Author Topic: Fluke 179 keeps blowing fues  (Read 1546 times)

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Offline BeBuLamar

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Re: Fluke 179 keeps blowing fues
« Reply #25 on: May 23, 2024, 10:32:26 am »
I would use the clamp and see about how much current is flowing. There must be significantly more than 10A to blow the fuse before you can get the reading.
 

Online J-R

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Re: Fluke 179 keeps blowing fues
« Reply #26 on: May 23, 2024, 07:04:12 pm »
In many cases you can use the mV range on the DMM and probe across the fuse.
You can, but you need an accurate measure of the resistance across the fuse and a bloody good microvolt meter or you're not getting an accurate picture of the current flow.

For context, a 100A main battery fuse will have a resistance of something like 1mΩ, and 50mA through that is gonna be a volts drop of only 50uV, well down in the noise for your average handheld.
My home-made shunt is 10mΩ, giving me a better chance of seeing the signal over the noise at lower currents.

You don't need precise numbers for this method.  The expected mV values are easy to find with an internet search.  You're also be typically probing fuses under 10A, not the larger fuses.

I've not found a clamp meter to be very useful on most vehicles when troubleshooting battery drain.  The wires are very well wrapped/bundled in addition to being hidden.   Resolution/accuracy isn't very good, especially when monitoring larger connections like the battery.
 

Offline robert.rozee

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Re: Fluke 179 keeps blowing fues
« Reply #27 on: May 24, 2024, 03:03:21 pm »
you can buy a couple of really big schottky diode for just a few dollars, rated at 40A or more. connect these in parallel to a 1 ohm power resistor (also available for a few dollars) such that no matter the polarity one or other of the diodes will be forward biased. mount the whole lot on a moderate-sized heatsink with a 30A or so cheap automotive fuse in series.

some big schottky diodes:
https://nz.element14.com/vishay/vt4045bp-m3-4w/diode-schottky-45v-40a-to-220ac/dp/2115196?st=schottky+power+diode
https://nz.element14.com/stmicroelectronics/stps60150ct/schottky-rect-dual-150v-to-220ab/dp/2806772?st=schottky+power+diode
a big 1 ohm, 1% power resistor:
https://nz.element14.com/arcol/hs25-1r-f/resistor-wirewound-1ohm-1-axial/dp/2478482?st=1%20ohm%20power%20resistor

the above assembly can then be connected in place of the earthing strap that runs from B- to the vehicle chassis. as long as you don't try cranking the engine, all will be well. if you do crank the engine, just the (cheap automotive) fuse will blow. the diodes will prevent the 1 ohm resistor ever seeing more than 1 volt across it, ie from ever dissipating more than 1 watt. and for currents less than half an amp or so, the diodes will have no effect on the measurements you will be making.

now just measure the voltage drop across the resistor, your meter will display 1mV per mA of parasitic current flow.


cheers,
rob   :-)
« Last Edit: May 24, 2024, 03:10:44 pm by robert.rozee »
 
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