Attached is the schematic for the HY1803D power supply (sold under different brands). I watched a video on explaining this schematic, which I mostly understand, but one part was left unexplained. Or, rather, it was explained as "it may be wrong"...

The op-amp on the right (U2A) controls a relay for switching two windings of a transformer: K1 at top-right is the coil and SK1 at center-left is the switch. Okay, fine. The op-amp's inverting input is a fixed voltage (~4.6V) set by a voltage divider. And the non-inverting input is using positive feedback (possibly for hysteresis?). But that's it. I don't see how that op-amp could be doing anything, because its inputs never change.

Can anyone shed some light on this?

 

The non-inverting input of U2A is referenced to the same poit that the positive supply output is connected (shown as what is usually an 'earth' symbol)
The inverting input has a potential divider between a fixed 12V positive above the positive supply output and the negative supply output. It is essentially measuring the output voltage and when this is above some level it switches the relay to increase the the voltage into the regulator.

 

Huh? Isn't the potential divider between the fixed 12V and the negative output terminal? How does that ever change?

 

Huh? Isn't the potential divider between the fixed 12V and the negative output terminal? How does that ever change?

Because the negative of the fixed 12V is connected to the positive supply output. So it sees the change as the output voltage changes.
Note the two symbols as below.

 

So the 12V rail doesn't move, but the negative rail does?

 

Thanks, but I'm lost on this. The 12V regulator is referenced to the same "ground symbol" as the op-amp inputs.

 

The three points, circled in red below, are connected together.

 

Ohhhh, I see. I missed how that output section works. I was assuming ground where there isn't one. That's a weird arrangement.

Thank you!