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 ==

In article ,
Sammy wrote:
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.

--
*A bicycle can't stand alone because it's two tyred.*

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" writes:

In article ,
Sammy wrote:
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.

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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Important: Anything sent to the email address in the message header above is
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"Sam Goldwasser" wrote in message
...

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.

On Sat, 11 Nov 2006 09:40:16 -0000, "Sammy"
wrote:

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 . . .

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"Sam Goldwasser" wrote in message
...

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

"Morse" wrote in message
...

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.

Zeners aren't right either.

Why not replace the diac?

"Sammy" wrote in message
...
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

"Homer J Simpson" wrote in message
news:Hvu5h.706$_Z2.376@edtnps89...

"Morse" wrote in message
...

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.

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" wrote in message
...


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



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

In article Hvu5h.706$_Z2.376@edtnps89,
Homer J Simpson wrote:

"Morse" wrote in message
...

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.

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.

--
*It sounds like English, but I can't understand a word you're saying.

Dave Plowman London SW
To e-mail, change noise into sound.

"Morse" wrote in message
...

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.

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.

"James Sweet" wrote in message
news:REw5h.2202$T_.422@trndny06...



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

Or cathodes together, either way.

would do this.

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

Morse

"Homer J Simpson" wrote in message
news:0pv5h.714$_Z2.50@edtnps89...

"Morse" wrote in message
...

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.


Ah right, that buggers up that theory then!

Morse

"James Sweet" wrote in message
news:REw5h.2202$T_.422@trndny06...

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.

Homer J Simpson wrote:
"James Sweet" wrote in message
news:REw5h.2202$T_.422@trndny06...


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.





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]

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

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

"Reinhard Zwirner" wrote in message
...

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?

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.

"Sammy" wrote in message
...
Thanks to all of you who replied.
The diac is split and measures 0.640 ohms in either
direction.

You may have a faulty triac then.

Homer J Simpson schrieb:

"Reinhard Zwirner" wrote in message
...

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?

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.

Hmm. As to my experience the triacs's gate normally is sensitive enough
to get the triac triggered by two zeners. BTDT: 24 V halogen lamp
controlled
by a triac triggered by a "two-5.6-V-zener-diac".

CU

Reinhard

Reinhard Zwirner wrote:
Homer J Simpson schrieb:

[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


Breadboard it up and find out, a light dimmer circuit only has a few
components.

"Reinhard Zwirner" wrote in message
...

Hmm. As to my experience the triacs's gate normally is sensitive enough
to get the triac triggered by two zeners. BTDT: 24 V halogen lamp
controlled by a triac triggered by a "two-5.6-V-zener-diac".

But how does the zener help?

Sammy ha escrito:

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

Please notice that a BTB04-600 *IS NOT A DIAC*. It is really a 4AMP
600V TRIAC. Check for yourself he
http://www.st.com/stonline/products/...btb04600sl.htm

Before replacing it, you should find why the original exploded.
Probably you have a shortcircuit somewhere in the bulb wiring.

"lsmartino" wrote in message
ups.com...

Sammy ha escrito:

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

Please notice that a BTB04-600 *IS NOT A DIAC*. It is really a 4AMP
600V TRIAC.

I had a bad feeling that he didn't mean a diac!

Check for yourself he
http://www.st.com/stonline/products/...btb04600sl.htm

Before replacing it, you should find why the original exploded.
Probably you have a shortcircuit somewhere in the bulb wiring.


Maybe, but IME they can and do burst like that with halogen loads because
the bulbs present a very low resistance when they fail. Almost every time my
500 watt uplighter bulb used to fail it would take out the mains fuse, the
triac or both.

Morse

Homer J Simpson schrieb:

"Reinhard Zwirner" wrote in message
...

Hmm. As to my experience the triacs's gate normally is sensitive enough
to get the triac triggered by two zeners. BTDT: 24 V halogen lamp
controlled by a triac triggered by a "two-5.6-V-zener-diac".

But how does the zener help?

If we talk about two back-to-back zeners: to provide a defined, constant
threshold at which the triac becomes triggered.

On the other hand: for me it's just a quick and dirty solution for
replacing a faulty diac if there's no original replacement at hand.

Bye

Reinhard

James Sweet schrieb:

....

Breadboard it up and find out, a light dimmer circuit only has a few
components.

As I wrote yesterday: BTDT ...

Bye

Reinhard

Correction to my last message. . . I should have said that
the faulty component (diac)
is marked Z2 07D431K. It is blue, with 2 leads. It measures
low resistance
in both directions. An exactly similar component on the
board measures high resistance both directions.
The component BTB04 600SAP (a triac, on a heatsink) is in
the 300w lamp circuit which is working ok.
In the 12v circuit, which has a transformer, are two
transistors? Q1 and Q2, which are marked U07772.
I don't have a schematic, but should manage a repair without
it. == Sammy ==

Reinhard Zwirner wrote:

Homer J Simpson schrieb:

"Reinhard Zwirner" wrote in message
...

Hmm. As to my experience the triacs's gate normally is sensitive enough
to get the triac triggered by two zeners. BTDT: 24 V halogen lamp
controlled by a triac triggered by a "two-5.6-V-zener-diac".

But how does the zener help?

If we talk about two back-to-back zeners: to provide a defined, constant
threshold at which the triac becomes triggered.

On the other hand: for me it's just a quick and dirty solution for
replacing a faulty diac if there's no original replacement at hand.

Bye

Reinhard

You have to connect Zener diodes in series, not parallel, if you want
it to clip at the same level in both directions. Connect the two
cathodes or anodes together, and use the other set of leads to connect
it into the circuit.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

On Mon, 13 Nov 2006 20:21:32 -0000, "Sammy" put
finger to keyboard and composed:

Correction to my last message. . . I should have said that
the faulty component (diac)
is marked Z2 07D431K. It is blue, with 2 leads. It measures
low resistance
in both directions. An exactly similar component on the
board measures high resistance both directions.
The component BTB04 600SAP (a triac, on a heatsink) is in
the 300w lamp circuit which is working ok.
In the 12v circuit, which has a transformer, are two
transistors? Q1 and Q2, which are marked U07772.
I don't have a schematic, but should manage a repair without
it. == Sammy ==


The 07D431K sounds like a 430V, 7mm diameter MOV. The fact that there
are two of them rather than just the one seems to confirm that it
cannot be a diac.

In fact go he
http://www.datasheetarchive.com/sear...actDS=Contains

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

"Michael A. Terrell" schrieb:

Reinhard Zwirner wrote:
....

If we talk about two back-to-back zeners: to provide a defined, constant
threshold at which the triac becomes triggered.

On the other hand: for me it's just a quick and dirty solution for
replacing a faulty diac if there's no original replacement at hand.

Bye

Reinhard

You have to connect Zener diodes in series, not parallel, ...

Hi Michael

That's exactly what I tried to describe with "two back-to-back zeners",
but, obviously, this description has led to a misunderstanding - sorry!

Thanks for clarification

Reinhard

On Tue, 14 Nov 2006 08:26:58 +1100, Franc Zabkar
wrote:


The 07D431K sounds like a 430V, 7mm diameter MOV. The fact that there
are two of them rather than just the one seems to confirm that it
cannot be a diac.

And they frequently fail shorted . . . and blow fuses

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"default" wrote in message
...

And they frequently fail shorted . . . and blow fuses

Or shoot flames out the top!

On Tue, 14 Nov 2006 15:45:32 GMT, "Homer J Simpson"
wrote:


"default" wrote in message
.. .

And they frequently fail shorted . . . and blow fuses

Or shoot flames out the top!

Place I worked had some Reliance DC drives 5-100 HP range. The MOVs
would be give an audible indication that the fuses were blown - not
even the leads remained.


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"Sammy" wrote in message
...
Correction to my last message. . . I should have said that
the faulty component (diac)
is marked Z2 07D431K. It is blue, with 2 leads. It measures
low resistance
in both directions.

So it's a MOV, not a diac, as Franc pointed out. It serves a passive role
until there's voltage spikes etc and then presents a very low impedance
across the mains and blows the fuse and usually itself.

You can leave it out really, there's no user safety issues I can think of,
though a bad mains spike could take out the triac without one fitted, then
you'd get an 'always on' lamp. It's not like it's a box of sensitive,
expensive electronics though so I wouldn't worry about it too much.

At the very least, leave it out, replace the fuse and test the light, it may
well work. Replace the MOV once you've cleared the fault, if indeed there is
one other than the MOV itself, if you really feel happier with one back in
circuit.

You're lucky you didn't put two back to back diodes or whatever in its
place, they would have exploded and possibly embedded their casing in your
flesh or eyes! What made you think it was a diac? Did you give the circuit
layout a look over before you came to that conclusion? Diacs are always in
line with the triac gate, never across the supply!

A friendly tip: Always be sure about components before naming them,
misunderstandings can be very dangerous when dealing with mains operated
equipment.

Morse

Another, more urgent repair has cropped up, so the faulty
halogen is put aside for the moment. Thanks again for your
informative and useful replies.

Morse wrote:
"Sammy" wrote in message
...

Correction to my last message. . . I should have said that
the faulty component (diac)
is marked Z2 07D431K. It is blue, with 2 leads. It measures
low resistance
in both directions.

So it's a MOV, not a diac, as Franc pointed out. It serves a passive role
until there's voltage spikes etc and then presents a very low impedance
across the mains and blows the fuse and usually itself.

You can leave it out really, there's no user safety issues I can think of,
though a bad mains spike could take out the triac without one fitted, then
you'd get an 'always on' lamp. It's not like it's a box of sensitive,
expensive electronics though so I wouldn't worry about it too much.

I thought overvolted triacs just turned on harmlessly, hence no need
for a MOV.


NT