Circuit diagram
A
milliamp meter can be used as a volt meter by adding a series
resistance. The resistance needed is the full scale voltage reading
divided by the full scale current of the meter movement. So, if you have
a 1 milliamp meter and you want to read 0-10 volts you will need a
total resistance of 10/.001 = 10K ohms. The meter movement itself will
have a small resistance which will be part of the total 10K resistance,
but it is usually low enough to ignore. The meter in the example below
has a resistance of 86 ohms so the true resistor value needed would be
10K-86 or 9914 ohms. But using a 10K standard value will be within 1% so
we can ignore the 86 ohms. For a full scale reading of 1 volt, the
meter resistnace would be more significant since it would be about 8% of
the total 1K needed, so you would probably want to use a 914 ohm
resistor, or 910 standard value. The milliamp meter can also be used to
measure higher currents by adding a parallel resistance. The meter
resistance now becomes very significant since to increase the range by a
factor of ten, we need to bypass 9/10 of the total current with the
parallel resistor. So, to convert the 1 milliamp meter to a 10 milliamp
meter, we will need a parallel resistor of 86/9 = 9.56 ohms.
Reprinted Url Of This Article:
http://www.hqew.net/circuit-diagram/How-to-build-Analog-Milliamp-Meter-Used-as-Voltmeter_6491.html
