Odd China component, or ??

Discussion in 'General Electronics Chat' started by Tuffe, Oct 1, 2011.

  1. Tuffe

    Thread Starter New Member

    Mar 9, 2011
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    Hello,
    Can anyone please help ?
    I managed to fry my dear car battery charger/starter.. It charge 12A and gives 75A start boost.
    I should help a friend to start his massey ferguson, and did not know it had reverse ground (+) :(
    I have added a picture of the to me, strange component . It is a link from the rectifier, and goes straight to output +
    I would say its a protection diode, but the four legs confuses me a lot...

    Best regards.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    This appears (to me) to be a really basic current measuring shunt. You take a high power low ohmage resistor, measure the voltage across it, and you have the current through the circuit.
     
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    It was a power rectifier diode.
    Now it is a blob of charred plastic. :)

    The cathode goes to the left. Right below it is a rectangle with a line through it (the line indicates the cathode end); and a reference designator "D12"; "D" is the prefix used for diodes.

    The item it melted to (brass rod with a nick in the left end) is a current shunt, as Bill_Marsden indicated. The nick was to calibrate the value of the shunt. Don't scratch the rod while you're removing the remains of the diode.
     
    Last edited: Oct 1, 2011
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  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think it is a measuring shunt with a diode glued to it.
    The diode is probably used to measure the temperature of the shunt.
    This is a measure for the amount of current flowing through the shunt.

    Bertus
     
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  5. Wendy

    Moderator

    Mar 24, 2008
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    I can buy that idea. The blob of glue looks deliberate to me.
     
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  6. Tuffe

    Thread Starter New Member

    Mar 9, 2011
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    Thank you very much everybody, for your replies!! You see, both the shunt and the diode is vanished on my charger..It was only a black mark where they had been.. I was happy to find a person who had a identical charger, who allowed me to take pictures inside.. I was hoping to find something I could identify, but as I said, it gave me no clue...
    Does some of you have an idea about what kind of diode it has been ? For the shunt, I guesss its enough to measure excact diameter for the cord.. It seems like a massive copper wire..
     
  7. colinb

    Active Member

    Jun 15, 2011
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    Interesting idea. However, its accuracy would depend on ambient temperature (unless another temperature sensor cancels out the temperature difference from ambient), and on airflow around the shunt, amount of glue on the diode, etc. so it seems like a really difficult way to measure the current, compared to simply measuring the voltage across the shunt.

    Why would one use shunt temperature to measure current rather than voltage across the shunt?
     
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  8. SgtWookie

    Expert

    Jul 17, 2007
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    They typically use brass for shunts. Brass has a higher resistance than copper.

    You will need to get the diameter and length correct, as well as the shape.

    As far as the diode itself - I think Bertus hit on the correct answer. You might be able to convince your friend to allow you to measure the forward voltage of the diode in their unit. If not, you might try using something like a 1N914 or 1N4148 diode, and use an adhesive like JB-Weld to secure it to the new brass shunt. The diode should be directly in contact with the shunt before you apply the adhesive.
     
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  9. #12

    Expert

    Nov 30, 2010
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    Clothes pins. I always carry clothes pins. Just the right thing to hold a diode on while the first layer of glue dries.

    The diode you buy will have a stripe near one end, just like the drawing below the shunt. Attach it with the stripe to the left.
     
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  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    How about using shunt temperature to tell when the shunt is too hot? Or maybe the shunt has huge temp. coeficient and needs to be compensated.
     
  11. tom66

    Senior Member

    May 9, 2009
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    Maybe it's a 4-wire kelvin current shunt? The diode designation could be incorrect.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    It seems to me a shunt with a diode glued with epoxy to measure the temp of the shunt to prevent it from melting
     
  13. debe

    Well-Known Member

    Sep 21, 2010
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    Unfortunately he hasnt shown the rest of the board.I would bet the diode senses the temp of the shunt to shut down the charger in an overload situation. The cheep chinese charge/crank chargers ive worked on had Klixon thermal switches on the transformer & the rectifier to shut it down.
     
  14. tom66

    Senior Member

    May 9, 2009
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    I don't understand why you would measure the temperature of the shunt. Design the shunt to survive the rated current, but sense the current using the shunt and shut down on overload... Then again, I've seen some very odd design decisions before.
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If the diode is used to sense shunt heating, its voltage drop is probably compared to that of another diode on the board (as colinb suggested), thus minimizing ambient temperature problems. This method might be simpler than measuring shunt voltage drop, which might have transients that would have to be dealt with.
     
  16. colinb

    Active Member

    Jun 15, 2011
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    @Ron H: Good point. The use of temperature sensing rather than direct voltage measurement introduces a natural low-pass filter in the form of thermal masses and thermal resistances that would filter out transients and provide a natural averaging filter so that the sample rate can be lower. Also, it provides electrical isolation between the load circuit and the measurement circuit, but in this instance, where low voltages are in use and within the same device, I doubt that is the reason.
     
  17. paulktreg

    Distinguished Member

    Jun 2, 2008
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    How about the diode gets a little warm so we'll stick it to the shunt and use it and the copper track it's connected to as an heatsink?

    It just seems a rather crude way of monitoring current through the shunt when, as in ATX power supplies, you just measure the voltage across them?
     
  18. tom66

    Senior Member

    May 9, 2009
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    Or in many supplies, you just rely on the MOSFET smoking before an overcurrent damages something else... especially imported junk.
     
  19. colinb

    Active Member

    Jun 15, 2011
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    That's the same question I've been asking. So far the only advantage of temperature-based current measurement instead of voltage-based measurement is that the temperature-based method introduces natural low-pass filtering so voltage transients/noise are mitigated. Also, it would provide electrical isolation from the load, but I don't see that as any advantage in this case.

    It still seems like direct voltage measurement would be easier, and an electrical low-pass filter could be used to filter out transients.
     
  20. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Perhaps it was designed by Rube Goldberg.:p
     
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