# ac transformer power consumption?

#### osha123

Joined Mar 28, 2024
9
sorry if this is not right place to ask but i still will)

i have broken 300w desktop power supply witch i took apart and it had this big transformer in it,
i have never worked with transformers and out of curiosity i measured resistance of coils i noticed that primary coil has only 2.6-ohms resistance.

and outlets here are 230vac, so i was wondering what would be idle power consumption?

i ask this because (2.6R*230V=20346W), i know its not 20kw with ac but as i don't know anything about transfromers and ac power calculations i don't wanna reuse it before im sure that it wont double my electricity bill by forgetting it on.

#### nsaspook

Joined Aug 27, 2009
13,418
sorry if this is not right place to ask but i still will)

i have broken 300w desktop power supply witch i took apart and it had this big transformer in it,
i have never worked with transformers and out of curiosity i measured resistance of coils i noticed that primary coil has only 2.6-ohms resistance.

and outlets here are 230vac, so i was wondering what would be idle power consumption?

i ask this because (2.6R*230V=20346W), i know its not 20kw with ac but as i don't know anything about transformers and ac power calculations i don't wanna reuse it before im sure that it wont double my electricity bill by forgetting it on.
That's a good question.
For AC we use impedance instead of resistance. The idle current for a typical 300w desktop power supply would be low with the idle power being only a few Watts of mainly reactive power.

#### crutschow

Joined Mar 14, 2008
34,680
It's not the resistance that determines the primary current with no load, its the inductance which gives a high enough inductive impedance (reactance) so there is just a small magnetizing current form the AC.

#### schmitt trigger

Joined Jul 12, 2010
918
To the OP: You are not talking into account the core’s magnetic properties.
ALL the magnetic materials suffer from losses when subjected to an AC excitation.

These are the so called core losses and are expressed in watts per unit of weight. Core weight, that is.

#### Irving

Joined Jan 30, 2016
3,979
The DC resistance has little to do with the power consumption. The transformer primary coil is an inductor so has reactance (the AC equivalent to resistance for DC). With nothing connected to the secondary windngs the only consumption is a small magnetizng current, typically 3 - 5% for a transformer of that size, say 10-15W.

#### Ian0

Joined Aug 7, 2020
10,006
I was amazed how hard it was to find a datasheet which gives useful information like no-load current and iron loss.
Eventually I found this one:
https://www.farnell.com/datasheets/3981391.pdf
For a 300VA transformer: No-load loss: 2.5W

 This one is even better:
https://www.farnell.com/datasheets/3773026.pdf
No load loss is the same as iron loss which is 1.8W
Magnetising current (which is the current that flows in the primary when there is no load) = 15mA
You can model the primary is a 29k resistor in parallel with a 57H inductor, all in series with the 3.56Ω DC resistance.

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#### osha123

Joined Mar 28, 2024
9
thanks for all the anwsers! opened up a lot about transformers in no time

#### Ian0

Joined Aug 7, 2020
10,006
By the way, don't go looking for the 29k resistor with your multimeter, as you won't find it.
That "resistance" appears as it takes some energy to rearrange all the magnetic domains in the core, so you can think of the inductance as being a complex number: 57-92j H in this case. Inductance gives a 90° phase shift to the current, the imaginary part of the inducatance another 90° phase shift in the opposite direction, so the current through it is in phase with the voltage, so it looks like a resistance.

#### LowQCab

Joined Nov 6, 2012
4,196
This Transformer is very likely a part of a SMPS, ( Switch-Mode-Power-Supply ),
which operates on completely different principles than a generic, Iron-Core-Mains-Transformer.
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#### Ian0

Joined Aug 7, 2020
10,006
This Transformer is very likely a part of a SMPS, ( Switch-Mode-Power-Supply ),
which operates on completely different principles than a generic, Iron-Core-Mains-Transformer.
.
.
.
I was assuming it was a 50/60Hz toroid as he described it as a "big" transformer.
If it is indeed a switched mode, most of post #6 is irrelevant.

#### crutschow

Joined Mar 14, 2008
34,680
This Transformer is very likely a part of a SMPS, ( Switch-Mode-Power-Supply ),
which operates on completely different principles than a generic, Iron-Core-Mains-Transformer.
.
If it's an SMPS transformer powered from the mains than it operates on the same basic principles, just at a different frequency, since it's not likely a fly-back type.
Why do you think otherwise?

#### osha123

Joined Mar 28, 2024
9
I was assuming it was a 50/60Hz toroid as he described it as a "big" transformer.
If it is indeed a switched mode, most of post #6 is irrelevant.
It is switched mode, it just was also very big one.

#### osha123

Joined Mar 28, 2024
9
If it's an SMPS transformer powered from the mains than it operates on the same basic principles, just at a different frequency, since it's not likely a fly-back type.
Why do you think otherwise?
SMPS it is

#### LowQCab

Joined Nov 6, 2012
4,196
Basic "principles" certainly,
but every specification is highly specialized for the individual application.
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#### Ian0

Joined Aug 7, 2020
10,006
It is switched mode, it just was also very big one.
Really? A 300W switched-mode will have a transformer no bigger than 50mm cube.

If this thread has proved one thing, it is that everyone's idea of what constitutes a "big" transformer is different.
Saying how big it is would have helped!
(My view is that a transformer is not "big" if you can lift it on your own)

#### LowQCab

Joined Nov 6, 2012
4,196
I've never seen a "Desktop-Computer-Power-Supply" that wasn't a SMPS.
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#### Ian0

Joined Aug 7, 2020
10,006
I've never seen a "Desktop-Computer-Power-Supply" that wasn't a SMPS.
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I interpreted "desktop power supply" as "bench power supply" not "desktop computer power supply". So, @osha123 , how about a photo of this transformer, so we all know what you are talking about?

"We demand rigidly define areas of doubt and uncertainty" -- Hitchhikers Guide to the Galaxy.

#### osha123

Joined Mar 28, 2024
9
Really? A 300W switched-mode will have a transformer no bigger than 50mm cube.

If this thread has proved one thing, it is that everyone's idea of what constitutes a "big" transformer is different.
Saying how big it is would have helped!
(My view is that a transformer is not "big" if you can lift it on your own)
Well it is 120mm