Possible? - skiping mains power circuitry and injecting power after rectifier

[mlindroth]

Hello all,

I'm trying to probe a malfunctioning PCB and would like to power it on the bench without having mains voltage on the board. Is it possible to inject 12V power after the diodes in the full-wave bridge rectifier and filter capacitor?

Thanks!

[bdunham7]

Yes, this is a common way to test boards out-of-chassis or when there is an issue with the mains supply.  You do have to know for sure what the voltage is supposed to be and have a current-limited or protected power supply.  You can also supply the power before the bridge rectifier if you remove the transformer leads.  In any case, you should be careful to first check that the problem isn't actually in the bridge rectifier itself.

[mlindroth]

Thanks, I have checked it in place hooked up to mains. The transformer is outputting 12V, and the FWBR is working. I believe that for some reason the microprocessor is not outputting PWM pulses, or not supplying 10V to the blower motor.

[fzabkar]

One possible, remote caveat is that some circuits monitor the frequency of the supply to synchronise their real time clocks or to detect zero crossings for TRIAC switching.

[Audiorepair]

Yes, and there is also the case where the circuitry monitors the AC  supply, so if this suddenly disappears, there is a "fast off" circuit that mutes the audio before Big Bangs blow the speakers.