How do I test how many amps a transformer actually puts out?

Reply Tue 10 May, 2016 08:03 am
Hi Everyone. Long time lurker, first time poster. I am a ME so my electrical experience is limited. I need to validate the 200 mA @ 24 VAC claim on a Transformer. I have some resistors and multimetter but I don't know how to proceed with the wiring and I'm a afraid of popping the fuse on the MM. If you guys can be as specific as possible it would be great.

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Reply Tue 10 May, 2016 12:25 pm
Gee Bal it's hard to be specific. But I'd connect my ac voltmeter and a variable load like a huge resistor. As I increase the load I'd note a point where the voltage begins to drop significantly

Remember that you'd probably hafta dissipate at least 5 w so at the later stages of your experiment don't leave power up too long

If however your load can take it, do leave it on for a while, occasionally feeling the winding. At 5w it shouldn't get very hot
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Tes yeux noirs
Reply Tue 10 May, 2016 01:26 pm
I need to validate the 200 mA @ 24 VAC claim on a Transformer.

To perform a load test on a transformer, calculate the required resistance using R=V / I where R is the resistance in ohms, V is the voltage in volts and I is the current in amperes.

R = 24 / 0.2
R = 120 ohms
Power in watts will be V x I
W = 24 x 0.2
W = 4.8

A 10 Watt rated resistor will be a sensible choice.

Or you could use a 24 volt 5 watt lamp bulb.


Connect the load in series with the multimeter, and preferably a switch also in series, and if you are nervous, a 500 mA fuse also in series, across the 24 volt secondary of the transformer. Apply power to the primary, close the secondary circuit switch, and watch the meter reading. Observe if the transformer appears to be overheating. Be alert and ready to disconnect power if necessary. If you are doing a proper test, you will have some way of monitoring transformer winding and/or core temperature. You can get temperature indicating stickers that change colour, or proper test equipment.

Have the resistor somewhere it can't scorch or damage anything, it might get a bit warm.

You need to understand something about electrical machines such as motors, transformers, etc. They generally have a 'rating' which might be in horsepower (for motors) or current (amperes) or power (Watts) which is generally supplied in a manufacturer's data sheet, or in sales advertising. Often there will be a plate or label on the item itself. The rating is determined by the acceptable level of temperature in the item in question. Sometimes an item will have multiple ratings depending on time. A one-hour rating, and a continuous rating. You can exceed the rating and cause overheating or damage, either short term for a big overload or long-term for a smaller overload. For example if a transformer is rated at 200 mA you could probably draw 250 mA for a short time without too much trouble. Some items are generously rated, and others not.

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Tes yeux noirs
Reply Tue 10 May, 2016 02:21 pm
Don't forget to switch the meter to the right range! Do it before applying power.

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Reply Wed 11 May, 2016 05:23 am
All good stuff.

Cheers lads. Greatly appreciated.
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