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Potentiometer cross-reference?

Discussion in 'Troubleshooting and Repair' started by SkyValleyRacer, Aug 19, 2011.

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  1. SkyValleyRacer


    Aug 18, 2011
    Rotary variable resistor 5% vs. 10%?

    * * * Please note: I meant variable resistor, not pot * * *

    I recently purchased a used Snap On brand timing light with advance. This feature allows you to pre-set the desired ignition timing degrees. Well when I went to use it, it would not work (read "flash") Long story short, I found the variable resistor for the advance would only allow the light to flash if you wiggled it.

    I removed the variable resistor, it is a Bourns 3852C-252-502H. I have searched online for this with no success. I did however, locate a datasheet that deciphered the part number. The -502 refers to "5k" and the H is for ±5%. I was able to find 3852C-282-502A. The A means ±10% Otherwise, all else is the same. My question is, if I use a 5k resistor with ±10% accuracy over the original ±5%, will this cause a huge discrepancy in ignition timing degrees? Please help, oh wise men! (and wise women)
    Last edited: Aug 19, 2011
  2. daddles


    Jun 10, 2011
    Without seeing a manual/schematic, it's impossible to tell, but I wouldn't recommend substituting a 5k resistor for the pot. I'm guessing the pot was used to set advanced and retarded timing values and substituting a resistor might "peg" the advance/retard value at one end of its range.

    You should post a picture of the pot and how it is connected in the circuit. Are all three terminals used or just two?

    If it were me, based on what I know at this point, I would look for a 5k pot I could substitute. The pot is probably a linear taper, but you should measure the old one if possible to give credence to this assumption.
  3. SkyValleyRacer


    Aug 18, 2011
    See, this is why I'm here, exactly the kind of input I'm seeking. I believe I've mislead you all a bit, with my terminology. I guess the correct term for the bad part is "variable resistor". Both the original part and the one I'm asking about subbing for it, are both 5k. Only difference is the original is ±5%, while the proposed replacement is at ±10%. BUT - both are 5k and variable resistors. Oh and there are only 2 wires; a yellow one, and a black one that goes across 2 solder tabs (?)
  4. daddles


    Jun 10, 2011
    OK, gotcha. The common terminology is a "potentiometer", virtually always shortened to "pot". "Variable resistor" also gets the idea across, but isn't common nomenclature, at least in my experience in US industry. "Resistor" by itself implies a fixed value.

    I'm assuming that the knob that turns the pot points to calibration marks on the case (probably in degrees advanced or retarded). For these calibration marks to still be relevant, you'd want the same pot behavior. You might (I repeat, might) not get this behavior with a pot that has a larger specification range. If you could measure the existing pot and then go to a store and select a suitable one, then you'd probably be in good shape. But that's usually an exception, especially if the old pot doesn't work well enough to characterize it (if it did, you'd still be using it!).

    It's probably going to be a crapshoot no matter what -- even if you got an identically-spec'd replacement pot, it might not work quite the same as the original. Thus, I'd suggest going ahead and trying a replacement pot. Note you could go here (e.g.) and buy a handful of 5k pots and use the one that works best, as they're pretty cheap.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It will almost certainly be fine. It is impossible to tell without viewing a schematic, but I wouldn't be worried.
  6. KJ6EAD


    Aug 13, 2011
    I can't see a problem with your proposed substitution. 3852C-252-502H isn't listed as obsolete, it just isn't listed. 3852C-282-502A differs in the precision you mentioned and potentially there is some difference in the bushing (25 vs 28). 252%22

    You could ask them.
  7. davelectronic


    Dec 13, 2010
    Horizontal preset

    Hi, my understanding of the termanolagy is horizontal or vertical preset potentiometer, pot for short, any despite the answers you have so far, i think a diferencial of 5% could pose a problem, having said that it might not, engine timming is fairly critical, hence the close tolerance component, i think its a try it and see thing, or use the correct tolerance preset potentiometer. Dave.
  8. daddles


    Jun 10, 2011
    So you put the new pot in and start using the timing light. How will you know that all the settings you make with it don't have a bias of, say, -3 degrees? To be able to check this, you'll probably require a scope or counter and some knowledge of the circuit's behavior or compare the measurements to a known-good timing light.
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