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Compaq laptop power supply for LED lighting

CompaqPS-outside.jpg
One day the power supply of the LED lighting in my kitchen broke down. As I wanted to make a warranty claim, I expected it would take a while before I had a replacement. So I dug in the junkbox to see if I had a suitable small power supply to serve as a temporary replacement. I found this 27VA Compaq power supply, dating 1994. Probably it belonged to a laptop and has landed in my inventory more than 10 years ago. With a minor modification, I could use it for my LED lighting.
According to the fine print on the back, this power supply delivers max 16V at 27VA. The reasons why I chose this one were (1) its rated power was not as high as most laptop power bricks (2) the case wasn't glued together and could be opened.


    Component side of the PCB.

Component side of the PCB.


    Solder side of the PCB.

Solder side of the PCB.


    PCB with shield mounted again.

PCB with shield mounted again.

I opened the case and found a compact unit packed in rf shielding. After removing the shield, I had a quick glance at the circuit board. It contained a single transistor for power conversion, controlled by a UC3843AN PWM controller. A CQY80 optocoupler bridges the gap between mains and output circuits to convey the control signal. On the solder side of the PCB, there are a number of SMD components, including an 8-pin IC carrying the type designation “358”, so I assume this would be an LM358 equivalent.

There is a trimpot near the output. I tried to turn it but it was sealed with a transparent varnish. Using plyers, I broke the seal, but I suspected this might damage the pot. So I replaced it. It was a 1k pot, I had a spare or two in store. After this, I connected a 8 Ω resistor to the output. The device initially delivered 16V. I could turn it down to 11V. I set it to 12V and connected the LED lighthing. Working fine!

Copyright © 2014 by Onno's E-page         published 2014-06-01