Finding a transformer's CT

You can use a resistance measurement to find the center tap, also, but you need to be able to measure lower values. The end to end resistance is about twice the resistance of either end to the center tap, but the total resistance may be below an ohm. Do you have access to a current regulated lab supply? If so, you can connect the winding in question across the supply, with the current limit set to a reasonable value (an ampere or a tenth of an ampere, or a guess of what the winding can stand without fusing, and measure the voltage across the winding with a volt meter. You can measure milliohms this way with reasonable accuracy. Just be sure to connect the current to the winding with one pair of connection points and the meter with a second pair of points, so the current path contact resistance voltage drop doesn't get involved in the resistance measurement. If you don't have a current regulated supply, you can just use a voltage supply and a series resistor at least 10 times the resistance of the winding, that limits the current to a safe current. The resistor will produce considerable heat, so use a large one and make the measurements quickly.

From a smashed VCR I plucked out its transformer that's got two primary taps and three secondary. Using the consistency checker on my DMM it looks like all three secondary taps are connected together. Something tells me that this means one of the three is center tapped, and to avoid shorting the mains when I use the transformer, I need to find out which one is the center one.

Is this possible to do with a DMM? I learned how to find the primary and secondary windings using the resistance meter on my DMM, but I do now know how to find a center tap in a winding.

No, there won't be a problem. The center tap is isolated from the mains on a working transformer. The worst that could happen is that you get 1/2 the voltage you think you'll get.

Just hook up a 1M resistor between all of the taps, and plug the other side in. Now, measure the voltages. If the terminals are A, B, and C, and the C terminal is the center tap, then the voltages will probably be

A-B = V*2 A-C = V B-C = V

for some V, defined by the winding ratio of the transformer. That implies that the the two terminals which are not the center tap will have twice the voltage across them.

Actually, the worst that could happen is that the VCR has a switch-mode power supply, and when he tries to connect 60Hz mains to it, it saturates, overheats, and flames.

a centre tap will show less resistance to either end of the winding than is measured from either end, also you may be able to observe two wires from inside the transformer going to the centre terminal, or observe that it enters the winding half way through its layyering, furthermore the centre tap is usually located in the centre.

don't connect any of the secondary terminals, connect the transformer to the mains and measure the AC voltage between pairs of secondary terminals.

it should become obvious which is the centre tap.

eg A-B 12V A-C 12V B-C 24V A is the centre tap. Bye. Jasen

Assuming its a 50/60Hz transformer, a handy way is to use a small transformer, say 6 volts AC, to energise one winding, and then measure the voltages that arise in the other windings.

All the best Ian Macmillan

Use a lightbulb in series with the primary to limit the current. That way you won't damage the transformer if you hook it up the wrong way.

Get a known good 6VAC transformer, and a .1A fuse for its primary. Pick any two terminals that have continuity, and put the 6VAC on them - that will be a winding or part of one.

Measure and record the voltages. All of them! :-)

Scale them, and Voila! You know what all of the windings' relationships to the others are, center taps and all. :-)

Have Fun! Rich