Question about a malfunctioning circuit board

Discussion in 'The Projects Forum' started by Tlman12, May 25, 2016.

  1. Tlman12

    Thread Starter New Member

    Nov 27, 2013
    I have a wine cooler that the circuit board keeps blowing it's fuse. I tested some of the components most of the diodes and found and replaced one bad diode but I don't really know how to test everything else or where I should start for the problem I'm having.

    I've attached an image of what I know. Is that one component a thermistor? how do I test it? from what I understand about thermistors this could very well be my issue but I don't know what a working one should test like. the numbers on it are "005 04". I replaced the one diode that tested at 0.002 both directions. it was a 1n4007 but all I had laying around was a 1n4001 (figured it would be enough to confirm the issue then I could replace it with a 7 later on) put it in and tested at 0.56 but didn't fix my issue, I replaced and blew another fuse.

    the amperage coming out of my power supply is ~12a (if i'm testing correctly) at ~118v. The fuse is rated at 250v 2.5a (same rating as the original fuse) and it's fast acting) but when i test across the fuse leads I'm getting the same ~12a that is coming from the power supply (again if i'm testing correctly) but I assume that should be dropped across the fuse some where at 2.5 or lower? or does a board like this require a slow blow? it looked like fast to me and there was no indication on it otherwise.

    I'm still pretty green with circuits. been a while since I took any sort of engineering but I do have a pretty good grasp on how "most" of the components should work just not how to confirm they are working the way the should.
  2. RichardO

    Late Member

    May 4, 2013
    Is the replacement diode still good? Putting in a 50 volt diode to replace a 1000 volt diode is not likely to work. I would be very surprised if it survived the test. :eek:

    There is nowhere enough detail in your picture to identify the "thermistor". Note that thermistors are normally low current devices and are not used in circuits where they can blow a fuse. If it looks like a thermistor then it is more likely a MOV that is used for surge suppression at the input of the power supply.

    I am confused about where and how you measured the currents. Can you give more details?
  3. SLK001

    Senior Member

    Nov 29, 2011
    I'm also confused. If you are measuring 12A at the fuse, then you have a dead short on your board. I'm surprised that you aren't tripping the breaker. As RichardO says, replacing a 1000V diode with a 50V diode can't lead you to any place good.
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    please learn how to properly use the meter before you continue your measurements are not correct I'm sure..
    and exercise extreme caution..
    You have already made a mistake replacing the diode with an underrated one.. Please don't continue..
  5. Tlman12

    Thread Starter New Member

    Nov 27, 2013
    It could be a mov that is a component I'm not fimiliar with..Ive done mostly dc low voltage so I'm fimiliar with diodes and resisters capacitors etc but I dont recognize some of the components in this board. I'm not sure why they would need 1000v peak diode in this circuit. I'm sure you'll probably say it was there for a reason for it and there probably is but the highest dc voltage/output i could find on the board was 12.5v so I yes I figured a 50v diode would last long enough to test. Not permanently but both were 1a rated. The loads are 2 12v dc fans 1 12v thermoelectric cooler and 1 LED diode. I will re-test the diode to see if it is still good.

    Testing the amperage I put my multi meter inline without the fuse installed which is how I usually test amperage but the the power across the fuse is ac I'm only used to working with dc devices so I didn't know if it would somehow be different or maybe my multi meter wasn't right for it. I do have ac settings on it though. I'm not stupid just a bit reckless and I love to tinker you can't assume because I don't know everything that I don't know anything. Worst case I'll buy a new board for $50 but I'd really like the satisfaction of being able to fix it myself. So go ahead and tell me I'm doing everything wrong but offer me some advice instead of assuming I'm just gonna blow myself up. At least I'd go out doing something I love lol.
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    What you have there is a switchmode psu, the mains is rectified by four diodes 1N4007, and goes across a smoothing capacitor, this will feed a mosfet pushing oscillator, chances are the fets are blown,and all four diodes will be too.

    If you want to measure the current, take the fuse out and put your meter on AC amps, and put the meter probes across the fuse holder, switch on and see what happens.
    Last edited: May 27, 2016