Derivation of Amperes Drawn

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With the recent topics about reading voltages (meters), especially the voltage before (Va) / after (Vb) the fuse, one should be able to derive the amperage drawn.  Being able to read the voltage delta at the fuse (Va - Vb) and calibrating for the fuse resistance one should be able to get the current drawn.   Ic = (Va - Vb) * Rf where Rf is the (calibrated) resistance of the fuse.

Using a meter app to read Va and Vb, the voltage delta across the fuse does seem to be react as a resistance and therefore the current can be computed.  Will this approach work or am I missing something?  Has anyone implemented this in an app?

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Posted 4 years ago

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Ed, K0KC

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Your assumption makes sense if the resistance of the fuse is constant. I have never checked it out, but I bet the fuse resistance with no current flowing is different than the resistance with 20 amps or so flowing through it. You would probably also need to take contact resistance into account if the fuse was in a fuse holder (which may also vary depending upon the current flow).

Ed, K0KC
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Mickey N4MB

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Fuse resistance varies with temperature, which rises significantly as you approach the amperage rating of the fuse, when the conductor is designed to melt. Unless you already know the current, you can't predict the fuse resistance very accurately... Although this method should get you in the ballpark, the delta in fuse resistance vs. current makes it less simple to get an accurate current measurement. Better to use an ammeter, which measures voltage drop across a calibrated resistance, either directly or across a current divider.
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Martin Ewing AA6E

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Yes, Ohm's Law works! You may run into problems when the fuse resistance is very low, as it normally is. What you need to measure is delta-V (voltage across fuse). If you only measure Va and Vb relative to ground, your meter needs high precision to get a good measurement of delta V.
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Greg Leber. (K9ON)

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Walt - KZ1F

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Its s still early yet here so it took a moment. Points for Greg, that's pretty good!
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Thanks everyone for the comments.  A constant voltage offset (contact potential difference) would be easy to calibrate out but the variable resistance based on temp doesn't sound so easy.  Oh well, back to the drawing board.

Perhaps "moving" the fuse to an outboard pigtail and inserting a proper current sense resistor would work.  To me it wouldn't be worth the effort though.

Kinda odd there is meter points on both sides of the fuse?  One could read the delta and know if the fuse is intact of blown, but if it's blown, then the system isn't operational to take the readings. Scratching my head on this one....

Thanks for the expertise...

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The fuse is effectively acting like meter shunt. I have a 100W UHF radio in my car and started experiencing some intermittent shut downs while transmitting. I measured the voltage drop across the fuse (holder) at the 12V connection to the battery. I found that the fuse I purchased at a local car audio shop had a metallic coating that introduced a slight resistance, acting to limit the current. I cleaned up the fuse with some emery paper and the problem went away.