troubleshooting. trying to identify a component and problem

Discussion in 'The Projects Forum' started by John68, Oct 27, 2006.

  1. John68

    Thread Starter New Member

    Oct 27, 2006
    3
    0
    Hi,
    I have a radio controlled helicopter problem. I accidentally hooked the battery up backwards and fried a MOSFET controller and did some other unknown damage. I already replaced the MOSFET, but I am having problems getting the servos power. The servo plugs have 3 pins each, for the wires. The 3 pins are positive, negative, signal. The signal wire is working fine, and there is proper ground. There isn't any power. The circuit board is laid out simply wnough for me to see that there is a trace from the battery positive (9.6V) through a component, to the positive pins for the servos. This component, which I believe to be some sort of diode, is there to limit the voltage as the servos can't have more than 6 volts running to them. The problem is, I can't identify the component, nor can I completely rule it out as being the problem as I did a few diagnostic tests and was just dumbfounded.

    test 1

    After establishing that the signal pin was functioning properly, and that there was continuity to ground to the negative pin, I plugged a 6 volt battery into one of the other servo slots, giving all the servo plugs power (the trace for the positive and negative is common to all servo plugs, respectively) this operated the servos without fault.

    test 2

    I took a few copper strands and tinned them together, and made a "jumper" wire to go directly from the positive battery terminal to the positive on the servo plugs. When soldered on, I got a response from the receiver (built into the board) that usually means that the receiver is not receiving signal from teh transmitter. I got no response from the controls, either. removing the jumper, and it was back to the same problem.

    test 3

    I desoldered the component from the board and plugged in teh battery. everything else worked fine, except the servos. (just wanted to eliminate that)

    test 4

    With the component desoldered, I made a jumper wire to go from one pad to the other, where the component was mounted. With this set-up, nothing worked.



    The results of my tests leads me to believe that this component is a type of diode that cuts down the battery voltage for the servos, but does not allow voltage to flow back through it. I believe the component is the problem, here, but I am open to suggestions, as the whole board was juiced backwards and anything could have happened.

    That's where I am at. I have a picture of the board, and the component, which, to describe it briefly, is a very small brown cube, that is soldered to the board by the opposing face sides. There are no markings on this component(if there were, I could look it up on digikey and get another one) but I would assume this is some type of zener diode, that keeps power flowing only one way through it. I don't have a good way to measure the component, but using the ridges on the edge of a dime, it is 5 ridges wide and 6 ridges long, and approximately the thickness of a nickel.
    As for the voltages in question, the battery pack is a 7.4 volt pack that charges to 9.9 volts when fully charged, and cannot drop below 6.2 volts when completely empty(without ruining it, it's a lithium-polymer). The servos are meant to run on 4.8-6.0 volts.
    I have a picture of the circuit board, if anyone wants to look at it, but due to the small size and nature, it is a very large file size picture, so I'd have to email it to you.


    Just wondering if anyone here has any ideas for me.

    Thank you!
    John
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    John68,

    Would it be possible to take another picture with your digital camera set on a lower resolution? It would be helpful to see both sides of the board if that is possible.

    hgmjr
     
  3. John68

    Thread Starter New Member

    Oct 27, 2006
    3
    0
    hgmjr,
    There is no connection to the other side of the board, where this component mounts, nor through the pins for the servos, or the trace on the board.

    I can't get the file size smaller than 114K and it still be recognizable. This is a very small board, with it's total area about that of a quarter. I can email the picture, to any one who wants to see it.

    Thanks,
    John
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Take your best jpeg and zip it. That should allow you to post it here since .zip files can usually be accommodated if they are not too large.

    hgmjr
     
  5. John68

    Thread Starter New Member

    Oct 27, 2006
    3
    0
    WOW, ok, problem solved. I actually talked to the electrical engineer who designed the thing. It turns out, the helicopter world is a small one, and I am in the heart of it all.

    Thanks for the replies, I appreciate.
    I am sure I will be bugging you when I start building my lighting control system.
     
  6. chuckey

    Well-Known Member

    Jun 4, 2007
    75
    10
    Can you not put your DVM to its ohms range and tell us its resistance? Accordinng to your figures thhere is about 1.5V to be got rid off, try two 1A silicon diode in series, should be near enough.
    Frank
     
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