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Topic Title: Call Out
Topic Summary: Bathroom Lights Transformer
Created On: 20 April 2013 06:17 PM
Status: Post and Reply
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 20 April 2013 06:17 PM
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Ampman

Posts: 1041
Joined: 06 February 2006

Evening / afternoon .

Got a call to look at bathroom lights not working , Went into loft & tested primary side of a 230/12volt transformer ,

I measured 55 volts when the pull cord for lights was switch off & 230 when pull cord on .

Nothing on the secondary side , So transformer knackered ?

Why the 55 volts when pull cord off ? i did not disconnect to test .

cheers
 20 April 2013 06:23 PM
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impvan

Posts: 826
Joined: 07 September 2005

Using a digital voltmeter? You got 55v because of inductive or capacitive pickup from adjacent cables. Try using a 'proper' meter, i.e. one with a needle; or load the dead wire with a 15W bulb.

If the transformer is an electronic one, you'll get queer readings even when it IS working, unless you have a true-rms meter which is accurate in the 10-40kHz range...
 20 April 2013 06:31 PM
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daveparry1

Posts: 6372
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Make sure there's a load connected to the transformer, ie not a broken/loose connection or all dud lamps as without a load on electronic transformers usually shut-down,

Dave.
 20 April 2013 06:36 PM
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Grumpy

Posts: 460
Joined: 09 January 2009

Could you recommend a "proper" meter? I've had issues with 'queer' readings in the past and ended up just replacing the transformer but I'd be happier being able to prove it was a duffer.
 20 April 2013 06:56 PM
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daveparry1

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You can't beat connecting a 12v 50w halogen lamp across the secondary to test the transformer,

Dave.
 20 April 2013 07:02 PM
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impvan

Posts: 826
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Look for meter with a "Low-Z" range; or do what I do and keep a 100K 10W wirewound resistor with leads and crocks attached.
 20 April 2013 09:54 PM
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SKElectrical

Posts: 911
Joined: 01 February 2009

Originally posted by: daveparry1
Make sure there's a load connected to the transformer, ie not a broken/loose connection or all dud lamps as without a load on electronic transformers usually shut-down,


Correct


Originally posted by: daveparry1
You can't beat connecting a 12v 50w halogen lamp across the secondary to test the transformer,


Sound advice.
Sound advice.
 21 April 2013 07:49 AM
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broadgage

Posts: 1356
Joined: 07 August 2007

I would say that the transformer was almost certainly dead.
The voltage measured across the input with the lightswitch off would be capacititive coupling or leakage between the switch wires.
For a voltage to be produced by this minute current the transformer primary must be open circuit.
I would expect that a sound but unloaded transformer would still draw a minute current and therefore have no measurable voltage across its primary in the circumstances described.

If the transformer was a copper/iron sort it probably had a blown thermal fuse in series with the primary.
If it was an electronic transformer it is likely that the reservoir capacitor failed short circuit and either an internal fuse went or the rectifier failed open circuit.
 21 April 2013 09:27 AM
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M.Joshi

Posts: 212
Joined: 10 January 2003

Originally posted by: Grumpy

Could you recommend a "proper" meter? I've had issues with 'queer' readings in the past and ended up just replacing the transformer but I'd be happier being able to prove it was a duffer.


Have a look at Agilent, Fluke or Gossen MetraHit True RMS multimeters. Not the cheapest though.

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