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I took a flyback transformer out of an old CRT television.

This is how it looks like on the top: enter image description here

And this is how it looks like on the bottom: http://i.imgur.com/8QYwPhQ.png

W1 is a thick red wire that ends with a suction cup; W2 is another red wire similar to W1, but thinner and without suction cup; P# are the pins.

In order to identify the 0 V pin of the secondary coil, I connected the + input of a voltmeter to W1, the - input of the voltmeter to the 0 V output of a power supply, and I tried touching each pin of the transformer with the 24 V output of the power supply.

I read +0.03 V on P0, +1.79 V on P4, 0.00 V on the others. However, I saw on the internet that by using this method I should read something between +5 V and +10 V on the 0 V pin of the secondary of the transformer. I couldn't get that voltage on any of the pins: am I doing something wrong? Is probably P4 the pin I'm looking for? How can I be sure? Thanks.

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The primary and secondary coils might not have a common pin. I have found this.

The only way to be sure is one of two ways:

  1. Put a small AC voltage across the assumed primary and measure it on the assumed secondary. Be careful not to blow whatever you're using to measure the secondary, because the voltage will be amplified by quite a few hundred times. You could make a high voltage resistive network voltage divider.

  2. Or you can do the more scary method of just sending DC pulses through the assumed primary, and waiting for sparks between the secondary's leads. With a DC pulse of 10v, which might consume 10 amps, you should get a spark between the secondary leads which can span anywhere from a few millimetres to a couple of centimetres. If you send DC pulses at say 20Hz, turn down the lights, you can look for a corona, which you may see between two or more pins - if the secondary has multiple taps, which it probably does. To work out which taps are which, either you can try to gauge by the brightness of the arcs, the distance the sparks travel, or for something more certain, use method 1, with a multi-channel oscilloscope to detect relative phases, voltages etc.

Either way, be careful - you can get a really nasty shock from these things. If you decide to put any significant AC or pulsing DC through it, it has the potential to become lethal. A single DC pulse, or extremely low voltage AC (millivolts) would be safer.

One way of putting very low AC through it would be to use a cheap power supply like that used for powering low voltage desk lamps. These might output about 12v at 50Hz. Use this, and put a 2k resistor in series with this and the primary coil. Assuming the transformer primary has an impedance of 1 ohm, this should present approximately 1mV at the primary, which would give around 5 or 10 volts AC at the secondary. (Plus or minus a lot).

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