Need to ID transformers

Discussion in 'The Projects Forum' started by scytheye, Dec 26, 2007.

  1. scytheye

    Thread Starter Member

    Nov 27, 2007
    Hi all - I've pulled some salvage transformers, about 4 of them, and cannot find their ratings online. Is there a way to check the ratings with a multimeter or other method? I'm trying to find a transformer that takes household current and steps it down to 24VAC @ 3A. Thanks much -

  2. SgtWookie


    Jul 17, 2007
    Well, for starters - you can get a fair idea of the VA rating by weight ;)

    The transformer you're looking for will weigh around 2 1/4 lbs.
    The frame of it will measure about 3" tall by 2 3/8" wide by 1 1/4" thick, the windings will e about 2" thick.

    But that's just to find the ballpark VA rating (Volts times Amperes)
    You're looking for a 72 VA transformer, but you want a specific voltage and current.

    Ohm out your transformers, find out which leads connect to what. If you salvaged transformers from tube amplifiers/radios, you should set those aside; as most of those have quite high voltage on the secondary windings (sometimes several hundred Volts) and won't be suitable for use with semiconductors. Unless you like to see bright flashes, hear loud pops, and enjoy acrid smells accompanied by black smoke.

    Once you find out which leads connect to what, you have part of the riffle down. Transformers can be wound in a practically bewildering array, but the most common types will have a single primary winding (two leads) or a double primary winding (four leads) and a single or center-tapped secondary winding. (two or three leads)

    In the case of a double primary winding, connecting the windings in series (with correct polarity observed) is for 230V operation, connecting the windings in parallel (with correct polarity observed) is for 115v operation. If you connect the windings out of phase (incorrect polarity) your transformer will output nothing but heat. Lots of heat. Oh, and maybe some smoke. :eek:

    Your primary windings will (normally) have more windings on them than the secondary windings (if it is a step-down transformer) and the wire on the primary side itself will be of smaller diameter (since it carries less current) - thus, an Ohmmeter will show the resistance is greater. However, you're going to need a good Ohmmeter, as the resistance readings may be VERY small, even just a couple of Ohms.

    Your secondary windings will read considerably lower than the primary windings.

    Safe way to test them is to use a low-voltage step-down transformer as the input to the side you believe to be the primary side. For example, you could pick up a 115v to 12.6v transformer from someplace like Radio Shack, wire it up (use a fuse or circuit breaker, make sure all line voltage connections are covered with shrink tube and/or electrical tape and that the GROUND WIRE IS CONNECTED BEFORE powering it up). Test your "known-good" transformer output with your meter to verify it's voltage. Power the KG transformer down, and connect it's output to what you think is the primary side of the transformer under test. Power the KG transformer up, and measure both the input and output sides of the transformer under test starting using the highest AC range of your multimeter. The line voltage divided by the ratio of input to output voltages (under load) will give you the output voltage when the transformer is line powered.

    Measured input is 12.3V, measured output is 1.4V no load
    Ratio is 8.79:1 (approximately)
    115 / 8.79 = 13.1 (approximately)
    That's at no load, of course.

    Don't forget, if you accidentally connect the secondary side of the transformer being tested to your KG transformer, the primary side of the transformer under test will have high voltage on it. Be careful.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    If you're really set up and have a variac, hook it up to the transformer secondaries. Ootch the voltage up until you measure 120 VAC on the primary leads. The voltage on the secondary will tell you what the transformer will output when the primary has 120 volts on it.

    If these are power transformers, the secondary wiring should be heavier than the primary, assuming a low voltage output. Step up transformers will be the other way around. The wire gauge will give some idea of the current capacity. If the two windings have very different gauges, then the voltage difference will be very high.
  4. lightingman

    Senior Member

    Apr 19, 2007
    Hi,Just a small tip...If you are testing unknown mains transformers, and have not got a variac, just conect them to the mains in series with a 100 watt lamp.If it is a 240 volt primary, the lamp should just glow slightly (with no load).This will save a lot of blown fuses and the smell of burning transformer varnish.Daniel.