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Limited Resistance Range on 500ohm Pot

Discussion in 'General Electronics Discussion' started by tdog, Mar 20, 2013.

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  1. tdog

    tdog

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    Mar 20, 2013
    I am building an Astrophotography Tracking Platform based off of this design.

    http://www.garyseronik.com/?q=node/52

    The circuit, at the bottom of the link above, looked simple enough for me to recreate. I have some limited electronics knowledge and have definitely done my fair share of Google searches in trying to learn how to do this.

    The whole point of this box is to create a steady 3v for a DC motor attached to a gear. I wasn't quite sure how the voltage regulator worked, or how to wire the pot so I rigged everything together with some wire just to make sure that I had everything right before I created a PCB to permanently mount everything. With everything rigged up and with both leads connected to my multimeter I was able to get a very steady 3.000v from the circuit. I had a little concern at this point because I was very close to one of the limits on the pot to reach the 3v, as I turned the pot to the right the voltage steadily decreased from about 5.3v down to 2.89v.

    When I made the PCB and mounted everything inside the box I decided to test everything again to make sure I still had the 3v I needed. This time around though I couldn't get the voltage to drop below 3.2v. I checked the resistance that my pot was producing and it only had a range of about 500 to 234ohms. I found this table http://www.reuk.co.uk/LM317-Voltage-Calculator.htm that shows that I need the pot to produce 210ohms of resistance for the voltage regulator to output 3v.

    It's my understanding that a 500ohm pot should have a range of 0 to 500ohms, which leads to my confusion about why I can't get the resistance down to the 210ohms that I need for the regulator.

    The easiest solution, I think, would be to buy another pot, but I want to make sure it will do what I need before I spend another $10. I would appreciate any help that I can get.

    Thanks,
    T-Dog
     

    Attached Files:

  2. davenn

    davenn Moderator

    13,798
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    Sep 5, 2009
    Hi tdog
    welcome to the forums :)

    yes you should be able to go to 0 Ohms :)
    firstly, how did you wire the pot ?
    I cant quite tell from that pic which terminal is which as far as wiper and track end terminals go

    Dave
     
    Last edited: Mar 20, 2013
  3. duke37

    duke37

    5,364
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    Jan 9, 2011
    The potentiometer is a multiturn type, it looks as if it does not go all the way to the end.
    You could use the other end of the pot but then the control would work in reverse.
    You could put a resistor in parallel to the pot to bring the resistance down.
     
  4. tdog

    tdog

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    Mar 20, 2013
    It is a multiturn pot. Right now I have the 1 terminal wired to the voltage regulator and the 2 terminal going to ground. The 3 terminal is not attached to anything (would that be called "open"?).

    I am assuming that the 2 terminal will always go to ground and that by switching you mean switching 1 to 3. That should solve my problem and also answers my question about why the resistance would go down when I thought I would be turning the knob in a positive (clockwise) direction (turning the knob clockwise currently makes the motor run slower, not faster).

    I will try switching the wires tonight and post back with my results.

    Thanks so much.
    T-Dog
     
  5. tdog

    tdog

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    Mar 20, 2013
    I moved the wire that went from the 1 terminal to the 3 terminal and it appears that my problems have only gotten worse!

    Using the LM317 Calculator I have estimated the resistance range of the pot to be from 338 to 604ohms. The control does now work in reverse like mentioned above(turning the pot clockwise makes the resistance and voltage go up).

    I'm not sure why there is added resistance between the battery and the voltage regulator. Could it be the PCB? Or maybe a poor soldering job?

    I'm open to any suggestions. Thanks again.

    T-Dog
     
  6. tdog

    tdog

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    0
    Mar 20, 2013
    Ok. So I checked the resistance on the pot with my multimeter and it appears that the pot is working as it should.. it has a low end of 0 and a high end of 494ohms.. I even adjusted it so that with either terminal it produced a steady 210ohms.

    I also checked the 150ohm resistor and that as well is at 148ohms. I can't seem to figure out what the problem is..
     
  7. BobK

    BobK

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    1,688
    Jan 5, 2010
    Check your wiring. Something is likely wrong. Check that you have used the pins correctly on the regulator. Use a multitmeter on continuity check to check that all points that should be connected together are, and none that should not be are.

    Bob
     
  8. tdog

    tdog

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    Mar 20, 2013
    Once I determined that it wasn't the pot I started to play around with the whole thing last night.

    I figured that since the regulator must be getting too much resistance I decided to see what would happen if I jumped a wire across the resistor, giving the circuit an easier path to the regulator. I took a small piece of wire and attached it to both leads of the resistor and BAM the whole thing started to work normally. I was able to produce 3.000v from the regulator.

    I don't know why this is though. Another thing I noticed was that the voltage output of the circuit would slowly tick down by a thousandth of a volt about every second.. at first I thought this was because the battery was being depleted slowly with the whole thing energized, but then I thought that the purpose of the regulator was to produce a steady voltage based on resistance, regardless of the input.. so as long as I have at least 5ish volts, the circuit should be able to produce 3v.. I checked the battery and it was showing about 8.9v..

    Can anyone explain why the voltage is tapering off? Is that how it is designed to work with a battery?

    Thanks again,
    T-Dog
     
  9. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    What resistor did you short out? I doubt that you have fixed the problem. You may have just created a voltage divider bypassing the regulator.

    Bob
     
  10. tdog

    tdog

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    Mar 20, 2013
    The resistor that I shorted out is the 150ohm resistor labeled R2 on the diagram below.

    How would I know if I am bypassing the regulator?
     

    Attached Files:

  11. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    The circuit is correct with R2 in. There is something else wrong. R1 and R2 form a voltage divider and the regulator will regulate the voltage at their junction at 1.2V.

    I still say you should check all your connections. Also remove the motor and see if that make a differenence. Maybe your battery cannot supply enough current for the motor.

    Bob
     
  12. tdog

    tdog

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    Mar 20, 2013
    Bob,

    You were right! I pulled the board out and checked all of the connections with my multimeter and the adjustment pin from the regulator didn't have continuity with the R2 resistor.

    I cut the wire off that was jumping across the R2 resistor and then re-soldered the connection on the regulator. Everything appears to be working as it should but the voltage still tapers off slowly once it's been adjusted.

    I am checking the voltage while the wires are not connected to the motor, so I don't think it's an issue of current.. I am using a cheapo drug-store battery that I snagged from a junk drawer so maybe that's the problem. The battery still reads around 8.75v right now though..

    I've been all over Google trying to see if there is some sort of issue with running the LM317T voltage regulator with a battery but haven't had any luck finding answers. I need a very steady 3v to run the motor at exactly 4rpm for about a half hour at a time. I am not sure how the slow tapering off will effect overall motor performance or how that will effect the tracking capabilities of the platform I'm trying to make.

    I really appreciate all of your help.

    Thanks again,
    T-Dog
     

    Attached Files:

  13. davenn

    davenn Moderator

    13,798
    1,939
    Sep 5, 2009
    I would suggest that your voltage drop off is definately due to your battery
    you really really shouldnt be using a small 9V battery in that circuit.
    It doesnt have the power to drive the regulator and the motor. The regulator has losses as does the motor when in operation.
    Those 9V batteries are only good for ~ 330mA maximum and not for a long period of time at that current drain.

    That cct really needs to be powered off a 12V 7.2AH sealled lead acid / Gelcell type battery that will give you plently of time using the camera with long exposures.

    cheers
    Dave
     
  14. BobK

    BobK

    7,682
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    Jan 5, 2010
    Glad to hear that you got it working. I second Dave on the 9V battery. What is the current draw of the motor? Even something like 4 AA NiMH batteries would be better, and rechargeable.

    Bob
     
  15. tdog

    tdog

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    Mar 20, 2013
    Thanks for the advice. I looked up some 12v 7.2ah batteries but they seem to be pretty bulky. I was hoping for something a little more lightweight, or even better, something that would fit inside the project box.

    The four AA NiMH batteries seem like it might fit but my only concern is that they would only produce 6v right? Everything I've read about the regulator says that you need at least 2-3v more input than output to account for dropoff.. so the 6v from the batteries would be cutting it close to my desired 3v output.. It looks like I could squeeze 6 batteries (9v?) in there.. so maybe that's what I'll do.

    Thanks for all of the help, I don't know how long it would have taken me to figure this thing out without everyone's help. I will have to check the current draw on the motor later tonight or tomorrow.

    T-Dog
     
  16. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Actaully, NiMH batteries are only 1.2V so it would take 5 min, 6 to be safe. You still haven't said how much current the motor draws. That is all important in choosing a battery.

    Bob
     
  17. tdog

    tdog

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    Mar 20, 2013
    I think I'm having trouble measuring the current properly. I keep getting only 0.06mA.. I looked up the specs for the motor from the website I bought it from and it says that it's no load current draw should be 6mA.
     
  18. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Yes, I checked the specs and it does seem to be 6mA. If that is the case, a 9V would be fine to run it.

    Bob
     
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