Other Parasitic draw currents by millivolt drop across fuse.

DanClarke · Sep 19, 2019

I can't put this anywhere else (no privileges) so it can go here.
Although I think Gary @gstuart posted this (or one similar) some little while ago too.
Its a chart for converting millivolt drop across fuses and converting that to current being drawn by a circuit.
This allows you to determine the current being drawn without pulling the fuse and waking any systems up.

marjon · Sep 19, 2019

That looks handy thanks Dan, now have it stashed somewhere safe

I am assuming yes I know that’s risky, those values also represent the bigger type blade fuse, or even older glass?

J

Datatek · Sep 19, 2019

Likely to be pretty inaccurate unless you have a very accurate millivoltmeter.

DanClarke · Sep 19, 2019

marjon said:
That looks handy thanks Dan, now have it stashed somewhere safe

I am assuming yes I know that’s risky, those values also represent the bigger type blade fuse, or even older glass?

J

I think that in the main it is the mini-blade fuse Jon.

DanClarke · Sep 19, 2019

Datatek said:
Likely to be pretty inaccurate unless you have a very accurate millivoltmeter.

Agreed, but (there's always a but, isn't there Keith?) it gives a reasonable indication that there IS a draw of some degree happening.
A DVM with a millivolt range is supposed to be able to do exactly that ...measure in millivolts. So the translation tables are based on the reading being NOT-ZERO and should give you a clue at least.

Here is a guy using it to good advantage. I think Gary posted it too.

marjon · Sep 19, 2019

Ah, so I see his chart was slightly different.

J

DanClarke · Sep 19, 2019

marjon said:
Ah, so I see his chart was slightly different.

J

This method even appears in VW tech bulletins, so it IS an accepted testing method.
I agree that the "derived" milliamps seems to vary across the charts from the various sources, but the principle of the testing remains the same.

Wazzajnr · Sep 19, 2019

Unless you know the exact resistance of the fuse, it's no good for calculating the current draw. Might give you some indication of "some current" but discerning between 20 or 40mA, not a chance unless you have a really accurate meter.
Also need to take resistance of leads into account.
Also, the dissimilar metals will create a thermocouple junction which will have some effect on the readings.
Wouldn't be my preferred way of measuring to be honest.

Datatek · Sep 20, 2019

DanClarke said:
This method even appears in VW tech bulletins, so it IS an accepted testing method.
I agree that the "derived" milliamps seems to vary across the charts from the various sources, but the principle of the testing remains the same.

Perhaps that's why they find it hard to fix things. On most commercially available DVM's the lowest range is 200mv
and you can get a reading from 2 wet fingers, plus the accuracy can be typically 1 to 5% +/- 1 digit, so it may or may not actually tell you there is current flow what it certainly will not do is give an accurate reading of the current flow.

DanClarke · Sep 20, 2019

Datatek said:
Perhaps that's why they find it hard to fix things. On most commercially available DVM's the lowest range is 200mv
and you can get a reading from 2 wet fingers, plus the accuracy can be typically 1 to 5% +/- 1 digit, so it may or may not actually tell you there is current flow what it certainly will not do is give an accurate reading of the current flow.

Fair comment Keith. I will give it a try to see what results I get (or not).

Other Parasitic draw currents by millivolt drop across fuse.

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