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changing my VMeter from 0.25A to 25A

Discussion in 'Electronic Basics' started by Hadry LittleWood, Sep 8, 2004.

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  1. hi

    I have a volt meter which can reads DC current up to 0.250A
    How do I make it able to read up to 25A

    maybe just adding some resistors?

    thanks

    jif
     
  2. Gene

    Gene Guest

    Use a shunt resistor in series with the circuit, then measure the voltage
    across the shunt and by applying Ohm's law you have the current. (for
    example 0.01 ohm shunt would give you 0.25V @ 25A)
    Gene
     
  3. Graham Knott

    Graham Knott Guest

     
  4. Graham Knott

    Graham Knott Guest

  5. CFoley1064

    CFoley1064 Guest

    Subject: changing my VMeter from 0.25A to 25A
    Hi, Jif. Most multimeters have a shunt resistor built-in, and use the internal
    voltmeter to read voltage across that shunt resistor. Technically, if you know
    that resistor value, all you would have to do is find a resistor 1/99th that
    value, and place it in parallel with the ammeter. Then 99% of the current will
    flow through your shunt, and 1% will flow through your ammeter. A reading of
    0.25A = 25A.

    Practically speaking, there's a bit of a problem, though. It's very hard to
    get precision oddball value low ohm resistors at reasonable price/delivery.
    Also, depending on the value, your R shunt value may be high enough to result
    in an uncomfortable amount of power dissipation.

    As a practical matter, it's a lot easier to just buy a 50A shunt and read the
    voltage with any standard voltmeter. The shunt has a 1 milliohm resistor,
    which results in 1mV per Amp up to 50A. This harkens back to the time when
    analog meters ruled the earth, and they had 50mV full scale deflection burden.
    Easy and straightforward.

    And, lo and behold, All Electronics, one of the better US electronics surplus
    outfits, has a 50A shunt available for $12 USD in stock. They claim 0.5%
    accuracy:

    http://www.allelectronics.com/cgi-bin/category.cgi?category=385&item=SNT-5
    0&type=store

    Remember to use the big screw terminals for the current, and the small ones for
    your voltmeter, if you need that accuracy.

    I guess today's your day.

    Good luck
    Chris
     
  6.  
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