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Thread: Resistors for current measurement

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    Supporting Member mklotz's Avatar
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    Resistors for current measurement

    I'm always nervous wiring my good VOM in series into a circuit to measure current. Another way to measure current is to measure voltage across a resistor that is series wired into the circuit in question. This way the VOM remains in high impedance mode and so is less susceptible to damage.

    Most circuits will not be adversely affected if the series resistor is very small in comparison to the circuit resistance itself. OTOH, the resistor must be large enough to produce an easily measured voltage when the current is flowing through it.

    I acquired three 50 watt, 1% resistors in resistances of 0.1, 1.0, and 10.0 ohms and attached them to a large aluminum plate to act as a heat sink.

    Given 50 watt capability, the maximum current and associated voltage for each resistor is...

    ohms amps volts
    0.1 22.4 2.24
    1.0 7.1 7.1
    10.0 2.24 22.4




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  2. The Following 4 Users Say Thank You to mklotz For This Useful Post:

    Paul Jones (02-19-2019), ranald (02-18-2019), Scotsman Hosie (02-19-2019), Seedtick (02-18-2019)

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    Supporting Member ranald's Avatar
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    ranald's Tools
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    I'll never do it but it is interesting. Thanks for sharing Marv.
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    Marv i may be wrong but i think an unpainted aluminum surface, better even with a little thermal compound should work better
    Ariscats

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    Supporting Member mklotz's Avatar
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    Quote Originally Posted by Ariscats View Post
    Marv i may be wrong but i think an unpainted aluminum surface, better even with a little thermal compound should work better
    You're correct, of course, but that bit of aluminum was lying around, I was in a rush, and all my applications involve currents far below the 50 watt limitation.
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    Quote Originally Posted by mklotz View Post
    You're correct, of course, but that bit of aluminum was lying around, I was in a rush, and all my applications involve currents far below the 50 watt limitation.
    O.K. Marv. I just made a comment.
    Ariscats

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    Supporting Member mklotz's Avatar
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    You made me feel guilty (and I deserve to be) so a few minutes with a sanding drum and we have some nice bare radiative surfaces...


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    The use of resistors in this situation is called a "shunt"
    Another method for measuring high current draw is to measure across the supply lead that oft times goes from the power source directly to the load. Disconnect, and, accurately measure the resistance of that lead, and you have a "known" shunt value. Reconnect, and then measure the current draw and you achieve the same result

    Cheers Phil

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    Quote Originally Posted by mklotz View Post
    You made me feel guilty (and I deserve to be) so a few minutes with a sanding drum and we have some nice bare radiative surfaces...


    Don't forget the compound. Otherwise! LOL
    Ariscats

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    Thanks mklotz! We've added your Current Measuring Method to our Measuring and Marking category,
    as well as to your builder page: mklotz's Homemade Tools. Your receipt:




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    Scotsman Hosie's Tools
    After the 1st number of years — and easily as many blown up meters — I discovered I could blow up a $5.oo meter, with about the same efficacy as I could a more expensive model. And since, truth be told, I rarely needed the kind of sensitivity of either a Simpson or a Fluke, I defaulted to always having an extra meter handy. Kinda like having an extra truck key. (And I probably saved a couple thousand dollars, over the last 40 years.)

    On another note, I did a helluva lot better, when I started marking the damned dials. So I could clearly tell what settings I had them on.

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