty?

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

 

[default]

I have a 12v halogen dimmer light - a The diac is faulty. I don't need to have
the dimmer function working.
It would be ok if I could just have it at full brightness
all the time. Could I substitute 2 diodes in parallel, one
in either direction, to achieve full
brightness?. Any particular diodes?. == Sammy ==

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 . . .

 

[Morse]

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.

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

 

[Homer J Simpson]

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.

Zeners aren't right either.

Why not replace the diac?

 

[Morse]

I have a 12v halogen dimmer light - a 'mother and child'
uplighter (240v). The diac is faulty. I don't need to have
the dimmer function working.
It would be ok if I could just have it at full brightness
all the time. Could I substitute 2 diodes in parallel, one
in either direction, to achieve full
brightness?. Any particular diodes?. == Sammy ==

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

 

[Morse]

Zeners aren't right either.

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

Why not replace the diac?

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

Morse

 

[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

 

[Dave Plowman (News)]

Zeners aren't right either.

Why not replace the diac?

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

 

[Homer J Simpson]

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.

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.

 

[James Sweet]

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.

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

 

[Morse]

Just two zeners in series with the anodes together

Or cathodes together, either way.

would do this.

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

Morse

 

[Morse]

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.

Ah right, that buggers up that theory then!

Morse

 

[Homer J Simpson]

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

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

 

[James Sweet]

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.

 

[Reinhard Zwirner]

[two zeners as replacement for a diac]

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.

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

 

[Sammy]

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

 

[Homer J Simpson]

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?

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.