input resistance of transformer based power supply

Thread Starter

Jorne

Joined Feb 28, 2020
22
When I measured the input resistance of some 230V AC to 12V DC transformer based power adapters, I got around 400 ohms. Isn't this quite high for the resistance of the primary winding? I expected no more than 100 ohms. The adapters are low power though (output power< 10W).

(Background story: I measured this because I heard that one way to distinguish transformer based power supplies from switched mode power supplies is to measure the input resistance. A transformer based power supply has a transformer at the input side, so you will measure the resistance of the primary winding. With a switched mode power supply, you would measure infinite or mega-ohms resistance because of the rectifier bridge and converter.)
 

LowQCab

Joined Nov 6, 2012
1,185
If you are going to try to "make a guess", then weight is a more reliable indicator.
But you are still just "guessing".
If this is an important thing for you to know,
then you shouldn't be assuming ANYTHING.
There's about a million different ways of designing a Wall-Wort,
and they've all been used somewhere.

Just for interest, why do you need to know ?
You can generally trust the Specs printed on the unit.
.
.
.
 

DickCappels

Joined Aug 21, 2008
7,867
I agree with you. Are you sure you are measuring the primary?

Is this a small impedance limited transformer? Their primaries can measure high in both resistance and inductance.
 

Thread Starter

Jorne

Joined Feb 28, 2020
22
I agree with you. Are you sure you are measuring the primary?

Is this a small impedance limited transformer? Their primaries can measure high in both resistance and inductance.
I'm measuring at the AC terminals of the adapter. So, I might measure some filter inductors also, but that shouldn't make much difference I think.
I don't now what a limited impedance transformer is. But you can see the symbol of safety isolation transformer with short circuit protection on the adapter in my picture.
 

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MrAl

Joined Jun 17, 2014
8,500
When I measured the input resistance of some 230V AC to 12V DC transformer based power adapters, I got around 400 ohms. Isn't this quite high for the resistance of the primary winding? I expected no more than 100 ohms. The adapters are low power though (output power< 10W).

(Background story: I measured this because I heard that one way to distinguish transformer based power supplies from switched mode power supplies is to measure the input resistance. A transformer based power supply has a transformer at the input side, so you will measure the resistance of the primary winding. With a switched mode power supply, you would measure infinite or mega-ohms resistance because of the rectifier bridge and converter.)
It depends on the input and output voltage also (which depends on the turns ratio), and the output current rating.

The secondary AC resistance goes by the square of the turns ratio times the primary resistance and when it is a step down transformer the secondary resistance is much lower than the primary resistance.
If the input was 120vac and the output 12vac and the input measured 400 Ohms with a dc test current then the secondary AC resistance would be 400*(12/120)^2=4 Ohms. With a 12 watt output that means about 1 amp output current, and 4 Ohms with 1 amp drops 4 volts which would be a little high i think. With 240vac input however, it would be not half but one quarter of that which puts it at around 1 Ohm, not unthinkable. 1 Ohm at 1 Amp only drops 1 volt which is certainly possible.

So the main thing to think about is the secondary resistance which is the square of the turns ratio times the primary resistance and the turns ratio is the number of turns of the secondary divided by the number of turns of the primary, or the rated output voltage divided by the rated input voltage.
(12/240)^2 is just 0.0025 so the secondary resistance is just 0.25 percent of the primary resistance which is a lot lower. For a rated 230vac input it will be s little bit higher but you see how this works with these simple calculations.

Of course we assume an ideal transformer for these simple estimates as there is also leakage inductance especially with AC to DC unregulated wall warts that use a low frequency transformer like at 50 or 60 Hertz. That will drop some voltage too but help to filter the output DC voltage and the usual internal winding construction makes it safer to use also.
Lastly, the secondary wire resistance itself plays a part in the total apparent secondary resistance but that is usually quite small on the order of less than 1 Ohm probably even less than 1/4 Ohm so the total secondary resistance might be 1 Ohm plus 1/4 Ohm or something like that.
 
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