Any electronic experts here?

Discussion in 'Mech Tech' started by wizrod, Aug 12, 2019.

  1. Hi,
    I've a problem with a brake warning lamp, as used in conjunction with 3-pin brake switches on a dual brake m/c.

    When I got the car, the lamp was inop. I've recently decided to get it working, so I found and fitted a replacement.

    It appeared to work fine. Ignition on and the lamp lit up.

    Engine on and the lamp stayed on, pressed brake pedal and the lamp blew up (well..there was a flash, and it went 'phut').

    I know the brakes need I expected the lamp to come on when I pressed the brake...the explody bit was a surprise, though.

    Anyway, I took the lamp apart to see the damage. There is a tiny circuit board inside, with some diodes, resistors & a transistor. Both diodes were broken and part of the copper circuit track was burnt & missing.

    (see attached image for the circuit in the lamp...crossings out indicate failures.)

    At this point, I opened the original lamp...same story.

    So, as far as I can see, the connections to the lamp were correct, what else could have caused this overload?

    (Note..things I still need to verify: the connections at the switches( right colour wires to appropriate terminals?) A d whether the brake lights (still) work.

    I fully admit that my electronics knowledge is a but rusty, but I can't see what cause the overload....

    TIA for any insight.

    BTW, this is an issue on my Thing, but just as applicable to a Late Bay...the replacement lamp I bought was from Late Bay.



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    Last edited: Aug 12, 2019
  2. That looks about right (although E to C diode is the wrong way round ;)). I'll try and dig out my pic. 61 as I recall is connected to the handbrake switch (if you've got one). Hard to think why it would go phut, if the lamp worked initially. Could try fixing it again and leaving 61 disconnected.

    The problem I found with the few I've seen is the bulb holder itself shorting, either in internally, or to other bits.
    jivedubbin and wizrod like this.
  3. Swap 31 and 15 and it will pop when 61 is earthed and 12 volts goes straight through both diodes..
    wizrod likes this.
  4. That would certainly match what I see...the connections were correct, so I'll investigate how the other ends of those wires are hooked up...

    Any idea on the diodes I'd use to fix them? The bulb is 1.2w, so draws 100mA, so anything rated for a greater forward current would be ok? These...?

  5. Yep. Really anything’ll do.
  6. TA

    I'll order some up and try a little surgery on the remains...
  7. For a 1.2 watt pea bulb thats abou 0.1Amps
    So chuck in a couple of 1N4001 1 amp diodes.
    Transistor may still be OK . Test by power o 15 ground 31. If transistor still works take K to +12 and light should come on if the transistor is working.

    BC548 will just about do..
  8. Thanks. The transistors seem ok...well, they haven't visibly broken...

    One is marked BC237, other I forget...I assume a BC548 is a rough equivalent?

    BTW...I'm not sure 1N4001s would physically fit...they might just if I were to mount them on the other (solder) side of the pcb...
    Last edited: Aug 13, 2019
  9. Probably 1n914 or 1n4148 generic switching diodes then.

    Bc237 is a slightly higher voltage rating, but just a generic silicon npn transistor..
    Valveandy and snotty like this.
  10. Well...diodes ordered...well see what happens..

    I can feel my o-level electronics slowly waking up!!
    Last edited: Aug 13, 2019
    Valveandy and jivedubbin like this.
  11. Keep going!
  12. So...diodes replaced.

    With 15 & 61 connected, the lamp goes on...ok so far.

    With 15 and 31 connected, and with 15 & K bridged (simulating a signal from the MC switches) the lamp goes on.

    However...this was on the bench with a test supply and the cover off.

    Different story back in the car. Here, with just 15 & 31 connected (ignition on) the lamp came on dimly an 'something' got warm...I guess there's a short somewhere...

    The thing is, my continuity tester shows a short between the base & collector of the transistor, which doesn't feel right...scratch that(perhaps)...I wasn't testing it properly...

    Last edited: Aug 19, 2019

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