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ATX PSU to battery charger conversion but it squeals like mad.
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Refrigerator:
Today i decided to modify an old ATX PSU i had into a lead acid battery charger.
It's some very old power supply but it has nichicon caps in it so it should be good quality.
It also has a 2kBit EEPROM chip in it with other weird stuff, but i digress.
I disconnected the voltage feedback opto and hooked my circuit to it.
It's basically to bypass all the complicated voltage feedback circuitry to increase the voltage to ~14V and to add current limiting, it's a flyback SMPS so increasing the voltage is fine.
It works great, i get 14.2V and it limits current at just over 3A but it squeals like mad at the most annoying frequency possible and i can't seem to figure out a way to stop it. :-//
I tried connecting capacitor/resistor combos between pin 1 and 2 but didn't get far.
Schematic of my voltage feedback circuit included, it was drawn with LM393 in mind, but after testing and trying to silence the noise i replaced it with a LM358 with two diodes added to outputs to only sink current.
With both ICs i get noise but i had better success silencing it with the LM393.
TheMG:
Have you looked at the feedback loop using an oscilloscope? What do you see?
Refrigerator:
Tried hooking the scope up but didn't see much, everything seems completely fine.
Could it be that the noise is inherent to the PSU and is not related to feedback?
Both with a 6.8 \$\Omega\$ load for CV and with 3.3 \$\Omega\$ for CC it regulates very well.
This is with a 1n cap across pin 1 and 2.
Yellow trace is V+ and red is the opamp output.
Ps: the ripple in the second pic might be my 3R3 resistor burning out.
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