Hello,
I'm having an issue with a friends desk lamp. When he initially brought it to me, he said he had put a 120 V AC rated bulb into the 12 V socket. After that it hasn't turned on. I don't know if that actually caused the problem, but please let me know.
Inside the lamp base is a small 120 V AC ---> 12 V AC adjustable output (via potentiometer) power supply. It's made by a Chinese company (
http://www.chiachien.com/English/en_Profile.asp).
After taking apart the power supply, I measured the main input resistor as open, even though there were no signs of damage. According the resistor color bands, it is 0.5 ohm. After replacing the resistor, the power supply started to show life again, but at a reduced voltage. The output never goes above 1-1.5 volts with the pot turned all the way up. I measured the resistance of the pot and it is very low when turned up. The bulb lights up, but only very dim. I know the bulbs work correctly, because I have tested them on a lab power supply. I checked for shorts between wires and the light frame, but it looks all good.
I decided it must be a cap or transistor, so I purchased replacement components. I replaced all of the capacitors, transistors, most diodes (could try all if you think necessary, but I checked them with a meter and they come back as good), and even the little blue diac. It still shows the same behavior though. I can hear switching noise when I turn up the pot, and one of the large NPN transistors gets hot after a few minutes. Does anyone have any idea why a PS like this would function like this? What do you suggest I try?
If you need a circuit diagram, parts list, or anything else let me know. Below are the transistors, and diacs (ordered two different manufacturers because I couldn't identify the original) I ordered.
https://www.digikey.com/product-detail/en/micro-commercial-co/DB3-TP/DB3-TPMSCT-ND/2513488https://www.digikey.com/products/en?keywords=497-3099-1-ndhttps://www.digikey.com/products/en?keywords=497-12230-ndhttps://www.digikey.com/products/en?keywords=2n5551bu-ndThe two white wires going to the middle of the board are from the potentiometer/switch.