hi there,
i have this PSU and try to figure out what the upper section of the circuit is doing. i understand on the lower side there is just a basic recitifier, some filtercaps, a regulator and some more filters. what i dont understand is the upper section that has the PLR output. can anybody explain what this circuit is supposed to do? or what PLR stands for? is this another regulator? an oscillator? current source? the PSU is from a MIDI controller keyboard..
thank you for any help!

 

It detects when the voltage on the main reservoir capacitor is >7V.
At a guess, it‘s to do with saving the settings on power down.

 

hi lan0,
thanks for this info. i can see that if i hook up my scope to the PLR pin, that about 2 seconds after switching the power off, there is a small negative pulse coming out (about -150mV). Do you think this is what its supposed to do? Thank you!
kind regards

 

T1 is a constant current source for the zener. T2 and T3 form a schimdt trigger which will switch states depending on the voltage across the reservoir capacitor. Withe supply switched on the PLR pin should be a positive voltage - I can't say what voltage without the circuit of whatever it is connected to. It should go low as the reservoir voltage falls.

 

You will notice that collector of transistor T3 is not connected. This is called open-collector output.
It needs a pullup resistor (about 10kΩ) to a positive voltage (+5V) in order to present a proper logic output.



The most likely function of this circuit is to detect falling supply voltage (power loss) and to present a logic LOW at PLR.
This is used to save the current settings to non-volatile memory just before the +5V supply starts to fall.