Hi all, I have browsed this forum go years but have never needed to post until now
I am after some help with a switch mode PSU, this is 3x identical PSUs in one unit designed to supply 3 X independent outputs.
The problem I have is its spec it's slightly wrong and it's over voltage, it's output is 26.57v, I need it to be 26.3v, that 0.2v is a problem as its a battery charger and the fuel gauge in the battery won't charg the packs above 26.3 and throws a battery error.
I have stripped the PSU and looked over, it's a pretty standard SW PSU with a OB2269 to control the HV side and it's using a LM358 OP Amp to sort the feedback, the problem I have is in unable to to work out what I need to change to reduce the output.
It looks like it may be in a bias current setup ?
On the op Amp only one is used, the + appears to be the ref but it's not got any voltage at input, it's going through A few resistors and a 5v Zenner is there as well, on the - Input it appears to get tied to the op output via 1k resistor then into a cap, it then T off again through a 1k resistor then to ground.
Attached are some pictures and I will try and draw out what I have been able to workout circuit wise shortly.
Any hep with this will be very appreciated as I need to get the output down to 26.3v dead.
I am after some help with a switch mode PSU, this is 3x identical PSUs in one unit designed to supply 3 X independent outputs.
The problem I have is its spec it's slightly wrong and it's over voltage, it's output is 26.57v, I need it to be 26.3v, that 0.2v is a problem as its a battery charger and the fuel gauge in the battery won't charg the packs above 26.3 and throws a battery error.
I have stripped the PSU and looked over, it's a pretty standard SW PSU with a OB2269 to control the HV side and it's using a LM358 OP Amp to sort the feedback, the problem I have is in unable to to work out what I need to change to reduce the output.
It looks like it may be in a bias current setup ?
On the op Amp only one is used, the + appears to be the ref but it's not got any voltage at input, it's going through A few resistors and a 5v Zenner is there as well, on the - Input it appears to get tied to the op output via 1k resistor then into a cap, it then T off again through a 1k resistor then to ground.
Attached are some pictures and I will try and draw out what I have been able to workout circuit wise shortly.
Any hep with this will be very appreciated as I need to get the output down to 26.3v dead.
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