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Preset Variable Resistor Markings

Discussion in 'General Electronics Discussion' started by Sudden, Aug 22, 2011.

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  1. Sudden


    Apr 27, 2011
    Do preset varable resistors have value marks on them.
    I have PSU ( 13.8vdc 7A, just had it second hand from carboot sale) which has stopped giving any output. I have looked at the pcb and tried the voltage adjuster with no luck and noticed that the pcb has label stating VR200R the actual VR has 5000 stamped on it. Is this the ohms value or the makers mark?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    On most components that have markings on them, those markings serve to denote either/both the manufacturer and component identification/value.

    Clearly 5000 is not a manufacturer, so it is likely to be the value.

    5000 could mean either 5000 ohms, or it could mean 500 x 10^0 ohms (500 ohms).

    Given that most pots have fairly wide tolerances, the most likely one of these is 5000 ohms (or 5k)

    However, if the marking on the board says VR200R, then there is evidence that the value is 200 ohms.

    None of these values (5k, 500R, 200R) is unusual or particularly unlikely, so it is somewhat difficult to say.

    The best way to be certain is to measure it.

    Do you have any reason to suspect that this pot is damaged? measure between the wiper and both ends. The resistance should rise and fall smoothly as you rotate the pot.

    Also remember where the pot (I'm assuming a trimpot) was set to. If you can't remember, set it to the middle before you apply power.

    I would be looking at other things first, most notably the transformer. Is there voltage on the secondary? Depending on your answer move forward or backward until you find the fault.
  3. davelectronic


    Dec 13, 2010

    Hi there.
    Not to put a damper on things, but check the transformers secondary output AC the low voltage side of the transformer, if it a laminated transformer, i expect it is, if there is AC power voltage there you have hope, as Steve said correcting the potentiometer ( preset ) will do the trick, but if there are no AC volts on the transformers secondary windings then you might need to change the transformer, or give up, i say this as transformers often have a thermal fuse buried in the windings during manufacture, if the power supply has been badly treated and over loaded on it power output this thermal fuse might have blown, its very hard for a novice to find it and then replace it, and even then the windings insulation might have been damaged, this can have that distinctive burnt electrical smell. But if there is AC volts on the secondary winding your in luck. Dave. :)
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