Trimmer heating up

Discussion in 'The Projects Forum' started by paulepc, Apr 30, 2014.

  1. paulepc

    Thread Starter Member

    Apr 2, 2014
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    I have a variable .1 to .3 dc voltage into my op amp and need +/- 10 volt out so at .2 volts in I need zero out. this circuit does work, but the trimmer gets very hot.

    I am not drawing much current, so why is the trimmer getting so warm?

    Power supply +/- 12 volt 5 AMP

    Thanks
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could have nearly 24V across the trimmer, which would drive 24mA through it. That means the trimmer would dissipate ~576mW of power; so it's not surprising it gets hot! Use a higher value trimmer, e.g. 10k, 20k or 50k.
     
  3. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Thank you. I used a 1K to get finer control of my centre voltage I'll change it to 20K

    Paul
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The way you are using the trimmer to tap off the opamp output will give you the same degree of control regardless of its value. To get finer control you would normally connect a given resistance trimmer in series with a higher resistance fixed resistor; or else use a multi-turn trimmer.
     
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  5. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Excellent! Thanks Alec. will modify now.

    Paul
     
  6. BobTPH

    Active Member

    Jun 5, 2013
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    Shouldn't the trimmer be connected to ground instead of -12V? That would help. But I agree, you should use a 10K trimmer. 1K is probably too low for the op amp output.

    Bob
     
  7. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Cant use ground as I need to get -12 Volts. Thanks.
    Paul
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The wiper needs to swing between +0.91V and -0.91V. This means that the pot must be at least 25% of the total resistance between amp 1's max output and -12V.

    Also, you now are pulling 15.3 mA from amp 1's output worst case.

    You can get greater adjustability, reduced stress on amp1, and lower pot current all by adding 2K to 3K in series with the low side of the pot.

    ak
     
  9. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Low side? you mean between the -12 and the pot. I have put in 15K but can't get the centre volts. so thats where you get the 25% from am I on the right track?
    Paul
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Yes, between the -12V and the pot. But lets back up a bit.

    Is the pot a calibration adjustment or an operational adjustment? That is, is it set once to map the 0.1 - 0.3V input to +10V - +10V output, or is it routinely adjusted as part of the use of the circuit?

    If the former, then I'm not sure howthis thing works. You need a DC gain of 100 and an input offset of -0.2V, but you have an amp gain of 121. So are you trying to get both gain and offset tweaks in one device?

    ak
     
  11. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Ok Thanks.
    It is a set once and forget, I hope! I am using LM324N Quad Amp but using just 2 out of the 4 at present. I don't mind adding to the circuit to get it done properly, If you thick I need another stage I am happy to do that. I see you worked out the maths, I have not long started with electronics so finding my way, always open to advice.
    Paul
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could provide the necessary gain and offset with a single opamp, like this:
     
  13. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Thanks Alec. I'll make up a circuit and give it a go.

    Paul
     
  14. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Alec. On the circuit you have shown. My input has chaneged to 1.4 to 3.2 volts what needs changing in the circuit.

    Paul
    Cymru!
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Are you sure you have R3=1k?
     
  16. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Sorry Alec, I have confused the issue. What I was trying to say was that originally I was out putting .2 volts mid range. I have changed the software and I am now getting 1.4 to 3.2.
    this is feeding into your circuit I have just completed, but due to the higher input I was asking what I need to change to cope with the new voltage I am generating.
    Thanks Paul
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Try these values:
     
  18. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Thanks, will try today.
     
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