PSU repair questions

[Chris Wilson]

I got an odd ball custom built PSU at a rally yesterday, for virtually nothing. Apparently custom built for some testing purpose in a production environment it basically consists of a computer SMPS unit in a nice case, with a bespoke PCB. feeding panel sockets. It appears in use it had USB control of some functions, and also has a serial port. The grey wire out the front of the panel terminates in a USB plug. The computer SMPS outputs are fed to the custom PCB and there are banana sockets on the front panel for +15V, -15V, +12V, -12V, +5V and -5V.

I am seeing the following under test from these sockets, in the same order as above: 11.6V, -0.1mV, 12.16V, -11.3V, 5.05V and -5.04V

I am guessing the 15v is derived from a DC to DC converter circuit from 12V perhaps, and that's what the coil and choke are for? You can see photos at http://www.chriswilson.tv/psu/psu.html

I will try and attach a few below, too. There are also some small IC's with two loops of insulated wire around them, and I have never seen anything like that before, any ideas why they have done this?

I was going to use the supply (forgetting about USB /serial control) just to run breadboard experiments using OpAmps that need  + and - 15 rail supplies. Without a circuit diagram I need assistance with ID'ing what part of the circuit may be at fault. The SMPS itself outputs sensible voltages to the PCB, although (assuming it's even used) the -12V is low at around -11.3V. I suspect they may have got the split 15V supply with a resistor divider network, hence perhaps the 4 big 1k resistors? Guessing though....  Thanks for any tips. I can upload more specific photos if desired.

[alanb]

ICs with wire round them - possibly Hall effect current monitoring.

[tron9000]

From the very small ID number on the photo's, I Did a google search - could they be these?
http://www.melexis.com/Current-Sensors/IMC-Hall-Current-Sensor-ICs/MLX91205-689.aspx

[Chris Wilson]

From the very small ID number on the photo's, I Did a google search - could they be these?
http://www.melexis.com/Current-Sensors/IMC-Hall-Current-Sensor-ICs/MLX91205-689.aspx

Oh! Wow! Well spotted, I have got a lens on them and they do indeed look like 91205 units! I guess the box of tricks was for testing some other device and moonitoring current consumed, maybe at different voltage levels, and logged via USB?

 After determining input voltages to the board wewre present I pulled the wired end electrolytics and tested them, all 4 were fine.

I attach another photo, I have marked interesting IC's with part numbers.


If we assume U12 is for current monitoring and it should have -15V on its current sense wire, it doesn't, I need to work out how the 15V levels are derived now.

Thanks for the replies.