I was checking the current discharge from the battery with the engine turned off, and since the earth strap was already disconnected I connected the multimeter between the earth strap and the negative terminal and promptly blew the fuse in the multimeter. I now know I should have done this from the positive terminal, but can someone explain in 'O' level physics terms why there's any difference?
Maplin has 22mm fast acting fuses rated up to 800mA. Anyone know a source for a 2A fuse?
It shouldn't have made any difference whether you used the meter in series with the +ve or -ve leads, probably something was taking several amps. Next time use a stop light bulb with wires soldered on in series with the meter, if there's not much current then the bulb stays cold and low resistance so doesn't affect your readings, but if something is trying to take lots of current then the bulb will limit it and glow (and save your meter).
Have you tried
I thought that, but I've only ever seen references to doing it from the
+ve side.
Something apparently was - over 2 Amps' worth!
I suppose I should have used the higher Amps scale on the meter first (0 - 10A) but I never imagined it would be drawing more than a few milliamps. Diodes in the alternator possibly?
Thanks. I've got some T (time delay) fuses with the same rating, but I thought they intentionally used fast blow ones to protect the circuitry.
Just convention probably.
Standard testing procedures when using any multimeter on any circuit is to start with the highest range. Current flow when not running can be surprisingly high in modern cars. It will certainly be higher than a few milliamps. Some of them turn off some circuits after a delay in order to conserve the battery.
A meter that is destroyed is generally more inconvenient however :-)
Indeed they do, and often super-fast rated ones. They don't always save the meter however, it depends on the magnitude of the fault current.
Chris
None. The DVM would simply have given a negative reading - if you had the leads round the wrong way.
Well, RS, etc. But of course they have a minimum charge.
Best way when measuring quiescent current from a battery is to use some form of a jump lead with the meter across it initially then disconnect leaving just the meter after things have stabilised. But I'd tend to go for a 10 amp DVM.
It's a rule of Electronics Engineering that an expensive piece of equipment will always blow rapidly to protect the 25p fuse inside it.
Always put a bulb in series if you suspect a high current may be flowing. It might blow, but it's a lot easier than getting a replacement fuse for the meter.
RS? Farnell?
:-)
What might blow? If you have a 12 volt bulb in series with a 12 volt battery and another load, the bulb wouldn't blow (well, not because of over current) even if the other load was of negligible resistance.
Chris
There isn't any. Sure it was on current setting and not voltage? Quite a few multimeters will pack up if you try to measure current with them set to voltage instead of amperage.
Transistor:- a component protecting a fuse.
? The other way round, surely.
Yup, on a voltage range the input resistance of a meter will be significant (I was going to say massive, but hey!) and you will have negligible current throw through the meter.
Otherwise, measuring the terminal voltage on a car battery would be a short-lived event!
ROFLMAO. Clueless...really clueless.
Not the ones I've seen blown.
Not the ones I've seen blown. Admittedly utterly cheap s**te though. Most decent autoranging ones will go "user is a f****it - I'll swap it over to what it SHOULD be".
If you try to 'measure current' with the meter set to voltage you're merely measuring the voltage of the battery less the series resistance of the load. Which in practical terms means you'll just get battery volts.
Damage - or a blown fuse - results when the meter is in current measuring mode, ie has a very low resistance shunt across it, and you then try to measure voltage from a source capable of suppling more current than the fuse rating.
I've got a 200 quid Fluke. Still blows its fuse if you make a mistake. And it's an expensive fuse...
It does, however, prompt you with a 'leads' readout when you swap to current measurement.
Conor wrote on Sat, 11 Mar 2006 11:18:05 -0000:
But if you try to measure current with a voltmeter, you're putting a big resistance in series with the circuit. Rather like putting the voltmeter across a car battery -- very little current will flow.
OTOH, if you try to measure voltage with an ammeter, you're putting a tiny resistance in parallel with the circuit. A huge current will flow until the fuse/meter melts.
Conor wrote on Sat, 11 Mar 2006 11:16:14 -0000:
Is that a jumbo-sized Whoosh I just saw going overhead?
No. I have my BTEC HND in Electronic Engineering. You?
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