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Help needed with replacement of burnt out transistor

  1. Jun 16, 2009 #1
    does anyone happen to know what resistor i would need for
    a pair of 850Mah rechargeable batteries as the one in
    my hair clipper burnt out when i replaced the old
    rechargeables which were 300Mah cheers
     
  2. jcsd
  3. Jun 16, 2009 #2

    negitron

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    Clarification, please. Your title mentions a transistor, but your OP asks for a resistor; which is it? Also, are these batteries NiMH or NiCd and did you replace the old ones with new ones of the same type? The two are NOT interchangeable because the charging requirements are different.
     
  4. Jun 17, 2009 #3
    whoops sorry title should be resisitor. The batteries are Ni-Cd Uniross the old ones are Sanyo Nikel Cadmium 600Mah not 300 as I thought before. (same as Ni-Cd i think!?)
     
  5. Jun 17, 2009 #4

    negitron

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    Tough to say. It sounds as though the new batteries have a lower internal resistance than the old ones. Unfortunately your clippers have cheap charging circuitry which uses an unregulated voltage source with a series resistor to produce a current source for the charging the batteries. Typical slow charge currents for charging NiCds are generally designed to met C/10 levels ( C being the rated capacity). For 600 mAH cells, you therefore want to aim for 600/10 or 60 mA charging current. You may need to experiment a bit to determine the optimum resistor value (don't forget about power rating!) but a safe place to start would be about double the value of the old resistors.
     
  6. Jun 17, 2009 #5
    im rather new at recognising resistor types especially the burnt out one, I really need to know what resistor i should replace with. I am buying a job lot of resistors from ebay but would like some rough idea before seeing if they can cope!?
     
  7. Jun 17, 2009 #6

    negitron

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    Can you make out any of the color bands on it? If not, you need to know what voltage the charging circuit is applying to it and whether the batteries are being charged in series or parallel. A safe assumption is to consider the battery as having zero internal resistance and the resistor will therefore need to be (V[charger] - V[battery]) / .060. This will necessarily result in a high value for R, but you can adjust it down from there. Remember, once you have your value for R, you need to plug it into the power equation ( P = I^2 * R) to determine how much power it will be dissipating so you can size it properly.

    NB: Note that without knowing the precise characteristics of the charger there is some risk of further damage here, so you have to decide if it's worth it or not.
     
  8. Jun 17, 2009 #7
    the charger puts out 5-6V Max 5watts input is 100-240V 50-60Hz 0.2amps. Without any real reference I think the batteries are being charged parallel as they are identically connected at both the positive and negative ends
     
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