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I have two old transformers that appear to be fine, however no DC voltage was measured on the secondary coil.

I have measured:

The input voltage: 229 AC the input coil ohms: 0.015 kOhms the two parallel output coil ohms: 0.008 kOhms each the multimeter base resistance when i touch the probes = 0.006 kOhms resistance between AC and DC coils - no continuity.

The other transformer is similar, continuity between the primary and secondary, AC 230 in, DC 0 out.

What kind of problem affects both these transformers?

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    \$\begingroup\$ Here's a little video for you to watch youtube.com/watch?v=ZjwzpoCiF8A \$\endgroup\$ – JIm Dearden Jun 29 '15 at 14:07
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    \$\begingroup\$ There's no problem. You'll find AC on the output. That's what transformers do. \$\endgroup\$ – Brian Drummond Jun 29 '15 at 14:13
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If they are transfomers rather than DC power supplies, then you would measure zero volts DC at the output even though the transformer is working correctly.

A DC voltmeter will show zero volts when measuring AC.

Switch the multimeter to AC volts to measure the transformer output.

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  • \$\begingroup\$ Thanks, that solved the problem, it now reads correctly :) \$\endgroup\$ – com.prehensible Jun 29 '15 at 14:15
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    \$\begingroup\$ @ufomorace Please consider marking this answer as accepted, since it helped you solve the problem. \$\endgroup\$ – Lorenzo Donati Jun 29 '15 at 16:04
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JRE's answer is correct and to the point, but I thought I'd add some theory.

As you probably know, a transformer is two coils wrapped around some kind of core. The important thing here is that the coils are not connected electrically. This provides complete DC isolation. In fact, this is why transformers are used to isolate power supplies from harmful mains voltages.

However, an alternating current can cross between the coils because of electromagnetic fields. The AC goes into the primary coil, creating a field. This field is picked up by the secondary coil, which turns this field into a voltage on the other side. As you know, the turns ratio determines what the output voltage is.

Also, an AC voltage "riding" on a DC voltage will be passed through a transformer, with the DC removed. For example, a sine wave oscillating between 10V and 20V is "riding" on a 15V DC offset. After being passed through a transformer, it will oscillate between -5V and 5V (if the turns ratio is 1:1). Is this way, it acts similar to a capacitor; it can be used as a DC blocker.

Long story short, you need to set your meter to AC volts, since transformers can only pass an alternating current.

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    \$\begingroup\$ 'After being passed through a transformer, it will oscillate between -5V and 5V` - only if the numbers of turns on the primary and secondary are equal. \$\endgroup\$ – abligh Jun 29 '15 at 18:48
  • \$\begingroup\$ Good point, answer updated \$\endgroup\$ – crocboy Jun 29 '15 at 18:51

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