Compaq laptop power supply for LED lighting
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.
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!
