Potentiometer Wiper Current

Discussion in 'General Electronics Chat' started by AutoNub, Oct 14, 2011.

  1. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    Hello! I just have a quick question about the wiper current of a potentiometer. I've never actually had to calculate wiper current before because I've never used a potentiometer in a current adjustment configuration in conjunction with a power supply before.

    Anyway, if someone could provide me with formulas or show me the calculations for showing current flow (directions and magnitudes), it'd be highly appreciated.

    (See attached image for wiring diagram.)


    Thanks!


    Edit: The voltage maximum is 15v and the current maximum is 52A.
     
    Last edited: Oct 14, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, not exactly certain what you're doing here, but I threw together a simulation on Falstad's interactive SPICE simulator:
    http://www.falstad.com/circuit/#$+1...+0.5 w+352+128+400+128+0 g+400+176+400+192+0

    You need to have Java installed to run the simulation.

    You have two VR1's in your diagram; that defeats the purpose of having reference designators - they should be unique. For example, the left one should be VR2 and the right should be VR1, or vice versa.

    The only path that might support current up to 52A would be through the two pots if they were turned down all the way; but just about any pot that you can buy nowadays would burn up with more than a few mA current through it.

    What is it that you're trying to do?
     
    Last edited: Oct 14, 2011
  3. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    I copied and pasted the first variable resistor to save time, but forgot to change its name to VR2. Sorry for the mistake. To clarify, let's call the 100 Ohm variable resistor VR2. I'd edit my original post but it seems the option is unavailable.

    The current adjustment configuration circuit adjusts the voltage to control the current. See the attachment for more details. At the bottom right of the second page, the default basic current adjustment configuration is shown. My aforementioned configuration is a custom alteration based on what is shown here (to allow for a fine and course adjust--which has been verified to work properly in lab testing). I just want to know how to determine the theoretical currents to each pin (pins 2, 6 and 7). I could just go out and measure them but I want to know the math involved.

    On a side note, is all of the SPICE software free? If so, I'd love to download it and give it a try. I currently don't even have a circuit design software like Circuit Maker or Multisim, so it'd be good to get something. My original image was created quickly in AutoCAD Electrical.
     
    Last edited: Oct 14, 2011
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Kilowatt-rated variable resistors do exist (I have used them myself, but at much higher voltages and rated just a few amps). One can sometimes get them configured as true pots. with two ends and a rotary or linear slider, even though they are often loosely called "rheostats". Sometimes they incorporate motor drives for automation, or because adjusting them is a bit tough for human muscles!

    These are hardly the latest technology however, and naturally consume a lot of power and generate lots of heat. Is there no less wasteful alternative?

    As for free Spice, you can get freebies like LTSpice, but things like PSpice will cost you plenty.
     
  5. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    We are getting off topic here. Let's forget about whether the pots are rated properly to handle the current or voltage. The only thing I care about is learning how to calculate the current, whatever it may be, in the configuration I showed in my original post.

    If it helps, let's imagine I never mentioned 15V and 52A... Pretend I said more satisfying numbers, like 1v and 20mA.... I just want to know the math formulas and procedures for calculating. I understand basic circuit analysis with regular resistors, but the potentiometers, especially with the wiper going off into a different pin of the power supply, has me a little confused when attempting to calculate the theoretical currents and directions of flow. Would any current at all be going through the wiper to pin 7, for example? If so, how much current would split off to pin 7 and how much would continue on to pin 2?

    Thanks for the replies.
     
  6. SgtWookie

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    Jul 17, 2007
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    Ahh, OK then!
    Kirchoff's Law still applies:
    http://www.allaboutcircuits.com/vol_1/chpt_6/1.html

    You can think of a single pot as two resistors that are both variable; the sum of which is constant.

    For example, if you have a 100 Ohm potentiometer, and the wiper is set to midpoint, the resistance from the high side to the wiper will be 50 Ohms, and from the wiper to the low side will also be 50 Ohms. The resistance from one end to the other doesn't change.

    If the high side resistor were called R1, the low side R2, and Wiper_Position a number between 0 and 1, then you can say:
    R1 = Rtotal * Wiper_Position
    R2 = Rtotal * (1 - Wiper_Position)
     
    Last edited: Oct 14, 2011
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  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The answers to those questions would depend on the currents, voltages, and impedances presented by the external circuit to the terminals of the network. You have given no information about this. We may guess VRef(+) is a fixed voltage level, but what is Ipgrm? Is it actually a current, or is it an adjustable voltage, perhaps determining the set-point for a current control loop?

    The level of current will depend on external conditions, as explained, but the division of this current can be found by treating the pots as pairs of resistors: A pot of R ohms at wiper position A% splits into two resistors, one of R*(A/100)Ω, the other of R*(1-A/100)Ω

    Edit: Beaten again...
     
  8. crutschow

    Expert

    Mar 14, 2008
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    In general, to calculate the wiper current you first calculate the equivalent resistance at the wiper based upon the wiper position which gives R1 and R2 as shown by SgtRookie.

    This equivalent wiper resistance is then Req =(R1*R2)/(R1+R2).

    Then you calculate the voltage at the wiper as Vw = Vs*R2/(R1+R2) where Vs is the voltage across the potentiometer.

    Finally you can calculate the wiper current as Iw = Vw/(Req + Rl) where Rl is the wiper load to ground.
     
  9. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    I believe SgtWookie answered all of my question perfectly in his last reply. SgtWookie, assuming you have AutoCAD or some other software capable of opening this file type, do you agree with the attached information? Thanks again everyone! SgtWookie, you are a brilliant man, my friend!
     
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  10. SgtWookie

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    Jul 17, 2007
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    No, I don't have AutoCad, DWG TrueView, nor PaintShopPro installed - I would seldom use any of them even if I had them.

    It's best to post images in .PNG format; they are very compact, not "lossy" like .jpg files, and require no software to view; one doesn't even have to download them.

    We tend towards the lowest common denominator on here; everyone who can get here must have a web browser, and that's all one needs to view a .png format file.

    For simulation, I usually use LTSpice; a very good and free PSPICE program available for download.

    When I draw a schematic for someone, I'll post a screen shot of the simulation, and frequently include the source file so that they can try modifications and simulate it themselves.

    I also use Circuitmaker, but it was discontinued a decade ago. You can still find the Student version available for download if you look for it.

    It's much easier to use tools that were designed specifically for the task of electronic schematic generation and simulation than to use a general purpose CAD tool - besides, you can actually test the results of your circuit construction electrically.
     
  11. CraigHB

    Member

    Aug 12, 2011
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    I actually a have an inexpensive AutoCAD program called TurboCAD. I use it a lot for different things. It really is a PITA to draw things with something like MSPaint. So much easier with a proper drawing program.

    Isn't JPG pretty much the defacto standard? I agree that PNG is much more compact, but generally, when posting images on the net, I rarely need more than a hundred KB for any image with JPG. Seems good enough to me.

    I just love things that are free (who doesn't) and when they work as good as, if not better, than something you have to pay for, what's not to love. LTSpice is definitely one of those programs. Another one I tried recently that I think is great is gerbv, the Windows version of the gerber viewer from gEDA.

    I actually still draw circuits in CAD, but yea, I need to find a good schematic editor I like. It's kind of kludgy doing it in CAD. The schematic generator in LTSpice is nothing to get excited about and I don't much like the one in Eagle. I only use them when I have to. When I'm drawing things out for the first time, I often do it in my CAD program.
     
  12. SgtWookie

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    Jul 17, 2007
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    If I need a CAD program, I'll just use the free Google Sketchup I installed. I've had no reason to use it as of yet. I usually just use MSPaint to crop/save screen dumps, or to touch up other people's images if they've posted in .png format and need some changes made. It's not a big deal for me.

    .JPG format files are great for photos, but stink for line graphics. Contrary to your assertion, .png format files will be large for photo-type images, or where lots of colors are involved. However, they are not "lossy" like .jpg files are. Artifacts from the lossy compression used are very visible for line graphics on even the first pass; and legibility generally goes right out the window if you try to edit/save a .jpg file a 2nd time. .png files don't have that problem, so work far better for schematic diagrams (which are basically line graphics).

    To each their own. I'll start out in either LTSpice or Circuitmaker (the latter only if I need logic IC's or certain other components that I don't have loaded into LTSpice) because I'm generally going to run a simulation on it.

    Then if I'm going to take it to a board, I'll re-do the schematic in Cadsofts' Eagle. I'd rather be able to just import the LTSpice schematic directly into Cadsoft Eagle, but haven't gone to the trouble of writing a conversion utility for it.

    If I were going to be publishing a book, I might consider using some other schematic drawing program, but the tools I use work well for what I need them for.
     
  13. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    Thanks for the replies! I used JPG format because I thought it was the standard. Anyway, I'll use PNG from now on.

    I'll attach a picture of the CAD drawing I put together. It isn't anything special--just a very quick guide for my mechanical engineer co-workers who might need to do some quick calculations. The drawing I put together isn't even a full schematic, but just a couple potentiometers in a current adjustment configuration for a power supply. I understand converting from actual to virtual as shown in the image isn't necessary for calculations, but it helps to conceptualize.

    If I want to use PSPICE for free, which version would you recommend? I noticed there is a student version. Would this be the best option?
     
    Last edited: Oct 17, 2011
  14. crutschow

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    Mar 14, 2008
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    I don't believe there is any legal, free version of PSPICE. The student version should work fine, if you can get it.

    A good, free version of Spice is LTspice from Linear Technology, which many use on these forums.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    I've been using LTSpice for several years now, and once you get used to it, it's very handy and powerful. As with everything, there is a learning curve.

    I briefly tried the student edition of PSpice, but found it rather unwieldy. I didn't have the time to learn a whole new package when the one I've been using works well enough for most things.
     
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