I'm attempting to repair (with not much success) a flyback power supply circuit. The circuit takes input of 240v AC and has output rails, from a flyback transformer, for 5v and 12v DC. I have traced the input up to the primary side of the flyback transformer where it measures 342v DC. There is, however, no voltage on the secondary side of the transformer.

Questions are thus:

  1. I have disconnected the secondary side of the transformer from the load, in an attempt to rule out a short on the 5v or 12v rails. There was still no voltage on the secondary side. Is this logic sound or would it be expected that a flyback transformer whose secondary side is not connected would have no voltage?
  2. What could cause no voltage on the secondary, when a DC voltage exists on the primary? Could the flyback transformer itself be faulty?
  • \$\begingroup\$ Since there is no voltage on either 5 or 12 V windings, if a transformer winding is open, it would probably be the primary. If you can disconnect one end of the primary and do a resistance test, you should be able to tell if the primary is broken or not. (I am assuming that the 5 and 12 volt windings do not share a common lead, which might be a bad assumption). \$\endgroup\$ Commented Nov 1, 2022 at 21:53
  • \$\begingroup\$ They do share a common lead. It’s a 3 pin output \$\endgroup\$
    – phil-lavin
    Commented Nov 1, 2022 at 22:04
  • 1
    \$\begingroup\$ Using an oscilloscope on the high voltage side of the psu is fraught with danger. A multimeter should be sufficient. Usual faults with flyback psus are open circuit resistors, electrolytic capacitors , failed diodes and things like blown up mosfets etc. Usually if the mosfet fails you’ll see a trail of destruction. Use a multimeter to check the diodes and resistors. Electros require an ESR meter to measure or test by substitution. Likely culprits are a small electro usually 22/47/100/220uF on the high voltage side. \$\endgroup\$
    – Kartman
    Commented Nov 1, 2022 at 22:17
  • \$\begingroup\$ @phil-lavin You don't say (or I am not finding where) but what's the application of this 'flyback?' \$\endgroup\$
    – jonk
    Commented Nov 2, 2022 at 2:51

1 Answer 1


If the driver isn’t switching then there will be no energy transfer to the output.

It’s also possible the transformer has a broken or otherwise defective winding. My money would be on the driver though.

  • \$\begingroup\$ Thanks. Can I test this easily? I have ordered some new drivers so will replace it when they turn up but it would be good to test and confirm if it’s switching. Would I be able to see this on the scope? \$\endgroup\$
    – phil-lavin
    Commented Nov 1, 2022 at 21:44
  • \$\begingroup\$ Yes, you should be able to observe the primary and driver waveforms with a scope. Note that your scope needs to have an adequate voltage rating to do so. \$\endgroup\$ Commented Nov 1, 2022 at 23:16
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    \$\begingroup\$ You can also use the AC setting on a volt meter to see if there’s anything registering. It give you a quick check to see if anything is going on. \$\endgroup\$ Commented Nov 2, 2022 at 15:08

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