Multimeter use

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I have a RSR MAS830 multimeter. I am trying to measure current drawn from my M48 circuit. I put the multimeter in series and have the probes on the 10A DC connection. My meter measures current only when the dial is set to 10A and does not work for the other settings. Current measured is about 30 mA and is pulsed current (M48 is switching ON and OFF a speaker load) On the current measurement socket it says 10A MAX UNFUSED.Why is this so? Why is the meter not measuring anything when the dial is set to 200mA and only measures when it is set to 10A? Thanks.

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Likely because you blew the fuse in the other ranges. If the 10A range is market as unfused, then the other ranges are very likely fused.

Stealing Proteus doesn't make you an engineer.

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Damn. Will have to look at the manuals and see how I can replace the fuse then. I thought the meter fuses would be a PTC or something so you could use it again(when the resistance was back to nominal after cooling) without manually having to replace the fuses.

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Quote:
I thought the meter fuses would be a PTC or something

I think that would be too slow, but for whatever reason, they never are PTCs. To make matters worse they tend to be an unusual size fuse for which you won't find a replacement easily. If you're lucky, you'll find a spare fuse inside the case when you take the back off.

After you replace it, always be careful not to leave the meter on a current range when you're done with it! It's too easy to probe a power supply voltage without thinking and blow it again.

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I will try opening the meter up. I had started measuring current with the lowest range on. I should have started with the highest range for current. Live and learn. Are there any auto ranging meters that measure current?

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Yes, there are meters that auto-range current. But, there is usually a high current unfused range that does not autorange.

The biggest cause of failure of current range fuses is leaving it on a current range and hooking it across a power supply as if you are measuring voltage. Autoranging will not solve this!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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peret wrote:
To make matters worse they tend to be an unusual size fuse for which you won't find a replacement easily.
Well, in cheap multimeters, like this Mastech stuff, they tend to use normal fuses. Which is good and bad.

Good, because you find replacements.

Bad, because the fuses they use are then often only rated up to 250 V. At the same time, the multimeters as such are often rated something like CAT II 600 V.

And here it gets interesting. A 250 V fuse is guaranteed to reliable cut off voltage up to 250 V. Above that it might not do it reliable, due to arcing. A multimeter specified with up to 600 V working voltage (and safe against even higher transients) should of course have a fuse being able to reliable cut off at least 600 V, shouldn't it?

Because of this mismatch I don't trust cheap multimeters for anything above 48 V.

On the other hand, if you find one of these "unusual size" fuses in a multimeter, there is hope that that fuse can reliably cut of higher voltages than 250 V (check multimeter documentation for fuse spec.), especially if it is longer than a normal fuse.

Stealing Proteus doesn't make you an engineer.

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Some multimeters like the Fluke 187 will beep nervously if you have the leads plugged into the current measuring ports and no current measuring function is selected.

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peret wrote:
To make matters worse they tend to be an unusual size fuse for which you won't find a replacement easily.
This was definitely the case with my Fluke 87 (one of my two Flukes). I bought it used from a friend and one of the fuses was blown. I couldn't find a replacement anyplace but through Fluke. However, they were reasonably priced so I ordered a replacement plus extras. The Fluke 87 has two different size fuses.

Don

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Well why not have the ESD diodes that they have in almost every i/o pin of commercial IC's, for these meters? Those have very high rating (>1kV)and also have fast response. If these are slow, you can also go for an ESD clamp.

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The problem is if when you inadvertently probe something with a meter set on current measurement you will create a short circuit.

If the 'victim' cannot deliver the current needed to blow the fuse, the meter really won't care, but the circuit might not survive.

So, ESD diodes won't help with this; those are meant for overvoltage protection and in fact will become more or less a short circuit when the voltage over them rises over the trip point. The excess energy is converted into heat.

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As far as the fuse voltage rating, I doubt that it will be a problem. The voltage differential at the probes for a current measurement is unlikely to be very high -- or the fuse will soon be toast!

As far as the actual rating, I wonder why it's usually so low. I doubt that a normal 3AG fuse will arc at 600v or even quite a bit more. I must be missing something here.

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ford2go wrote:
As far as the fuse voltage rating, I doubt that it will be a problem. The voltage differential at the probes for a current measurement is unlikely to be very high -- or the fuse will soon be toast!
Ah, you didn't get it. When you make a mistake, when you contact a multimeter set to a current measurement range to a voltage source, the fuse has to cut off that voltage. If it does not, you continue to have a short circuit over the voltage source. When this happens, something else has to give in. This is the moment it becomes dangerous.

ford2go wrote:
As far as the actual rating, I wonder why it's usually so low.
It is only so low in cheap multimeters. You won't find a 250 V fuse protecting the current range in a 600 V quality multimeter.

ford2go wrote:
I doubt that a normal 3AG fuse will arc at 600v or even quite a bit more. I must be missing something here.
A fuse is a safety device. You never ever underspecify a safety device. You do not compromise on safety. If a fuse manufacturer says his fuse is good for 250 V than that's it. 250 V, period. If it is not speced for the voltage it might have to cut off it is not good for the job.

BTW, a fuse can arc easier compared to an air gap of the same distance. The vapor forming from the vaporizing metal is conductive. Fuse manufacturers counter this to some extend. But still, if a fuse is rated only at a certain voltage it is a bad idea to consider it to be good for more than twice that voltage.

Stealing Proteus doesn't make you an engineer.

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As Arnold mentions - an arc forms when the fuse attempts to interrupt the current flow. Problem is the arc is ionised gas and is conductive aided by the metal vapour - the fuse still looks like a short circuit. HRC (high rupture current) fuses are filled with sand and this helps to extinguish the arc quickly thus being able to handle a higher fault current. You'll find the good multimeters use such fuses. The Fluke fuses are expensive for a reason.