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Turn Your Power Supply into an Ohmmeter - It's Free!

Discussion in 'Electronic Basics' started by Watson A.Name - 'Watt Sun', Jul 16, 2003.

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  1. I got tired of switching the leads of my DMM. Suddenly if dawned on
    me that I can just set the power supply to 10.0V for exaample, and
    read the current, and then divide the voltage by the current to find
    the resistance. Like I put a resistance on the PS, it reads 10.0V and
    the current is .018A, so 10 / .018 gives 555.6 ohms. Must be a 560
    ohm resistor.

    I turned my PS into an ohmmeter - FREE!

    Hee-hee - Work smarter, not harder!

    Of course, make sure the current stays low so the resistance doesn't
    overheat. For low resistances use a volt or less.

    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  2. buck rojerz

    buck rojerz Guest

    Also, you must now consider the combined accuracy, of two meters, instead
    of just one.

    Yes it works, but, so do pliers on a hex-nut. Something about the
    right tool for the job.
     
  3. Congratulations. You proved Ohm's Law works. Of course it's only as accurate
    as your meters, then you have to use your calculator.

    Sorry, but give me a decent digital multimeter.

    Your method is certainly worth remembering in a pinch.

    Mark Z.
     


  4. Yep, that's how ohmmeters work actually. I'm currently designing a digitally
    controlled PSU which includes a dot-matrix LCD that shows a lot of info
    about the state of the PSU. For e.g., it multiplies V * I so the user can
    see the load power in real-time. Simple, but very handy. I suppose I could
    also have it display V / I to show the load resistance in real-time. I think
    the PSU approach is good for measuring very small resistances (when you need
    to generate a lot of current to have a voltage drop large enough to measure
    accurately.

    cheers,
    Costas
     
  5. Al

    Al Guest

    At a local electronics store, there was a sale of multimeters for $5. I
    bought a bunch and have velcroed some to my workbench. I set them for
    voltage, current or resistance and leave them there. Good cheap way of
    doing some quick and dirty measurements. Surprisingly accurate too. If I
    smoke one, I just toss it. After all, it is a toss away world nowadays.

    Al
     
  6. Ratch

    Ratch Guest

  7. Sofie

    Sofie Guest

    Watt Sun:
    Just a little cumbersome.... isn't it??
    As you indicated, there is a risk of smoking low ohm, low wattage resistors
    unless you already know the value... if that is the case then why are you
    measuring it??......
    .....and the accuracy is compromised because you are measuring the voltage
    and then measuring the current..... and you are at the mercy of the
    regulation of your power supply.
    2 meter operations instead of one... the inherent innaccuracy of one of the
    readings is further compromised by the inaccuracy of the 2nd reading.....
    give me a DMM or VOM with a dedicated OHMS function any time..
    Actually, the much more used and handier version of this is the "flip-side"
    where you measure the voltage across a known-value resistor in the circuit
    to determine the approximate current.... most techs do this all the time
    while routinely troubleshooting.
     
  8. Ian Stirling

    Ian Stirling Guest

    This isn't especially usefull usually.
    However, with low ohm resistors, it can be.

    Given a constant current of an amp, the $5 meters mentioned elsewhere
    can now measure with a resolution of .1mohm.
     
  9. Yes, and Ohm's Law describes the interaction of resistance, voltage, and
    current. Not just resistance.
    I'm not an engineer, granted, but I don't require an education on Ohm's Law.

    Mark Z.
     
  10. Ratch

    Ratch Guest

    I belive that you are missing the point. The resistance (or impedance)
    formula V=IR (or V=IZ), describes the describes the interaction of
    resistance (impedance), voltage, and current. While correct and true in all
    cases, those formulas are NOT Ohm's law, and it is wrong to call them that.
    As shown in the second link I gave, Ohm's law is a property of resistive
    linearity in a material. Just as the specific gravity of a material is a
    property. If it conforms to Ohm's law, it is ohmic. Otherwise it is
    nonohmic. Ratch
     
  11. AC/DCdude17

    AC/DCdude17 Guest

    X-No-Archive: Yes



    Dangerous and inaccurate. Inaccurate, because the resistor will warm up
    and the resistance will increase.

    Using a volt meter and ammeter is certainly cheap and practical for
    measuring a very low resistance that the typical DMM can not.

    Say you have a 25ft 12AWG extension cord and you want to know the total
    resistance. The average DMM has a resolution down to 100mOhm. The
    resolution is nowhere near what you need.

    Connect it to ~12V DC(you don't want to use 120V, because high floating
    voltage will throw off accuracy on the low voltage meter range) and
    connect an ammeter in series.

    Connect a load on the other end that takes about a few amps.

    Measure the voltage across both ends of one conductor in mV range.

    If you read 54.6mV and the ammeter reads 2.12A, you can figure out the
    resistance by:


    0.0546/2.12=0.02575

    three sig dig=0.0258ohms=258mOhms x 2(to accomodate for return
    path)=506mOhms
     
  12. Ratch

    Ratch Guest

    I belive that you are missing the point. The resistance (or impedance)
    formula V=IR (or V=IZ), describes the describes the interaction of
    resistance (impedance), voltage, and current. While correct and true in all
    cases, those formulas are NOT Ohm's law, and it is wrong to call them that.
    As shown in the second link I gave, Ohm's law is a property of resistive
    linearity in a material. Just as the specific gravity of a material is a
    property. If it conforms to Ohm's law, it is ohmic. Otherwise it is
    nonohmic. Ratch
     
  13. And both Mark Z and Ratch are barking up the wrong tree. If they were
    to reread my post below, they would see that I used the formula,
    R=V/I, which in both URLs above was shown first and named Ohm's Law.
    Doh.

    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  14. Less cumbersome than removing the DMM leads and then reconnecting
    them.
    Yes, as I indicated.
    If you already know the value, then there'd be no point in measuring
    it. So you would be measuring the unknown value to determine it.
    It still finds the value with a reasonable accuracy.
    The regulation of the power supply makes no difference.
    You're repeating yourself. As I said above, it still finds the value
    with reasonable accuracy.
    If you reread my post, you would see that I already have the DMM. I
    was using an alternate method.

    If the VOM you mention above is an analog wiggle stick meter, it may
    be less accurate - maybe only 3% - than using my PS method.
    Right. Now you've stated something useful.
    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  15. I bought a few of those $5 DMMs from Futurlec a few months ago,
    actually I think they were about $6. 9V vattery included(!)


    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  16. Ratch

    Ratch Guest

    No, both the above URLs make a point of saying that R=V/I is not Ohm's
    law, and the first refers to R=V/I as the resistance formula. In other
    words, Ohm's law referring to V=IR is a misnomer. The second URL points out
    that Ohm's law really and truly refers to the resistive linearity of a
    material. Dah.

    By the way, you did not turn your electrical energy supply into a
    ohmmeter. You applied a method of using the energy supply to determine
    resistance. Ratch
     
  17. Or use a constant current PS and short the far end of the cord, and
    measure across both conductors on the near end.
    Or you can just look up the resistance in a wire table and find that
    12 AWG has 1.59 milliohms per foot. Then multiply by twice the
    cord length.

    As for being "dangerous and inaccurate", electronics experimenting is
    frought with danger, one being 'letting the smoke out.'

    As for "Inaccurate, because the resistor will warm up and the
    resistance will increase", you could have the same problem if you were
    measuring a resistor with a DMM, and the equipmewnt with the resistor
    had been powered on before you did the measurement. The resistor
    could already be hot.

    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  18. This is *still* pointless. I *never* claimed that I was doing
    *anything* with _Ohm's_Law_!! Quit trying to put words in my mouth!
    See my OP below.

    [snip]
    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  19. Chuck Harris

    Chuck Harris Guest

    It doesn't matter what your original post said, the OM found a factoid,
    and wants to show it off!

    -Chuck, WA3UQV
     
  20. Ian Stirling

    Ian Stirling Guest

    Very handy indeed.
    The ones I bought were 3 pounds 99p, ($6us?) I have around 8.
    Soon after buying one, I thought I'd discovered that they have an
    overvoltage LED.
    However, the smell of burning FR4 soon made me realise otherwise.

    It's amazing how many multimeters you can use when you have them free.
    I was just discharging a series string of Li-Ion batteries, to measure
    capacities.
    In the past I would have taken measurements every 5 minutes to ensure
    none had approached 3V.

    Why bother, just hook up 5 of them, and glance over every once in a while.,

    --
    http://inquisitor.i.am/ | mailto: | Ian Stirling.
    ---------------------------+-------------------------+--------------------------
    Lord, grant me the serenity to accept that I cannot change, the
    courage to change what I can, and the wisdom to hide the bodies
    of those I had to kill because they pissed me off. - Random
     
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