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John Hunter
 
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Default strange problem connecting light to dimmer

I am trying to change the light fixture on a ceiling light controlled
by a dimmer that has an on/off push control, and a rotary dimmer
control. I've changed this light twice before with no problems
(dimming worked as expected), but this 3rd time when I threw the
breaker back on I got no light. I'ver verified that the light works
by connecting it to another power source.

For the record, on this 3rd try to change the light, I was working
with the breaker on and and one point caused a brief short that caused
some sparks to fly and the breaker to trip. Since then I've always
turned off the power when working. Also, the original wiring coming
through the ceiling is very old and brittle with lots of exposed
parts. That's how I caused the short; by moving the wires around some
exposed wires must have brushed up against one another.

Tonight I tried to diagnose what was wrong, pulling out my trusty
fluke voltmeter. This surprised me: when the power is on, the minimum
voltage across the two wires I connect the light to is 60V. I get
this when the dimmer switch is off, or on at its minimum position.
When the dimmer switch is on and at its maximum position, I get 120V.
I was expecting to measure something close to 0V in the off position.

Now more wierdness. When I connected the light to the ceiling wires
(white to white, black to black, same ones that have worked for me
twice before without incident), I got nothing (as described above).
At first I assumed it was just a poor connection (I'm using wiring
nuts) but undid and redid everything and got the same result. When I
measure the voltage across the wires the light is connected to with
the light connected I get 0V, whether or not the switch is on or off,
minimal or maximal.

I have repeated all of these measurements multiple times (ie, turning
off the power, unwiring the light, powering back up, measuring,
turning off the power rewiring the light, powering back up, measuring
again). Always the same result: 60 to 120 V min to max when the light
is not connected, 0 volts when it is.

Can anyone explain to me what is going on, or what I should do to
diagnose this problem?
  #2   Report Post  
indago
 
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Default strange problem connecting light to dimmer

040117 1921 - John Hunter wrote:

I am trying to change the light fixture on a ceiling light controlled
by a dimmer that has an on/off push control, and a rotary dimmer
control. I've changed this light twice before with no problems
(dimming worked as expected), but this 3rd time when I threw the
breaker back on I got no light. I'ver verified that the light works
by connecting it to another power source.

For the record, on this 3rd try to change the light, I was working
with the breaker on and and one point caused a brief short that caused
some sparks to fly and the breaker to trip. Since then I've always
turned off the power when working. Also, the original wiring coming
through the ceiling is very old and brittle with lots of exposed
parts. That's how I caused the short; by moving the wires around some
exposed wires must have brushed up against one another.

Tonight I tried to diagnose what was wrong, pulling out my trusty
fluke voltmeter. This surprised me: when the power is on, the minimum
voltage across the two wires I connect the light to is 60V. I get
this when the dimmer switch is off, or on at its minimum position.
When the dimmer switch is on and at its maximum position, I get 120V.
I was expecting to measure something close to 0V in the off position.

Now more wierdness. When I connected the light to the ceiling wires
(white to white, black to black, same ones that have worked for me
twice before without incident), I got nothing (as described above).
At first I assumed it was just a poor connection (I'm using wiring
nuts) but undid and redid everything and got the same result. When I
measure the voltage across the wires the light is connected to with
the light connected I get 0V, whether or not the switch is on or off,
minimal or maximal.

I have repeated all of these measurements multiple times (ie, turning
off the power, unwiring the light, powering back up, measuring,
turning off the power rewiring the light, powering back up, measuring
again). Always the same result: 60 to 120 V min to max when the light
is not connected, 0 volts when it is.

Can anyone explain to me what is going on, or what I should do to
diagnose this problem?


Turn off the circuit; take the dimmer switch out of the wall and disconnect
it from the circuit and wire nut the two wires together and connect the
fixture to the wires in the ceiling. Then turn the circuit back on again
and see if the light works. I am thinking that the dimmer has failed and
you need a new one.

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Jeff Wisnia
 
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Default strange problem connecting light to dimmer

You have probably fried the dimmer.

If your Fluke is a digital meter it probably has an extremely high input
impedance and any voltages you measured were obviously present across the
meter leads, but might not have more than a miniscule amount of current
capability, hence dropped to zero when loaded with a lamp bulb.

Try what indago said to try.

Jeff

--

Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can smile when things are going wrong, you've thought of someone
to blame it on."


John Hunter wrote:

I am trying to change the light fixture on a ceiling light controlled
by a dimmer that has an on/off push control, and a rotary dimmer
control. I've changed this light twice before with no problems
(dimming worked as expected), but this 3rd time when I threw the
breaker back on I got no light. I'ver verified that the light works
by connecting it to another power source.

For the record, on this 3rd try to change the light, I was working
with the breaker on and and one point caused a brief short that caused
some sparks to fly and the breaker to trip. Since then I've always
turned off the power when working. Also, the original wiring coming
through the ceiling is very old and brittle with lots of exposed
parts. That's how I caused the short; by moving the wires around some
exposed wires must have brushed up against one another.

Tonight I tried to diagnose what was wrong, pulling out my trusty
fluke voltmeter. This surprised me: when the power is on, the minimum
voltage across the two wires I connect the light to is 60V. I get
this when the dimmer switch is off, or on at its minimum position.
When the dimmer switch is on and at its maximum position, I get 120V.
I was expecting to measure something close to 0V in the off position.

Now more wierdness. When I connected the light to the ceiling wires
(white to white, black to black, same ones that have worked for me
twice before without incident), I got nothing (as described above).
At first I assumed it was just a poor connection (I'm using wiring
nuts) but undid and redid everything and got the same result. When I
measure the voltage across the wires the light is connected to with
the light connected I get 0V, whether or not the switch is on or off,
minimal or maximal.

I have repeated all of these measurements multiple times (ie, turning
off the power, unwiring the light, powering back up, measuring,
turning off the power rewiring the light, powering back up, measuring
again). Always the same result: 60 to 120 V min to max when the light
is not connected, 0 volts when it is.

Can anyone explain to me what is going on, or what I should do to
diagnose this problem?





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Chris Lewis
 
Posts: n/a
Default strange problem connecting light to dimmer

According to Jeff Wisnia :
You have probably fried the dimmer.


Yup, probably.

If he pulled enough current thru the dimmer to blow the breaker, the
dimmer triac almost definately blowed up. Triacs, being semiconductors,
are relatively fragile that way.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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Anthony Diodati
 
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Default strange problem connecting light to dimmer

Could the short have opened up a connection/splice somewhere?
Tony D.
"Chris Lewis" wrote in message
...
According to Jeff Wisnia :
You have probably fried the dimmer.


Yup, probably.

If he pulled enough current thru the dimmer to blow the breaker, the
dimmer triac almost definately blowed up. Triacs, being semiconductors,
are relatively fragile that way.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.





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John Hunter
 
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Default strange problem connecting light to dimmer

indago wrote in message ...

Turn off the circuit; take the dimmer switch out of the wall and disconnect
it from the circuit and wire nut the two wires together and connect the
fixture to the wires in the ceiling. Then turn the circuit back on again
and see if the light works. I am thinking that the dimmer has failed and
you need a new one.


That did it, thanks. New dimmer and all is well.

John Hunter
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Chris Lewis
 
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Default strange problem connecting light to dimmer

According to Anthony Diodati :
Could the short have opened up a connection/splice somewhere?


It _could_, but dimmer triacs are MUCH more sensitive to high overloads
than house wiring. Semi-conductors (even power devices) are excellent
"very very fast blow" fuses on high overloads. They can blow in
microseconds, whereas breakers take considerably longer even in the highest
of overloads.

If you get a short on a dimmer controlled device, even a momentary one,
chances are that the dimmer is dead even if the breaker doesn't trip.
Indeed, I'm a little surprised that the breaker _did_ trip. Somehow
that triac held the current high long enough for the breaker to notice...

[Likely the junction in the triac vaporized and maintained conductivity
just long enough for the breaker to start moving. Then the conductivity
failed when the vapors condensed.]

When I saw "nonfunctional", "short circuit" and "dimmer", "fried dimmer"
is the conclusion I immediately jumped to. The breaker tripping was
essentially irrelevant in determining the most likely cause.

The idea course of action is to test the dimmer ("Do I have good voltage
on the input and not the output?"), but, without some knowledge and proper
instrumentation (a high impedance voltmeter isn't it), swapping
the dimmer (or simply taking it out of the circuit temporarily) is
probably easiest for most people.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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