Variable resistance tester

Discussion in 'The Projects Forum' started by GUQUIK, Mar 29, 2011.

  1. GUQUIK

    Thread Starter New Member

    Mar 29, 2011
    6
    0
    Hi all. New here and not a tech, hence me joining up to ask for your help please.

    I need to make up a tester that can provide 0-100ohm to test a circuit. I think a pot such as on here would suit. I need it in a box with leads. Preferably changable from probes to aligator clips as needed. And would like to have a digital display of what it is outputing instead of using a multimeter.

    I do have a basic understanding and am happy to make it if someone can guide me a little. But if a product already exists I'm happy to buy one if it's suitable.

    Thanks in advance.
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi

    Would resistance decade box do the job? You can build or buy.
    http://ca.mouser.com/Search/Refine.aspx?Keyword=resistor+decade+box

    It would have at least 3 ten position knobs, one for ten 0.1Ω steps, one for ten 1.0Ω steps, and one for ten 10.0Ω steps. The total would be 99.9Ω. You could add an eleventh position to the first knob to get 100Ω. It is simple for a novice to build and you can read the value from the settings. No batteries required.

    A down-side might be that the resistance change would not be smooth, but in steps with an perhapes an open circuit in between. Would your application except this?

    If you use a pot you can ever know exactly what value it is at unless you measure it. To measure it you must apply a voltage, which I assume must be removed when the resistance is connect to the device being tested.

    How accurate do you need to be? Would two pots ganged together work? One for the test circuit, and one for the readout.

    Regards,
    Ifixit
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    You can buy multiturn pots, typically ten turns wire wound, that can be used with turns counting dials. http://www.bourns.com/pdfs/3549.pdf
    http://www.potentiometers.com/DB10.cfm

    NB I do not particularly recommend the linked products, these are just examples found on the web as illustrations.

    These things are not cheap, and their power rating is a bit limited - what sort of power rating would you require?

    Edit: These are likely to be seriously expensive if you get them new from a professional supplier - possibly more expensive than a low-cost decade box, but you would get smoother adjustment. The cost goes up dramatically if you want better than 5% accuracy.
     
    Last edited: Mar 29, 2011
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Please describe your application.
     
  5. GUQUIK

    Thread Starter New Member

    Mar 29, 2011
    6
    0
    Thanks guys. That is a bit expensive looking at the links above.

    I guess application would make the difference. I need to apply 0-100ohms max. to circuit for fuel sender unit in vehicle. Most guages read 0-90ohm, with lpg 90-0ohm.

    So a tool that can reproduce that is handy so it can be put in place at various points along the loom and tested at various points too. Can also test the guage itself obviously.

    The reason the digital display is handy too, is I can dial in various numbers to suit vehicle and watch multimeter at other end without the need to have it at both ends. Just nice to have, read lazy.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    The simplest method is to plot a resistance arc (using an Ohmmeter) and stencil it directly under the pot knob. That's how we did it back in the old days using dry transfer lettering. A linear taper pot is best for this application. ;)

    If you post a schematic we can possibly give you a more elegant solution.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I forgot to ask.. Do I understand it correctly that you're testing fuel gages and not the sending units, thus the pot is simulating the sending unit?

    EDIT: I just realized that if your sending unit is wired like I remember it on my boat it's opposite of how I drew it here. In other words the sending unit went to ground and the fuel gage+ went to the battery. I won't be able to change that unit I get back from the pub... It's Wednesday!
     
    Last edited: Mar 30, 2011
  8. GUQUIK

    Thread Starter New Member

    Mar 29, 2011
    6
    0
    Mmm, pub.

    Yes it is the guage I want to test. Sender is just done with multimeter. Just want to ad resistance along loom to test what comes out at guage and other points.

    Sender is earthed via earth wire to it internaly and also an additional one externaly.
     
  9. debe

    Well-Known Member

    Sep 21, 2010
    947
    184
    Had a tester made up for fuel gauges & temp gauges in my tool box. It was a Wire wound 200ohm pot & ijust made a scale for it. Simmilar to the one in the pic.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
  11. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    One of the basic tenants of using an Ohmmeter is to never attempt to measure a resistance in a live circuit. This is because an Ohmmeter circuit (whether analog or digital meter) is basically an Ammeter with it's own voltage source. If the meter is analog the current through Rx is supplied by the internal battery and the resultant current is converted to an Ohms scale which is non linear. Likewise a digital Ohmmeter supplies its own voltage and will also measure the current the same way, providing a digital read out through an (ADC) Analog To Digital Converter.

    Since resistance is defined as E/I (voltage across Rx divided by the current through Rx) it would seem logical that an OpAmp circuit could be employed to give you a scaled reading on a DVM, where Vx=Rx.

    Quite frankly we have OpAmp people here that are better versed with their use than I am. I'm surprised non of them have popped in here. This topic a refreshing change from endless 555 circuits, so I'm surprised that there hasn't been more interest in it.

    I've attached a conceptual diagram that converts Rx to Volts. Rx= (Vo x 10). It would nice to a have a direct reading like 100mV = 100 Ohms but working with millivolts would introduce its own issues. My diagram uses a 1Ω resistor as a current shunt.

    For certain, this topic interest me and I will be researching my conceptual diagram and will post a circuit as soon as I can. Now that I've thrown out the hook I fully expect one of our OpAmp wizards to bite before I get it done. ;)
     
  12. GUQUIK

    Thread Starter New Member

    Mar 29, 2011
    6
    0
    Looks the goods to me guys. Is there an easy way to use a larger box and add a digital readout to it?

    Would be real nice to hook it up to, say point A, and have multimeter at point B. Then point C etc. That way I can simply read what tester says and read what multimeter says without moving tester along. So would test guage but also the entire loom/circuit.

    All make sense?
     
    Last edited: Apr 1, 2011
  13. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Did you read my last most or am I talking to myself? I clearly addressed digital readout. BTW, do you mean "loop"?
     
  14. GUQUIK

    Thread Starter New Member

    Mar 29, 2011
    6
    0
    Sorry but as I stated in my first post.......

    And "loom" as in "wiring loom". I deliberately stated loom so people could picture the fact that while the tester may be at one end of vehicle, the testing could easily be done at the other end of vehicle, potentialy 6 meters away or 19.68498 feet.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A digital display would add a great deal to the cost, and decrease the reliability, with a minimum of benefit.

    I like the idea of just using a 100-200 Ohm pot, and use a large knob with a pointer, and make up a scale under the knob so you can have an idea what the resistance is. Increments of 5 Ohms should be good enough.
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    'Wiring Harness' would be more apropos. ;)

    Anyway, I don't want to spend any time on the OpAmp/DVM concept that I posted unless I'm certain that you have the skills to solder up a perf-board using OpAmps. The 'Pot In The Box' with a directly marked scale is reliable and fairly idiot proof. Are you sure that you need more than that?
     
  17. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The thing is, adding a resistance meter will cause the meter to go offscale when it's connected to the auto/truck, as current will be flowing through it.

    If you set it up for voltage instead, that might be useful - but you would not know what the resistance was.
     
  18. GUQUIK

    Thread Starter New Member

    Mar 29, 2011
    6
    0
    Can post diagrams if needed. Give me a little bit to sort them from manual.

    In Australia we call the harness a loom. ;) But either way I know what is meant by either. Tomato - tomarto (pronunciation not spelling)

    Idiot proof is fine but was happy to make tester if nothing adequate is available. A little support from here and I could build anything.:cool:

    Not quite the range I was after but what about this
    http://www.jaycar.com.au/productView.asp?ID=RR0700&keywords=RR-0700&form=KEYWORD
     
  19. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    That product uses 1/4W resistors (too low for you application) and the range to 1MΩ is not expanded enough for you. Be patient and you'll get your digital version.
     
  20. RFactor

    Active Member

    May 1, 2009
    33
    3
    It is my understanding that older fuel gauges used a rheostat in the tank and an analog current meter as the gauge. The problem with this type setup was that the gauge will fluctuate due to fuel sloshing in the tank, baffles will reduce this somewhat but I think that most modern fuel gauges are stepping motors controlled by the ECU. Another type of gauge uses a bi-metalic heater in the gauge, the time constant of the heater will damp the needle to reduce fluctuations.
    I assume we are talking about automobiles here but GUQUICK didn't actually say that.
     
Loading...