Author Topic: Multimeter VOM  (Read 166364 times)

Fu-Kwun Hwang

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Multimeter VOM
« on: January 29, 2004, 10:05:50 pm »
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Ammeters measure current, voltmeters measure voltage, and ohmmeters measure resistance.

These devices are often combined into one instrument called a multimeter or VOM (volt-ohm-milliemmeter).
Analog versions of ammeters and voltmeters typically utilize a galvanometer, which relies on magnetic effect.
A needle is deflected by an amount proportional to the current passes through the device (coil).
A good ammeter should have a resistance that is small compared to other resistance in the circuit.
A good voltmeter should have a large resistance.
This java applet shows how to utilize a galvanometer into a multimeter.
The full scale current for the galvanometer is 50uA with internal resistance R=2k ohm.

 If galvanometer was connected in serial with a 198kohm resistor, it becomes a 10V voltage meter.
Because total resisitor become 200k, it need 10 V between two ends to produce 50 uA current.
 And the current flow will be proportional to voltage across the device.
 If a smaller resistor were connected in parallel to galvanometer, it can be used to detect more current than 50uA.
 For example: if R/9 were connected in parallel to galvanometer, then the total current flow through galvanometer is 1/10 total cureent flow through both elements (the whole device).

The following applet shows the simplified version circuit diagram for a real multimeter.

You can operate at different mode:
1. select the Mode (Voltage/Current/Resistance) :
2. select the appropriate range for that mode.
3. You can change values of the DC voltage source V and resistor R:
    You can also modify V/R by click the mouse button at the device.
    Right/Left click mouse button once: to increase/decrease its value.
    Right/Left click mouse button twice: to change the increment value.
    If you press down the mouse for more than 1 second, it will change V's (or R's) value automatically.(every 0.5 sec.)
     When you have the value you like , release the mouse button.
    The unit for resistor is killo-ohm.
    The Text Field shown can also be used to enter value for R, R2 or V,
     depends on where you click the mouse button. ( "," input will be treated as "." e.g."2,3" will be read as "2.3" )
     if you click the voltage source, then the value in TextField is new V.

4. Hit the OFF/ON toggle button to connect/disconnect the meter to the test circuit.
    If the meter were overloaded, the fuse (green line in white background) will break.
     Please turn off connection and change range. (It will turn off connection automatically in 8 sec.)

    Change the range to a better one and click the button again.
    The effective (simplified version) circuit diagram is shown at the lower right edge.
    The internal resistance of the meter is also there.

    1. Mode: Voltage VR to measure voltage across an element (placed in parallel with that element)
      There are two resistors R and R2 connect in series. (R2 can only be modified by mouse click)
      The meter will measure the voltage across resistor R when it is connected.

    2. Mode: Current IR to measure current flows in a circuit. (placed in series with the circuit).
      It will measure the current in the test circuit when connected.

    3. Mode: Resistor R to measure the resistance for a unknown resistor.
      The meter provides the DC voltage source (3V+9V battery) needed for the measurement.
      The galvanometer first forms a voltmeter (Vo=3V or 12V), Unknown resistor is in series with a known resistor and then connected to the voltage source.
      From the voltage measurement, we will know the value for the unknown resistor
       V= R_known/(R_known + R_unknown) *Vo
      This is not a linear equation for R_unknown, so the scale in the meter is not linear.
      % shown in the scale represents infinity.

    4. The Rm checkbox is checked as default.
     If unchecked, the meter has infinite internal resistor (which is not real).
     This mode is useful for teaching the student when they first learn the meter. (make it easier).
     It should be checked in order to simulate a real multimeter.
     (when the internal impedance of the multimeter can not be ignored).
     Try it and find out the difference.

    **** When operated in resistance mode:
      The red (+) connector has a lower voltage related to the black(-) connector. Do you know why?

5. Learn / Exercise Mode :
Cilck the mouse at the center of the meter (within the white circle area)
 will toggle between exercise and learn mode,

 Learn mode: ( default )
 When the meter is connected to the test circuit, the needle of the meter will be shown.
Exercise mode:
 The needle will not be shown, and you will have 3 chances to enter the correct value into the textField.

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« Reply #1 on: January 30, 2004, 01:48:40 pm »
Subject: More Java suggestions
Date: Tue, 14 Sep 1999 14:38:08 +0100
From: Jeff Forssell <>
To: "''" <>
CC: Claes Ahlin <>
Back in school after a tough summer. My 81 year old mother was visiting
from USA and fell down a stairs and needed help for the 2 months she was
here. Bad luck! Didn't get much done this summer. But the weather was
nice at least.

Hope you´ve had a good summer.

I was checking to see if you multimeter aplet could be used to see in
what situations it would be incorrect (parallell with a LARGE
resistance) but it didn't take that into consideration- it gave the same
reading at 2000 kohm as 10ohm.
See the accompanying picture.

While I´m at it heres the interpreter for Q basic
in case you want to try the files I sent before.

I've been looking at Walter Fendts Javas. some are quite illustrative.
One that wasn't was about the "radioactive decay series". Do you have
anything planned there?

I'd like to find an alternating current aplet that would show the charge
acculation resp magnetic field buildup graphically.

I sometime try to show my pupils a water -electricity anology where
pump=voltage spource
small or clogged pipe=resistance
double chamber with elastic membrane=capacitor
turbine with inertial load=inductance
That might be something that could make an interesting Java aplet

I put together a very rough sketch of the C-L anology:
Jeff Forssell
personal homepage:
my village technology homepage:
Swedish National Institute for Distance Education (SSVH)
Box 3024 SE-871 03 Härnösand /Sweden
tel +46(0)611-55 79 48 fax +46(0)611-55 79 80
schools homepage


Subject: SV: re send multimeter plus more ideas
Date: Thu, 16 Sep 1999 12:40:29 +0100
From: Jeff Forssell <>
To: "''" <>
Thank you!
The aplet seems to work very well.
Some minor problem and a couple of wishes:
1) if I change the range while the meter is connected, the measured
voltage is not updated. (Hmm I tried it again and now it works. But if I
reload the page and turn "ON" directly it reads 4,67V on 10 V range. If
I change to 50 V range,4,76V stays in stead of 4,95V The Equivalent
circuit is updated directly.)
2) I would prefer a much faster component value change while
continuously pressing mouse <0.1s per change. One can always "fine tune"
with single klicks.
3) I would like a possibility of writing in R2 values also.
(4) Fu Hwang 1999 in lower left, in stead of 1998)


Subject: SV: SV: re send multimeter plus more ideas
Date: Fri, 17 Sep 1999 07:46:05 +0100
From: Jeff Forssell <>
To: "''" <>
CC: Claes Ahlin <>
Thank you!

I still have the problem with voltage update on changing range even on
the new aplet (i've tried in both NS 4.5 and IE 3.0).

with a newly opened page:
press ON
V= 4,76V 10 range
change to 50 V V still 4,76 (equivalent circuit Rs update
change to 10 V v changes to 4,95
change to 50 V changes to 4,76

The write in window for the values works well. I would (on behalf of
Sweden and some other countries) like even comma "," to be interpreted
as a decimal sign.

Other clarity wishes: prefix the box with "R2" when updating R2 and
suffix the box with "kohm" when R values being set.

V [ ]
R [ ]kohm
R2[ ]kohm

I think the current circuit would be easier to see if you had a wire
connecting rather than 2 ground symbols.

When the Rm box is not checked, it would be good for the Eqivalent
circuit to be altered perhaps like this:
Hope you don't think I'm complaining! I use your aplets a lot! I wish
someone would go through my stuff to help me develop it!


Subject: Very Nice ..... but....more ideas
Date: Sat, 18 Sep 1999 22:12:40 +0100
From: Jeff Forssell <>
To: "''" <>
Very nice!

But I still get the update problem when changing range (both IE 3.0 &
Nets 4.5). Have you tried as the FIRST step after opening the page? I
know that the problem goes away if I do a bunch of other stuff (change
values I think).

One other problem: I think that the multimeters that I´ve used have had
the red lead positive when in the resistance mode (which makes it
easier to interpret when measuring on active components like diodes). I
havesn't checked that in reality since I had the thought though.

Reviewing my own work:
I'm mainly interested in getting my web resources polished up for my
Swedish students which mean that it is in Swedish which makes it
difficult for non Swedes to say much about it.

  • Guest
« Reply #2 on: January 30, 2004, 05:53:47 pm »
Date: Thu, 9 Dec 1999 17:35:47 +0000
Subject: Request to link to your site:- Our ID: 446; Page: Multimeter
    VOM; URL:

The Scottish Further Education Unit is currently developing the Virtual
Learning Centre. In this we have created an Information Gateway which
contains links to Internet and CD-ROM resources. These have been evaluated
to set criteria, reviewed and mapped to the Scottish vocational education
and training curriculum. We would like your permission to include your site
in this Information Gateway.

Please do not hesitate to contact me if you require any additional

The site is located at
Please reply using your reply button so that we can cross reference the
information contained in the Title.


Jean Henderson


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Re: Multimeter VOM
« Reply #3 on: September 22, 2009, 05:25:27 pm »
I think for most casual usage there's no real advantage to the Fluke unless it has features you can't get in a "generic" meter.

Any more I think they're like calculators, no one cares about the guts because the all give you the same answer. So unless you need all the graphing and advanced scientific functions of something fancier, there's no reason to spend more.

Flukes are nice but I don't see the justification for something I use once a month at most. If I made my living off of it then I'd own one. If the difference in price was small I'd go for it, but for the money you can replace the cheap one many times.

I'd rather have a decent oscilicope and a cheap multi-meter. For automotive diagnostics any more they're much more helpful, almost as good as a decent diagnostic scanner.


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  • ...
Hello world!
« Reply #4 on: October 04, 2009, 01:18:31 pm »
echo '<b>Hello world!</b>';

Fu-Kwun Hwang

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Re: Multimeter VOM
« Reply #5 on: October 04, 2009, 03:16:10 pm »
Are you trying to hack this system?
This is an educational web site. Please leave us a clean space. O.K!
Please stop!