Multimeter Problems

[Bob_Roller]

I have a digital multimeter from Radio Shack, don't know if they have retail outlets outside of North America or not .

About 6 weeks ago when I connected the test leads to the battery on the R65, got '0' volts, tried it on a resistance scale, got off-scale, or open circuit indication when the two leads were touched together .

I tried just using a bare wire between the input jacks on the front of the meter, still an off scale high reading on the ohms scale .

Took the back off of the meter, and tried using a jumper wire on the soldered terminals on the PC board, no change in the readings .

Even replaced the 9 volt battery, no help .

Was resigned to the fact that I needed to get another meter, as this one was 'toast' .

I noticed a small glass tube type .5 amp fuse on the pc board, I thought that since the display is functional fuse must be still good .

I removed the fuse to look at it, and the display didn't go blank .

I put a short length of bare wire in place of the fuse, and the meter started working as it should .

Fuse was melted .

New fuse, everything works normally now .

Don't know how other meters are constructed, but just a bit of information, if you have a meter go bad on you .

[darrylri]

I have an analog RS meter that I must have bought 35-40 years ago.  It also has a small fuse, but the fuse is to protect the meter from overcurrent when using it as an ammeter.  I think the scale is rated for 500 mA.  That's so small an amount of current that I don't think I've ever used that scale.  Maybe the fuse protects other functions as well...

[nhmaf]

You blew the fuse by trying to measure the resistance of the battery, and had two voltage sources (the one in the multimeter and the battery itself) fighting each other.   Or maybe you had it in ammeter mode and essentially shorted out the battery by touching both leads to the terminals - which would blow the fuse.   The fuse is part of all multimeter designs to protect the rest of the device - ar at least any semi-decent multimeter design.
Always check that fuse if the multimeter appears to be operating, but not measuring things correctly.

[Landlubber]

You can never connect the multimeter across the two poles of a battery, that will cook it for sure

[Darwin_R65]

Actually landlubber that comment should be "you can never connect a multimeter across two poles of a battery when set to resistance, or current" If set to DC volts you measure voltage by connecting across the two poles.

John

[Bob_Roller]

I don't think I had it set to the resistance setting, as the dc scale is the first from the off detent, and the ohms scale is 5 detents away .

Regardless, of how the fuse go melted, I was surprised that the fuse protected the incoming electrical signal and not the entire meter .

I was of the frame of mind, if the fuse was bad, the meter display should have been blank .

Now I know better .

No wiring diagram came with the meter, if it had, I may have caught this right away .

[nhmaf]

Most meters - or at least, more expensive ones, anyway, have BIG, high voltage rating fuses on the input circuits as there are safety standards and creepage distance/clearances to be met for any circuits that may be exposed to high voltage.   Because of this, those fuses are isolated/insulated from the rest of the multimeter to meet these international safety criteria.   For example, my Fluke multimeter has 6A fuses rated for over 600V on the inputs - which occupy a large percentage of the plastic case.  On some pricey benchtop meters they may also have an isolated circuit which shows an error LED when an input fuse is blown, but all this would be WAAAAY to0 expensive to do on a cheap $7 meter made in China, and sold in RadioShack for $25 (a normal markup increment).   Once you get up into the $350 and over meters, some of these features start to get added in.

[Landlubber]

Robs initial comments...
when I connected the test leads to the battery on the R65, got '0' volts, tried it on a resistance scale, got off-scale, or open circuit indication when the two leads were touched together .

Yeah. I was referring to these comments regarding the multimeter, ta.

[Bob_Roller]

That could be misleading, what I should have said, is removed the leads from the battery selected the ohms scale then touched the leads together .

[Bengt_Phorqs]

I've blown a Radio Shack multimeter before myself but that was many years ago.  Never knew what caused it but I believe it was when I had it on the ohmeter and touched the probes together.  Didn't make sense to me then but that's what happened.

  And to answer your question, yes they do have international outlets.  I once shopped at a R.S. store in downtown Sana'a Yemen.

[Rob Valdez 79 R65]

I thought you are supposed to touch the probes together in Ohms mode, to adjust to zero, if analog, and just to see the numbers go to zero, if digital.

[montmil]

I thought you are supposed to touch the probes together in Ohms mode, to adjust to zero, if analog, and just to see the numbers go to zero, if digital.

That's right.

[wa1udg]

Multimeter markings  usually make it clear what the max current range is.  Fuses are to protect us from Murphy, who was an optimist.

[wa1udg]

In "automotive" service, a high quality muiltimeter with HIGH input resistance on the voltage ranges will often show full voltage on a circuit which has cruddy connections since only a few electrons need to flow for the meter to read voltage.  I find the inexpensive ones which actually draw a little current can be more accurate, however the best way to check things is under load, meaning measure VOLTAGE when the starter, ignition, or lamp is supposed to be drawing rated current.  Cars at one time had "amp" meters which measured current flow into and out of the battery (except for starter current)   The entire non starter current flow had to go through the meter.  Thick wire costs $$$ so later amp meters used a resistor of a very low value
in the circuit close to the battery and measured the "voltage drop" across the resistor with a VOLTmeter.  (Voltage (E) = I (current) times R resistance -- good ole Ohm's Law)  If you know the R and measure the voltage you can calibrate for current flow.  I suppose the resistor got too expenssive for the car makers because later they went to simple voltmeters which tell us almost nothing about battery condition unless you apply a load like the lights to a battery with the engine off.  Don't use the radio as a load because it doesn't present enough load to indicate anything useful.
One good thing to remember if you are new at 'lectricty stuff.  Despite what is done backwards in a few entry level textbooks to make it "easier", current flow is from the NEGATIVE terminal in battery circuits through the device powered, and BACK to the POSITIVE terminal.  Yup, you would think the + terminal would be the provider, but it is not.  Flow direction means nothing to a simple bulb,  and it is very diffucult (!)  to install a starter motor backwards since it is bolted into the negative circuit by being attached to the engine, but if you want to use a "one way street" - a DIODE  for a slick  modification you need to know this.