Multimeter question. - Page 2 - Electronic Circuits Projects Diagrams Free

rippa32 · 1st July 2009, 04:09 PM

Quote:

Originally Posted by Boncuk

Hi Rippa,

before you experience severe trouble with your multimeter take a look at the attached circuit which is drawn in three possible conditions.

It's quite easy to see that Ohm's law is one of the most important when dealing with electricity.

Assuming an inner resistance of almost zero Ω a shorted 9V-battery (also shorted by an ammeter) might deliver a current of 900A at a resistance of 0.01Ω of the ammeter (theoretically).

Voltmeters are correctly wired across the load resistance to measure the true voltage drop (without losses within the cable), while ammeters are connected in series with the load resistor. Their resistance must be as low as possible to avoid any additional voltage drop because of resulting erroneous voltage drop readings across the load.

If you happen to connect your multimeter parallel to a supply source using the 10 or 20A range you might prepare to purchase a new one. (The fuse is only used for the 200mA range in most DMMs.)

Boncuk

Thanks for all that info, I understand what went wrong now. Also thankfully it is just a blown fuse, I checked it.

AlainB · 1st July 2009, 04:12 PM

Hi,

A few years ago, I was claming to be the fastest fuses blower east of the Pecos. Faster than my shadow, I was! A fuse a day, almost!

Checking some current, then some voltage = Poff...!! "S&#$"! Forgot again!!

Now I am older and wiser.

I check a current then immidiately after, really, I mean immediately after, I put back this bloddy red wire into his voltage hole.

Alain

ericgibbs · 1st July 2009, 04:13 PM

Quote:

Originally Posted by rippa32

Thanks for all that info, I understand what went wrong now. Also thankfully it is just a blown fuse, I checked it.

hi rippa,
I did say the fuse had most likely blown when I replied to your first post.

Many of us have done the very same thing,

, always check that the meter is set to the correct mode before measuring.

audioguru · 1st July 2009, 04:51 PM

On Energizer's website they have a datasheet for their 9V alkaline battery. It is shown to supply 500mA for half an hour.
Into the almost dead short of a current meter it might supply 2A for many seconds.

JimB · 1st July 2009, 08:17 PM

Oh for the Avometer!
There must be a few of us here old enough to remember.

No fiddling about moving the leads around to measure current (only if you wanted to use the 2500volt range).
No fuses to worry about, just a mechanical cutout which open circuited the whole meter circuit if you overloaded it.
Overload in this case was when the meter needle was driven hard upscale or downscale.

JimB

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1st July 2009, 04:12 PM	#17
AlainB	Hi, A few years ago, I was claming to be the fastest fuses blower east of the Pecos. Faster than my shadow, I was! A fuse a day, almost! Checking some current, then some voltage = Poff...!! "S&#$"! Forgot again!! Now I am older and wiser. I check a current then immidiately after, really, I mean immediately after, I put back this bloddy red wire into his voltage hole. Alain Last edited by AlainB; 1st July 2009 at 04:16 PM.

1st July 2009, 04:51 PM	#19
audioguru	On Energizer's website they have a datasheet for their 9V alkaline battery. It is shown to supply 500mA for half an hour. Into the almost dead short of a current meter it might supply 2A for many seconds. __________________ Uncle $crooge

1st July 2009, 08:17 PM	#20
JimB	Oh for the Avometer! There must be a few of us here old enough to remember. No fiddling about moving the leads around to measure current (only if you wanted to use the 2500volt range). No fuses to worry about, just a mechanical cutout which open circuited the whole meter circuit if you overloaded it. Overload in this case was when the meter needle was driven hard upscale or downscale. JimB __________________ Experience is directly proportional to the value of the equipment ruined.