potentiometer burnout

Discussion in 'General Electronics Chat' started by Synaps3, Nov 20, 2013.

  1. Synaps3

    Thread Starter Member

    Jun 5, 2013
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    I bought a circuit that I'm trying to modify a bit. One part of the circuit had a small trimpot on the board. I wanted to replace this with something easier to use, so I bought some 1/2W 50k potentiometers. No matter how I solder them up, they begin to burn out passed half way. I understand that the first pin is ground and the other two are power. Not sure if it matters which way pin two and three are attached. It seems to work fine as long as I don't go past about half way. Could it be that I need a higher power potentiometer? I don't think so though because the original was a small multiturn one. Do those go above 1/2W anyway?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,123
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    What was the resistance of the original part? There are various ways to wire a pot. Perhaps a photo would help.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,547
    2,373
    It sound to me as if you have the pot slider attached to one outer connection?
    You will reach a point where the supply to the pot is shorted.
    Max.
     
  4. #12

    Expert

    Nov 30, 2010
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    I think you have a problem in the circuit and the pot is just the victim. You will have to post the circuit here before anyone can do better than guess at an answer.
     
  5. BobTPH

    Active Member

    Jun 5, 2013
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    You understand wrong.

    Bob
     
  6. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    I don't understand what you mean by "one outer connection".

    Well, I tried reversing the input and output pin this time and same result. I think I've tried every possible connection now. One way, it just blew out as soon as I turned it on, so I got another one. All other ways began to burn out at about half way.

    Not sure I do. It doesn't make sense to me that it would matter which way the current flows through the pot as long as it's flowing through the right connections. I tried it anyway like I said and same result.

    My only guess now is that the pot isn't a high enough wattage. The circuit I'm working with is a 1W amp and this pot controls the power output of the circuit from 0 to 1W. The pot I have is rated at 1/2W so maybe it's not powerful enough? Not sure.

    It's a premade circuit, so I doubt there is anything wrong with it. I even contacted the guy that built it and he's not sure whats wrong.

    This is what I replaced:
    http://4.bp.blogspot.com/-5uXxf4agW2I/T6jxYMbK32I/AAAAAAAAAiE/XfvaAW0QgcI/s1600/trimpot1.jpg

    This is what I replaced with:
    http://static.atomsindustries.com/assets/images/items/1046/1046.jpg

    I mean, just based on the size, you'd think the replacement would have an ample power rating.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,770
    970
    The bourns is a 1/2W pot.. so your guess is incorrect..

    Whats the resistance range of the old and new?

    Without seeing how you connected it and a schematic, your on your own now..
     
  8. tshuck

    Well-Known Member

    Oct 18, 2012
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    It sounds like, from your misconception of the potentiometer pin out, you've attached both one of the pins on the outside and the middle pin to the supply voltage, with the last pin connected to ground. If that's the case, it is no surprise your pots are burning out - you connect a 50k Ohm load across the supply and then decrease the resistance.

    You aren't trying the same smoked pot, are/have you?
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
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    Post a schematic; hand drawn is okay. You will get a proper answer very quickly.

    Or post a link to the circuit that you bought, and tell how you want to modify it.
     
  10. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    Ah ok, so your saying I need a 50k resistor in addition to the pot and I need to wire it between the input and output pins? I don't see how the original pot worked without that then. No, I am not trying the smoked pot nor have I :rolleyes:. I went through four of these so far. The last one burnt for a second, but it still seems to work ok, so I'm going to use that until it breaks completely because I'm running out.
     
  11. tshuck

    Well-Known Member

    Oct 18, 2012
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    The middle pin is attached to a wiper that contacts the 'resistor' inside, which, for your application, would probably be used to provide a voltage to whatever is connected on that pin between Vcc and ground - this device likely controls the output given this voltage. Between the two outer pins is a constant 50k Ohm resistor.

    As per the others' request, a picture of your application and/or a schematic would help us help you...
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A pot doesn't have an input and an output; it has three terminals and can be set up in several different ways, one of which is as a rheostat, which sounds like it may be what you want. But a 1/2 watt pot cannot function as a high power rheostat. As you are finding out, they will overheat and be ruined.

    Why not post a schematic and stop making smoke?
     
  13. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
  14. #12

    Expert

    Nov 30, 2010
    16,294
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    OMG. This thing is pure RF and has a butt load of trimmers. I'm out.
     
  15. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    I only need to replace one.
     
  16. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    What are the specifications of the pot you are using as a replacement? If it's a one turn pot, that will be a problem. It will also be a problem adding wires to an RF transmitter circuit.

    Somehow, I am guessing you are trying to increase the RF output. Yes?
     
  17. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    50K 1/2W 1 turn. The wires that carry RF are all coax type 50ohm. The pot itself shouldn't have any RF going through it (I think); it just varies the power to the transistor, so normal wire is fine here. I'm not trying to increase the power more than what the circuit was designed for. I'm just trying to replace the original pot with one I can mount on a case.
     
  18. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Can you post a closeup of the side of the board that the pot is not on?
     
  19. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
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    I'm going to try what tshuck suggested first.
     
  20. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    Keep on wiring pots the way you are doing and you are going to burn out every pot regardless of the wattage. And every burned out pot is now garbage.

    As the doctor says, if it hurts when you do that then stop doing it.

    Tell us what the unit is. Show us some photos. Better yet if you can provide a circuit diagram.

    Edit: Sorry, I didn't notice the photo. Can you show us a photo of the bottom side?
     
    Last edited: Nov 20, 2013
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