Please help me read/improve this schematic

Discussion in 'General Electronics Chat' started by meestahelectro, Sep 23, 2010.

  1. meestahelectro

    Thread Starter Member

    Aug 10, 2006
    I basically drew this schematic from an actual circuit board I am working with. this is only the top half of the circuit board, there is still another side that I will work on tonight. can you all please give me some insight as to what is going on here. I am fairly new to this stuff.

    the chip at the bottom is an LM2902 (I just show the top half of the chip) pins 1 - 7.

    I believe power comes into the board from the Red wire in the upper right. Power flows through to the V+ on the chip and to the coil on the relay. I'm not sure what the two capacitors do on the right side do.

    Also I am not really clear what the transistors on the left are meant to do.

    Again Irealize this is kind of hard with just the one side, but I want to understand a litle more as I proceeed.

    If anything ook wierd, please let me know. this took a long time for me to do and was quite tedious. thanks.
    Last edited: Sep 23, 2010
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    Not a clue yet, could be anything.
  3. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Where this circuit board from, what equipment?

    What is it suppose to do? Is it now not working?

    What supply voltage and what the relay supposed to control?
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Yes- we need to know more to provide clues that are of any help here.

    It seems from what you have drawn that the only thing for sure that it does is control something by switching a relay. That is surely less helpful than you wish. We need to know more...
  5. meestahelectro

    Thread Starter Member

    Aug 10, 2006
    I took another crack at drawing the schematic and gave myslef more space. The Tip120 on the original was in the wrong configuration.

    Here are more details about the inputs, what this does and what I have already figured out from staring at this thing all night. : )

    Black - Ground
    Yellow - +vdc (comes from a small light bulb)
    Pink - Ground
    Org - brings in +13vdc
    Purple - Vout (either to ground or +13vdc depends on the state of the relay)
    Red - +vdc (comes from the power supply, through the fuse block)

    essentially what this does is check temperature and if the temp is too hot it causes certain events to happen. at 900 degrees it flashes the bulb, at 940 it keeps the bulb on permanantly and at 980 it turns the bulb on and engages the relay...

    LM2902 is a quad op-amp chip (thanks Retched)... I can see that output 2 from the chip affects both the relay and the bulb circuit (this must be triggered at the 980 temp). Outputs from 3 and 4 seem to affect just the bulb circuit, this is probably related to the temp threshhold at 900 and 940. I am still not sure what output #1 does...

    can someone give me a little more insight on what caps being used on the red wire. is that as a precaution to a voltage spike. how would thi handle a voltage spike, if so?

    seems like the two BC237B transistors act together. the lower powers the upper and then connects it to ground, which cuts power to the lower... seems like this would cause a pulsing effect. not sure why it would do this...

    thanksfor the inspiration...
  6. meestahelectro

    Thread Starter Member

    Aug 10, 2006
    this is my first drawing of the bottom of the board... honestly it was lucky I started with the top or I would have probably abandoned ship. this side seems a lot more complicated... please help me understand this. thx.