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I have two under counter lights in my kitchen which stopped working at the same time. I assumed some kind of fuse needed changing, and on opening the little box connecting them to the mains I found the following.

circuit board for two under counter lights

I presume this is where my problem is, but what is it? And what do I ask for to get a new one that works?

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DO NOT TOUCH ANYTHING UNTIL IT IS UNPLUGGED FOR AT LEAST 48 HOURS. The two black cylinders in the middle right of the pic are capacitors and can retain a charge for up to weeks but that size should be drained after a day or so. Just unplugged even that size has enough charge to put you on your butt, melt a screwdriver tip, and is potentially lethal. –  Jason May 8 '13 at 19:36
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What type of lights are they powering? Neon or florescent is called a ballast or transformer. DC incandescent or LED are typically called a driver or power supply. –  Jason May 8 '13 at 19:38
    
It's a step-down transformer (Mains voltage to low voltage), possibly a rectifier (AC to DC), and maybe some other control circuitry. Look on the "little box", that contained the circuit board. It should have some manufacturer information, possibly a part/model number and/or serial number. –  Tester101 May 8 '13 at 19:58
    
@Jason: "that size has enough charge to put you on your butt..." - That would depend on the voltage across the capacitor. Since this appears to be a low-voltage DC power supply, it is very unlikely those capacitors present any danger; though of course it never hurts to exercise a little caution. –  BlueRaja - Danny Pflughoeft Sep 26 '13 at 20:21
    
Also just FYI, the nearby resistors are likely bleeders so the capacitors will discharge is seconds, not hours. –  BlueRaja - Danny Pflughoeft Sep 26 '13 at 20:24
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1 Answer

Of course it is impossible to tell for sure without looking at the circuit in more detail but it looks like a 120V AC to low voltage DC power supply. A way to troubleshoot it would be the following:

  1. look up the voltage rating on the lights. Using a multimeter, check if this voltage is present at the output of the board. If it is, the problem is with the lights/contacts, etc.
  2. check for a blown fuse on the board (if present). If you find a blown fuse, get a couple of new ones and replace it. It might have been blown in some sort of a fluke but if it keeps breaking there is a problem with a circuit. If no fuse:
  3. Switch the meter to AC Voltage measurement and measure the output of the transformer (large part with green core). If there is no voltage, the transformer is bad. If there is, you would have to redraw the schematic and troubleshoot every component (caps are usually the suspect). At this point it might be cheaper to get a new unit.

As with any high voltage board be very careful when troubleshooting a powered board!

PS: I believe Monso is wrong here - the voltages after step down transformer are typically 5-24 V which is not enough to cause harm (after the board is powered down).

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