Mains transformer with 1.4 ohm primary winding?

  • #1
48
2

Main Question or Discussion Point

I don't understand how this is possible.

This is a rather large transformer in a stereo system amplifier. The transformer is a simple step-down with 120V mains primary to a center-tapped secondary providing both 38V and 76V.

According to ohm's law, at 120V this thing should draw about 85 amps, which obviously isn't happening. What could be going on here?

I have measured for myself 120V in and 76V out of the transformer with the equipment running, and confirmed my ohm meter accuracy against an assortment of resistors.

There is also no additional resistance between mains 120V and the primary winding, just a relay.
 

Answers and Replies

  • #2
779
526
You are measuring a DC resistance of 1.4 ohms for the primary winding.

The gist of it is, a winding is an inductor, and presents an additional form of resistance to AC current flow called impedance. Impedance increases as frequency increases.
 
  • #3
vk6kro
Science Advisor
4,081
40
Consider just the primary winding on the transformer. It has an iron core and it has many turns of wire on it, so it has inductance.
It has an applied AC voltage with a known frequency, so it has reactance.

Reactance is the AC equivalent of resistance and it can be used in Ohms Law calculations.

You can measure the inductance of the winding, but assume it is 10 henries.
Reactance=2. * pi. * f * L
Pi=3.14159
F= 60 hz
L = 10 henries
So reactance = 2 * 3.14159 * 60 * 10 = 3769 ohms

Strictly, we should include the 1.4 ohms resistance in this, but we won't. Too small to matter.

So on a 120 volt 60 hz supply, the current would be 120/3769 amps or 31.8 mA.

The actual current would be higher than this, because the amplifier will use some power, but you can see that the inductance of the winding stops huge currents flowing.
 
  • #4
48
2
You are measuring a DC resistance of 1.4 ohms for the primary winding.

The gist of it is, a winding is an inductor, and presents an additional form of resistance to AC current flow called impedance. Impedance increases as frequency increases.

That makes perfect sense but I have measured the primary windings of other mains transformers with the same meter and got reasonable values of around 200 ohms. I wonder why this one is different? Just larger as far as I can tell.
 
  • #5
48
2
Consider just the primary winding on the transformer. It has an iron core and it has many turns of wire on it, so it has inductance.
It has an applied AC voltage with a known frequency, so it has reactance.

Reactance is the AC equivalent of resistance and it can be used in Ohms Law calculations.

You can measure the inductance of the winding, but assume it is 10 henries.
Reactance=2. * pi. * f * L
Pi=3.14159
F= 60 hz
L = 10 henries
So reactance = 2 * 3.14159 * 60 * 10 = 3769 ohms

Strictly, we should include the 1.4 ohms resistance in this, but we won't. Too small to matter.

So on a 120 volt 60 hz supply, the current would be 120/3769 amps or 31.8 mA.

The actual current would be higher than this, because the amplifier will use some power, but you can see that the inductance of the winding stops huge currents flowing.

I had no idea inductance played such a big role here. Thank you for the explanation and I'll be committing that equation to memory.
 
  • #6
597
289
This knowledgable plonker covers the subject well...

 
  • #7
sophiecentaur
Science Advisor
Gold Member
24,811
4,618
I wonder why this one is different?
All the others were cheapo low current transformers. Ideal for low power purposes. They may just get a bit warm but who cares?
 

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