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# Measuring Transformer Current

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#### M.Joshi

##### Member
This may sound like a basic question but here goes!

How do you measure the current output of a transformer?

I always thought that you connect a resistor to the transformer and then place an ammeter in series with the circuit, but the current reading changes with different resistor values?

M.Joshi said:
This may sound like a basic question but here goes!

How do you measure the current output of a transformer?

I always thought that you connect a resistor to the transformer and then place an ammeter in series with the circuit, but the current reading changes with different resistor values?

Remember: V = IR, so of course the current rating will change. Current doesn't just magically flow, it needs a load, so depending on the resistor in series, the amount of current that flows will change.

Maximum output current

How can I determine the maximum output current of a transformer?

Re: Maximum output current

M.Joshi said:
How can I determine the maximum output current of a transformer?

The easiest way is to check the specs on a transformer. It can be calculated, but unfortunately I don't remember the equations, but if I remember right, the correct equations are rather lengthy and involve omega.

A reference book of mine indicates that there is a relationship between the cross sectional area of a transformer core and it's power rating. If you know the voltage and the power then amps are easily calculated.

The graph provided does not show a linear relationship however I'll give you some points so you can draw your own. It starts at 50 watts for a 1.25 sq in cross section, 200 watts at 2.5 sq inches, 300 watts at 3.1 sq inches, and 500 watts at 4 sq inches.

There is no mention made of the duty cycle rating. If the winding conductor size is smaller than calculations might indicate then limit the power based on the wire size.

This isn't great but it might be more than you've got otherwise.

Another (similar way) is to measure the size of the transformer (as above), then look at transformers in a catalogue for one as close in size as possible - it's likely to have a similar wattage rating. You can do the same thing by weight.

Bear in mind, these crude methods give an indication of the total wattage of the transformer, if it has more than one secondary winding this wattage will be shared between them - how it is shared is dependent on the thickness of the wire, and how many turns there are.

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