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  1. #1
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    Circuit suggestion for an current limited power supply application

    I need to make a 5V 20mA current limited for use with 10kOhm and 20kOhm potentiometer-type devices. Because of the potentiometer low resistance at the ends of its ends, if I use an unlimited power supply, and even I set it as low as a few volts, it can destroy the pot due to heating of the element.
    Since the pot can go as low as a few Ohm, for a good linearity I would like to have a low output impedance as well.

    So the potentiometer should be supplied by around 5V but the current should not exceed 20mA.

    What type of circuit I should begin with?

    - - - Updated - - -

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    The problem is when the wiper passes through a and b it damages the device. What kind of interface between the 5V supply and the pot would limit the current to 20mA?

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  2. #2
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    Re: Circuit suggestion for an current limited power supply application

    What is the pot wiper connected to?
    Zapper
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    Re: Circuit suggestion for an current limited power supply application

    Viper outputs the potentiometer voltage to a DAQ.

    What do you think if I limit it just by a resistor as follows and measure Vout / Vreference:

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    Im just wondering maybe I dont need an active circuitry?



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    Re: Circuit suggestion for an current limited power supply application

    Hi,

    I assume you overcomplicate things.
    But indeed I don't know what you want to achieve.

    Where and how do you expect overcurrent?

    Klaus
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    Re: Circuit suggestion for an current limited power supply application

    If the pot wiper is going to a DAQ, where is all this current going to go that you are worried about?
    I see no path for that current.
    Show us that path.
    Zapper
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  6. #6
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    Re: Circuit suggestion for an current limited power supply application

    Quote Originally Posted by crutschow View Post
    If the pot wiper is going to a DAQ, where is all this current going to go that you are worried about?
    I see no path for that current.
    Show us that path.
    I have this one rail to rail ti.com/lit/ds/symlink/lmc6482.pdf

    The following currnet_source circuit can supply constant current 0.1mA to a 20k poti:

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    And here is the plot load versus current:

    Click image for larger version. 

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    How can I improve this circuit? My aim is to use this constant current source for a remote poti.
    I want the voltage stable and better noise immunity. And most importantly do you think for this application the opmap I use is fine?

    I would be glad for any suggestions.



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    Re: Circuit suggestion for an current limited power supply application

    Hi,

    Instead of a resistive divider use a voltage reference IC like the TLV431 or the TL431, or any by any manufacturer you find with similar or better specs.

    I see no problem with the 646x op amps for a DC (static voltage) current source, they do their job well at low frequencies andd operate up to about 16V. The "better" the op amp, the greater the cost and a Ferrari OA is likely unnecessary here. Op amps are like screwdrivers, don't you think, each serves a purpose, some can cover various needs, some are unsuitable for certain circuits - that's what hateful in-depth and confusing OA datasheet perusals and comparisons are for. Certainly adds new meaning to the expression "No pain, no gain."

    100ppm resistors are not especially expensive and won't eat into your budget noticably, 50ppm and lower may begin to feel a painful consideration and unnecessary I would say.

    The LM334 is one current source IC, and the datasheet is good for tips on reducing temperature variations.

    I'd observe results experimentally with the temperature feature of a simulation tool to compare various current source circuits to see which is closest to your real needs.

    Is this current limiting you are after or a current source/sink?



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