What does this circuit do?

Discussion in 'General Electronics Chat' started by camjerlams, Nov 6, 2014.

  1. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Im repairing this circuit, the transistor was torched and all markings burnt away so I'm just taking a guess at what to replace with. I can't figure out what should be gate, collector and emitter though, what is this circuit is trying to acheive?.. my hunch is the 18V with the 15 ohm resistor is for the gate, and the op amp connects to collector but I still don't see why.
    Is anyone familiar with this kind of schematic?

    https://drive.google.com/file/d/0B24ZN7xs6vYKM3VnUk1kampXQlU/view?usp=sharing

    https://drive.google.com/file/d/0B24ZN7xs6vYKM3VEMEptckt3NXM/view?usp=sharing

    Seems to have something to do with charging a 6V back-up battery, it went flat and that's how I found the burnt transistor. So I'm thinking the OP amp may be something like a voltage to current converter???

    http://www.allaboutcircuits.com/vol_3/chpt_8/7.html
     
    Last edited: Nov 6, 2014
    bordonbert likes this.
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
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    Have you tried contacting the company? Forma is now handled by Thermo Scientific.

    Ken
     
  3. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,647
    759
    Hola cam

    For starters: it is collector, base & emitter OR drain, gate & source.

    Not digged yet at all in your schematic; please recheck carefully all around the LM10.

    What that circuit does?
     
  4. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Interesting.. there's a Thermo Scientific across the road from me, although I know they don't make these freezers anymore.. I'll ask them on Monday thanks.

    I'll recheck around LM10, there's more i could include in my diagram so i should do that again too.
     
  5. antonv

    Member

    Nov 27, 2012
    149
    27
    Could it have been a LM7805 or some other 3-pin regulator instead of a transistor?
     
  6. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    That's what I thought too but the PCB is labelled with a Q at that location same as the other transistors, the regulators are labelled with U.
     
  7. camjerlams

    Thread Starter Member

    Dec 23, 2012
    56
    1
    Update:
    So I have just returned to this repair, I replaced the damaged transistor with a PNP (TIP32C) power transistor. The comparator output to the base, as I'd found this type of schematic on a lot of battery charger circuits just from google image search. It's float charging the battery and I'm not getting any alarms as yet. Thanks for discussing this with me, I know my schematic was useless, there was a relay in there that is switched by the transistor.
    Thanks again!
     
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