Multimeter
VOM
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 2k ohm.
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.
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.
(placed in series with the circuit).
- It will measure the current in the test circuit when connected.
- 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.
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?
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.
Thank you, Jeff Forssell, for your valuable suggestions.
Your suggestions are highly appreciated!
Please click hwang@phy03.phy.ntnu.edu.tw
for your feedback!
AuthorกGFu-Kwun Hwang, Dept. of physics, National Taiwan Normal UniversityLast modified : More physics related java applets