Transistor fried, illegible- help with replacement and explanation of it's intended purpose.

Discussion in 'The Projects Forum' started by camjerlams, Oct 19, 2014.

  1. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Hi, I'm a noob with electronics attempting to fix a battery charger circuit, the old transistor on the circuit is fried beyond recognition, no model numbers any more. I dont know what to replace with..

    The battery is backup power for a freezer, just so it can provide an alarm state if theres no power.
    Battery went flat, the charge leads have no voltage, a transistor is destroyed.

    Leg 1 (left) goes to an OP amp IC output amongst othwer things, Leg 3 (right) goes to the same OP amp IC, but input. It also parallel with 6 other larger ICs that I havent checked yet (hard to desolder). Leg 2 (middle) has a series connected resistor, looks like a 1 watt, 15 ohm, looks to be linked into the positive rail, close downline from a recitifier IC. (I'm assuming thats gate, so when power comes on transistor switches on the 5V signal??) Meaning I sdhould be looking at a PNP transistor??

    I'm not familiar with this schematic and havent worked with transistors before, does anyone have an idea of how tghis circuit should function and what transistor I should look into?

    Many thanks!
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    See if you can provide a make and model number of your battery charger.
    Photos of outside, inside and the circuit board top and bottom may also prove to be useful.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    Without some visual aids it is going to be very difficult to offer help and advice. As general rule circuits are not subject to spontaneous combustion. "Fried" implies that excessive current passed through part or all of the circuit for some reason that you either don't know or have not shared with us. As with most devices of this type just replacing the fried component is unlikely to make the device "good as new" again. The reason for this is that other components may have been damaged or stressed beyond their absolute maximum ratings. At some point it will be easier to to replace the whole device.

    If you do replace the device maybe you should buy two so you'll have a spare, and can take one apart and document it before it fails.

    One more thing. Even if you could identify the failed transistor you may not be able to obtain a replacement. Manufacturers stop making things like that all the time.
     
  4. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    https://drive.google.com/folderview?id=0B24ZN7xs6vYKLVhsajFXT1haYTg&usp=sharing

    OK so here are some images, thanks for your time, this is the PCB for the freezer, it controls the compressors, alarms and battery charging as far as I can see. That's why I'm keen on trying to fix; it's not necessarily as easy as buying a battery charger when I need the alarms to be working too like low battery alarm so I'm going to give it a good crack.
    Aaand the PCB is no longer made.
    The 2 big smoothing caps were gone, i replaced the transistor and resistor onn the black part of the PCB, both were burnt, the resistor seemingly only by proximity to transistor.
    I agree that the fault may not be in the trans, there are 6 ICs powered from one leg of the trans as I mentioned above; I'm thinking I may have to desolder those ICs to test them. Is it possible to get them out neatly with a soldering iron and sucker? Or should I just cut them off then desolder the legs, buy new ones?
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    It looks like a straight-forward linear power supply, likely making several voltages using linear regulators (not transistors). I would trace out a schematic of just the burned section, and go from there.

    I'm guessing that the fault that originally caused the fire is not on this board...
     
    Last edited: Oct 20, 2014
  6. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Ah ok, when I connect the cable going to the control panel, the voltage on the faulty part of the circuit goes from 5V to 0V. Also I'm getting 18V across the battery charge leads with the same cable discnnected and 0.2V with it connected. The battery is only 6V.. So I probably need to try a linear regulator that will produce 6V im guessing.
    I'll trace it out and go from there, thanks.
     
  7. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    I've traced out the main circuit elements and added another image to https://drive.google.com/folderview?id=0B24ZN7xs6vYKLVhsajFXT1haYTg&usp=sharing .
    Would you still agree that the element in question was a voltage regulator?
    I'm just not familiar with most electronics schematics so not asking anyone here to solve my problem, but to help me find out how it should behave.. Why is one side of the regulator connected to the neutral side of 6 ICs? What do the regulator and OP amp acheive working in tandem like that?
    The smoothing capacitors were both found to have no capacitance, could this have caused ripple to the reference voltage of the regulator and caused a high voltage/current situation through the regulator?
    The only other electrical components linked to this board are in the freezer control panel, I visually inspected it and saw no discolouration or heat damage..
    Any light you guys can shed on what kind of circuit this is would be appreciated, as mentioned above, the battery no longer charges.
     
    Last edited: Oct 22, 2014
  8. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Could this be a negative regulator?
     
  9. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Or perhaps the common ground?
     
  10. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Oh yeah..
    OK thanks guys, ill just trial and error different regulators I spose.
     
  11. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    I'm still having issues with this circuit, I've traced it out and added the diagram here https://drive.google.com/folderview?id=0B24ZN7xs6vYKLVhsajFXT1haYTg&usp=sharing
    It's looking like it is a transistor not a voltage regulator because of the 'Q' label on the board, also next to the other transistors. I've got a new PNP transistor but I still dont see how the circuit should work, this is the first time I've worked with transistors too btw, if you could take a quick look and share your thoughts I'd really appreciate it.
    The 3 voltages at the bottom were measured with open circuit where transistor should be.
     
  12. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Last edited: Nov 5, 2014
Loading...