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Electronics => Beginners => Topic started by: MikeK on March 31, 2021, 02:14:02 pm

Title: HY1803D power supply schematic confusion
Post by: MikeK on March 31, 2021, 02:14:02 pm

Please see the attached schematic.  The op-amp on the right controls a relay, but I don't see how the op-amp inputs will ever change.  Neither input seems to be referenced to anything that changes.  Am I wrong?

[attachimg=2]

Title: Re: HY1803D power supply schematic confusion
Post by: bd139 on March 31, 2021, 02:34:03 pm

The entire supply's internal ground is referenced to positive output of the supply. The power supply varies the negative rail rather non intuitively (consider voltage drop across the pass transistor). The opamp is a comparator in a schmitt trigger arrangement (to give it some hysteresis otherwise it'll chatter the relay to death) that samples the difference between the internal + reference from the 7812 and the negative rail.

Fixed a couple of these and it gave me a headache the first time or two (and now!)  :-DD

Edit: let me guess you're fixing one that has blown up the transistor? This is super common and usually happens because the relay stops working, failing to the highest voltage across the pass transistor (stupid design), followed by it dropping dead.

Title: Re: HY1803D power supply schematic confusion
Post by: MikeK on March 31, 2021, 02:38:39 pm

Thanks!  Didn't get it yet.  I have the chance to pick up a non-working unit and wanted to understand the schematic first.

Title: Re: HY1803D power supply schematic confusion
Post by: bd139 on March 31, 2021, 02:51:44 pm

If it's non-working it'll probably be easily repairable. They are made of entirely standard bits. Only risk is the transformer being toasted. Good luck with it  :-+