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How to test an AC/AC doorbell transformer?

Postby itm60 » Wed Aug 18, 2021 2:04 pm

I've had two brand new AC/AC transformers delivered to me in the last week, one from Amazon and one from CPC Farnell, and neither of them seem to work. So now I'm beginning to wonder if there is a problem with the way that I'm trying to test them.
I'm connecting the output cables of the transformers directly to a multimeter, and setting the multimeter to AC/15V, but I get no reading. I've even tried 2 multimeters - one an old Micronta model and the other a Hilka device. Both give normal readings when I test for DC voltage from an AC/DC transformer.
I've also tried connecting them up to a video doorbell, but they don't power that up either.

The first transformer is this one:
https://cpc.farnell.com/webapp/wcs/stor ... vbXg%3D%3D

The second is this one:
https://www.amazon.co.uk/gp/product/B09 ... UTF8&psc=1

It seems very improbable that I've been shipped two dead transformers by two different suppliers in a week. Am I going about testing these the wrong way?
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Postby Mr White » Wed Aug 18, 2021 10:44 pm

itm60 wrote: I'm connecting the output cables of the transformers directly to a multimeter, and setting the multimeter to AC/15V


But each transformer has an output of 24v
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Postby itm60 » Thu Aug 19, 2021 8:12 am

Also set meter to 60V but zero reading
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Postby Mr White » Thu Aug 19, 2021 10:18 pm

If you set you multi meter to 15v then connected it to 24v you MAY have blown your multi meter, or you may have blown a fuse.
You should connect your meter to a known working voltage and establish does your meter still work.
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Postby itm60 » Fri Aug 20, 2021 7:20 am

The meter is fine - it reports correct voltage on a different ac/DC transformer
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Postby brian_mk » Fri Aug 20, 2021 9:56 am

I would start by checking that the 13A mains socket is working by temporarily plugging another appliance such as a table lamp into the same socket.

If that's ok then check the resistance of the transformer primary and secondary windings using the multimeter (obviously with the transformer not plugged in). The actual resistance values measured will depend on the diameter of the wire used for the windings which varies depending on the transformer current rating.

To check the primary side, measure the resitance between the Live & Neutral pins of the mains plug. This would typically be somewhere between 40 and 200 Ohms.
To check the secondary, measure the resitance across the transformer output terminals. This should give a low reading - typically somewhere between 0.5 and 5 Ohms.
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Postby itm60 » Fri Aug 20, 2021 5:22 pm

Thanks for your reply. The resistance reading for the input side (i.e. the 240V cables is 0.L (the letter L).
The resistance for the output side is 0.6
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Postby itm60 » Fri Aug 20, 2021 5:23 pm

Oh and the 13 amp socket is fine - I plugged another transformer into the same socket and it tested fine. I also did a continuity test on the fuse in the plug that I'm using and it was fine.
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Postby Mr White » Fri Aug 20, 2021 10:04 pm

itm60 wrote:The resistance reading for the input side (i.e. the 240V cables is 0.L (the letter L).


There is your problem.

It looks like when you connected your meter which you set to the wrong value overloaded the transformer Primary coil, since OL on a digital multimeter means Over Limit (It is over the limit of resistance it can measure)
I would however check to see if it does have an internal fuse, and double check the plug top fuse.

If both are ok, you need a new power supply.
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Postby itm60 » Fri Aug 20, 2021 10:13 pm

Can that be right? In the past if I have set my multimeter to a lower range than the transformer is delivering the reading simply goes off the scale in the multimeter - it hasn't done any damage to the transformer. I have done this many times
????
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Postby Mr White » Fri Aug 20, 2021 10:28 pm

itm60 wrote:Can that be right?


I can only go by what facts you have said.

itm60 wrote:I'm connecting the output cables of the transformers directly to a multimeter, and setting the multimeter to AC/15V


Even though you know it is a 24v output transformer.

itm60 wrote:In the past if I have set my multimeter to a lower range than the transformer is delivering..................... have done this many times
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Postby itm60 » Fri Aug 20, 2021 10:31 pm

Mr White wrote:
itm60 wrote:Can that be right?


I can only go by what facts you have said.

itm60 wrote:I'm connecting the output cables of the transformers directly to a multimeter, and setting the multimeter to AC/15V


Even though you know it is a 24v output transformer.

itm60 wrote:In the past if I have set my multimeter to a lower range than the transformer is delivering..................... have done this many times

Yes - I've found in the past that the range setting on the multimeter hasn't been critical if all I want to do is check if a transformer is producing any voltage at all
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Postby itm60 » Fri Aug 20, 2021 10:33 pm

...seems to be confirmed in the "Overload" section on this page:
https://learn.sparkfun.com/tutorials/ho ... of%2Drange.
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Postby brian_mk » Sat Aug 21, 2021 8:58 am

It sounds like a mains fuse inside the power supply may have blown or the transformer primary winding has failed open circuit.

Some transformers incorporate a thermal protection device that may have tripped if there was a short on the secondary.

It's very unlikely that this would have been caused by having the meter set to the wrong voltage range.

However, if you had accidentally set the meter to measure current then it would have effecively shorted the transformer output which would have definitley caused damage or blow a fuse.

Most digital meters have a separate fused input jack for current measurement. If you had plugged a test lead into the wrong jack then there is a good chance that it would have also blown the fuse inside the multimeter. That fuse is probably only used when measuring current so the meter would still appear to work for other measurements such as voltage and resistance.

I would check the fuse in the multimeter and check if there is a replaceable fuse inside the transformer.
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Postby itm60 » Sat Aug 21, 2021 9:14 am

Thanks for the feedback. The multimeter is still working fine (i.e. the fuse has not blown) - I can still test other (AC/DC) power supplies with it and get normal readings. It's just these two AC/AC transformers that I can't get a reading from.
I have always been extremely careful about selecting the correct function on the multimeter (before connecting the device to be tested, so can be pretty certain that I did not at any stage select a current measurement function.
I just find it hard to believe that two consecutive brand new AC/AC transformers have been DOA.
With the 2nd transformer (24V/1A) I connected it to a 3-pin plug with a 3A fuse and plugged it into a normal mains socket. I continuity-checked the fuse in the plug and it's fine. Presumably this would not pose any problems?
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