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:
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.
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)
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).
3. Mode: Resistor R to measure the resistance for a unknown resistor.
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.
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.