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Rheostat confusion

Discussion in 'General Electronics Discussion' started by berkowwm, Jun 12, 2012.

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  1. berkowwm


    Jun 12, 2012
    Hello all,

    I am new to this forum (and have very little knowledge of electronics) so bear with me as I try to describe my problem! I am not sure if this is a stupid question, or if it's even the right question I'm asking...

    I am currently trying to repair a halogen light source on an 80's-90's era microscope, and I know that the rheostat has gone bad on it. The problem is that the rheostat has almost no markings on it (aside from "813r- 500KB" and "ALPS JAPAN 3") and I am trying to find out the correct part to replace it with.

    The overall light setup is as follows: The small halogen bulb is a small 2-pronged 20w 6v bulb, attached to a halogen socket which is wired to an unmarked transformer. The rheostat is apparently a circular knob-type and is wired between the wall plug and the transformer.

    I have checked the bulb and transformer, and they are working perfectly.

    Is there a way I can find the optimal Ohm rating for a replacement rheostat? Also, is the wattage of the rheostat very important, or is it okay as long as it's rated greater than the wattage of the bulb?

    I am a veterinarian and know almost nothing about electronics... Please help!
    I appreciate your time and consideration,
  2. CocaCola


    Apr 7, 2012
    From the part number the pot/trimmer is likely 500K in value... Do you have any pictures of the pot/trimmer? Also what country are you in so we know the mains voltage?
    Last edited: Jun 12, 2012
  3. john monks

    john monks

    Mar 9, 2012
    Do you have a model number for your microscope?
  4. berkowwm


    Jun 12, 2012
    Thank you for your quick replies.

    1) I am in the USA
    2) The device is (hopefully) pictured below.
    3) The microscope itself appears to be a relatively cheap medical model made in Japan and sold under a few different brand names. It has a serial number (861758) but no model number. There were two on the shelf in the hospital's surplus closet, nearly identical to each other, with one having an Abco brand name and the other an Edmund Scientific label (Funny thing too, both of them have broken potentiometers). So far, I have not been successful in my internet searches.

    I hope that helps. I'd like to get this microscope working so that it can be put to use for some field work and/or hobby stuff.

    Thanks again for your time!

    Attached Files:

  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    That doesn't look like a simple rheostat, more like a dimmer circuit, where the poti only controlls the triac.
    You could try to replace the whole bit by a standard household dimmer.

  6. berkowwm


    Jun 12, 2012
    Thank you for your replies. I may try a standard household dimmer, if I can get it to fit into the microscope base housing.

    Are there any potential problems with that method, that anyone knows of?
  7. john monks

    john monks

    Mar 9, 2012
    Yes. You must make sure that the dimmer is compatible with an inductive load. Remember, you are driving a transformer, not a light bulb.
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