Just got a brand new toroidal transformer rated at 230V primary 24V secondary @ 2.5V and before hooking it up to 240V (UK) supply decided to test it with a low voltage AC wallwart supply. There was 11.08 VAC on the primary and I measured 1.336 VAC on the secondary (i.e the primary seems to represent about 15% of the input).

So, my question is whether testing at low voltage like this is representative of the percentage voltage drop when running from the specified mains voltage. That would be a bit worrying as 15% of the input voltage (240 VAC) would result in an output on the primary of circa 36 VAC rather than the rated 24 VAC.

I'm hopeful that someone can reassure me that low voltage tests cannot be translated to the outcome at full mains voltage. Thanks for reading.

 

You will see some voltage inflation until the transformer is loaded to around 80% of full rating.
The ratings are also RMS and your meter may not be giving true RMS.
The ratio you get is about 1 in 12, so you should see about 29 VAC on your unloaded secondary. That seems perfectly normal for such a standard transformer.

 

You will see some voltage inflation until the transformer is loaded to around 80% of full rating.
The ratings are also RMS and your meter may not be giving true RMS.
The ratio you get is about 1 in 12, so you should see about 29 VAC on your unloaded secondary. That seems perfectly normal for such a standard transformer.

Thanks @Kermit2 . That goes a long way to reassuring me; particularly the part about "inflation". I hear what you are saying about RMS and you're probably correct that my meter does not give true RMS. Just wondering how you derive the "1 in 12" ratio figure and therefore come up 29 VAC unloaded rather than my 36 VAC calculation. Forgive my ignorance on the math (never been my strong point).

Edit: Just wondering if your calculation has something to do with the 80% loading you referred to?

 

Transformer secondary winding voltages are usually specified at full load. At light loads, or unloaded, their output voltages will be somewhat higher. You took your measurements unloaded:

Secondary/Primary ratio = 1.336 VAC / 11.08 VAC = 0.1206

Applying that ratio to a 230 VAC input on the primary, we get

230 VAC * 0.1206 = 27.7 VAC (unloaded),

which is about what I'd expect for a transformer rated at 24 VAC output.

 

Thanks for the detailed explanation @OBW0549 . Using the math you have provided I can now see how @Kermit2 calculated 29 VAC as UK mains is normally 240 VAC.

 

Uk Ac voltage range is 230V +/-6 %