Fluke 179 keeps blowing fues

[Paul LGF]

Hi All,
Hope someone can help?
New here, but have read a lot of the different post on the 179 looking for answers. Very helpful information.
I bought this Fluke on ebay about 3 years ago. everything apart from current/amp draw testing worked. (didn't find this out until i went to find a parasitic draw on a car a few months after buying). Both fuses were blown, so replaced them. Went to test for a draw again, with no luck. Tested fuse again, and both had blown.
Have just put the smaller of the 2 fuses in, the milliamp one. Tested it with the Fluke itself and it reads 1.9 0hms. Acording to the manual this should be around <12 Ohms.
Is this right or do i have a short on the board
Hope to have the 10A fuse this week coming so will se what that reads when fitted
Any help very much appreciated

[Gyro]

Welcome.

It's because, when you break the battery connection and re-make it (via the meter), many things draw surge currents and perform various high current consuming functions before the car shuts down to 'idle' / parasitic current.

To stand a chance of measuring parasitic current, you need to work out a way of implementing a shorting wire to bypass the meter until the car has fully shut down and then cut it to take the reading. Note that any intermittent connection will cause the fuse to blow. The car could take anything up to 20 minutes or more to fully shut down. Also beware of things like bonnet switches which could affect the process while the bonnet is open.

[Paul LGF]

Thank you for the quick reply Gyro,

Should of said...I'm a mechanic/auto electrician, have been for over 30years now.  Also for a hobby i repair pinball machines. Here in France they call them Flippers.

Yes I am aware of the info but thank you again

[Paul LGF]

Just thought that 1.9 Ohms was a bit to close to short for comfort

[Fungus]

The only way to blow a fuse is to pass too many amps through it.

Those fuses are expensive and designed to save your life in high power situations.

People generally use current clamps for automotive work where there can be hundreds of amps and a high risk of blowing expensive fuses.

Have just put the smaller of the 2 fuses in, the milliamp one. Tested it with the Fluke itself and it reads 1.9 0hms. Acording to the manual this should be around <12 Ohms.

Less is better.

Fuses should have as little resistance as possible to minimize the effect on the circuit.

[Gyro]

Thank you for the quick reply Gyro,

Should of said...I'm a mechanic/auto electrician, have been for over 30years now.  Also for a hobby i repair pinball machines. Here in France they call them Flippers.

Yes I am aware of the info but thank you again

Haha, ok.

If you are using the fuse test method shown in the manual (insert the volts probe into the current jack, with the meter set to resistance range), the meter is measuring the combination of the current sense resistor and the fuse. In an 'ideal' current meter, this value would be zero, but obviously both have resistance. No it doesn't indicate a short - the reason you are seeing 1.9R with the mA fuse in fitted in the Amps fuse position (which is what I think you meant*) is because the Amps current sense resistor is very low in value, so you are mostly just measuring the fuse resistance. The current sense resistor on the mA range is higher in value, hence the <12R indication in the manual.

Nothing to worry about (aside from the cost of replacement fuses!).


EDIT: *If this is not what you meant, then maybe there is a problem.

[Fungus]

EDIT: *If this is not what you meant, then maybe there is a problem.

I don't see how any possible problem with the meter would blow fuses. The current range is a low impedance range by design.

[Gyro]

I'm just working from the fuse test procedure and expected reading in the manual (assuming the OP did a fuse position swap)...

...
Have just put the smaller of the 2 fuses in, the milliamp one. Tested it with the Fluke itself and it reads 1.9 0hms. Acording to the manual this should be around <12 Ohms
...

[Fungus]

I'm just working from the fuse test procedure and expected reading in the manual (assuming the OP did a fuse position swap)...

...
Have just put the smaller of the 2 fuses in, the milliamp one. Tested it with the Fluke itself and it reads 1.9 0hms. Acording to the manual this should be around <12 Ohms
...

1.9 is less than 12

[Gyro]

1.9 is less than 12

That's helpful, did you work that out for yourself?  What do you think the mA shunt resistor value is on a Fluke 179? My guess is 10R.

[Fungus]

1.9 is less than 12

That's helpful, did you work that out for yourself?  What do you think the mA shunt resistor value is on a Fluke 179? My guess is 10R.

No need to guess, you can work it out from the spec.

(nb. The spec will include the fuse ... which measured 1.9 Ohms)

[bdunham7]

Just thought that 1.9 Ohms was a bit to close to short for comfort

I don't know where you see the "<12 ohms" in the manual, but I just tested a genuine Bussmann DMB-44/100 and it had a resistance of 0.733 ohms, which is pretty close to what I'd expect.  The overall resistance of the meter in this range should be under 2 ohms AFAIK (I don't have a 179 on hand) and "being close to a short" is irrelevant--lower is better.  The part of your question that has everyone puzzled is the notion that the meter is somehow at fault.  There is no possible malfunction of the meter that would result in blowing that fuse inappropriately. 

[Gyro]

Page 14 of the 175/177/179 Users Manual (May 2003, Rev 3,4/23 from the Fluke site), "Test the Fuses". The procedure assumes fuse and shunt in series.

(Don't ask me why it's so high)

[bdunham7]

Page 14 of the 175/177/179 Users Manual (May 2003, Rev 3,4/23 from the Fluke site), "Test the Fuses". The procedure assumes fuse and shunt in series.

Hmm.  Maybe a typo?  To get the specified burden of <2mV/mA, the actual value should be less than 2 ohms and I don't see why there would be a signficant deviation measuring it that way unless they're adding in up to 10 ohm for crappy test leads.  In any case, 1.9 is less than either 12 or 2.

[Gyro]

Hmm.  Maybe a typo? ...

Beats me. 1.2R would be too low for the 2R shunt. 2R would still be too low if you include fuse and lead resistance.

[AG6QR]

For what it's worth, I own a Fluke 179, and just tested the fuses using the method described in the manual.  The 400mA range shows 1.8 Ohms, while the 10A range shows 0.1 Ohms.  If I connect the same probe directly from the V socket to the COM socket (not going through either fuse or current sense resistor), the meter fluctuates between 0.0 and 0.1 Ohm.

I also have spare fuses, the same Buss fuse part numbers as came with the meter.  The 440mA fuse measures 0.8 Ohm, and the 11A fuse measures so close to the probe resistance that I can't distinguish it from shorting the probes together.  I don't have a convenient four-wire Kelvin resistance setup, but it doesn't matter -- I'll just say that the resistance of that fuse is small compared to what the Fluke 179 can measure.

I think that posted page from manual is correct, in that 1.8 is indeed less than 12.  But I can't figure out why they placed such a high upper bound on the resistance.

But as has already been pointed out, an ideal ammeter has negligible resistance.  A fused ammeter must be used in a circuit such that the other items in the circuit will limit the current to something below the fuse rating. If it's blowing fuses, it's because you're passing too much current through it, so the fuse is doing exactly what it's designed to do, and your meter is not at fault.

[Paul LGF]

Thank you for clearing this up, and thank you AG6QR for testing your 179.

This has put my mind at rest

Thank you all.

[bdunham7]

Beats me. 1.2R would be too low for the 2R shunt. 2R would still be too low if you include fuse and lead resistance.

AFAIK it is a 1 ohm shunt, so that would all stack up correctly.  The burden voltage includes everything other than the leads.

[Paul LGF]

Its good to know that what they say in the manual might be some what off of reality, and to have all of you lot help to confirm what i was reading on my meter was about right.

Also after reading on here about fake fuses. The ones that i bought were from Amazon and not as cheap as the chines copies.

[Fungus]

Also after reading on here about fake fuses. The ones that i bought were from Amazon and not as cheap as the chines copies.

"Reassuringly expensive" isn't a foolproof guide.

(unless Bussman/Fluke have an Amazon store...)

[AVGresponding]

Also bear in mind it can be problematic to get reliably accurate low-ohms measurements without a 4-wire setup.

I prefer to use a shunt for measuring car battery drain currents; I have a home-made one that is good for 70 amps or so, and anything above that it's just easier to use a clamp.

[Fungus]

A clamp is the answer and you can get a plenty-good-enough one for the price of two or three of those fancy Fluke fuses.

eg. The Uni-T.

[Paul LGF]

Clamps not to good when looking for a parasitic draw on the car battery, as most cars draw under 50ma.  You have to be so careful not to touch the clamp and change its position while checking, and make sure there is no magnetic influence near by.

[J-R]

In many cases you can use the mV range on the DMM and probe across the fuse.

[AVGresponding]

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.