be minimum. If the meter scale is showing a voltage
range, it is probably reading correctly.
This can obviously be checked by comparing
the power supply voltage with the reading on the
meter when the series resistance is low. To make
sure that it is not an ammeter, the voltage across
the resistor should always be checked and the
circuit current calculated.
Once the full-scale meter current and the meter
internal resistance have been determined, then
external resistance can be added in series to
measure voltage or in parallel to measure current.
Full-scale meter values can vary widely. The
lowest current (most sensitive meter) that I have has
a full-scale range of 20 microamperes. The meter
used in the Bird 43 wattmeter, for instance, has a full-scale
deflection current of 30 microamperes.
The more sensitive a meter is, the more options it
offers when designing it into a circuit. This will be
To configure a meter to measure voltage, a resistance
is usually placed in series with it. If a meter has a full-scale
reading of one milliampere, then the series resistance is
calculated at 1,000 ohms per volt (minus the internal
resistance). (One volt across a 1K ohm resistor equals one
milliampere). For instance, if a full-scale voltage reading of
10 volts is required and the internal resistance of the
meter is 50 ohms, then the series resistance equals:
10 x 1000 - 50 = 9950 ohms
For the same voltage range, if the meter has a full-scale reading of 50 microamperes, then the series
resistance is calculated at 20K ohms per volt (one volt
across a 20K ohm resistor equals 50 microamperes). If the
internal resistance is 2,500 ohms, then the series
10 x 20K - 2500 = 197,500 ohms
When higher voltage ranges are required — especially
when greater than a few hundred volts — it is always a
good idea to place the series resistor on the higher voltage
side of the meter and connect the opposite terminal to
ground as shown in Figure 5.
At voltages greater than 250 volts, then more than
one resistor should be used in series — especially if the
resistors are rated at 1/4 watt since this is their maximum
recommended continuous voltage rating.
A better solution is to use a voltage divider network
with the meter connected across an additional small-value
series resistor connected to ground (Figure 6).
Obviously, this can only be done if the voltage source
can tolerate the additional current load.
It is also important not to exceed the wattage rating
of the series resistor. As an example, if a one milliampere
movement is being used and the meter range of 300 volts
is desired, then the series resistance would be
approximately 300K ohms. If a single resistor is used to
drop the voltage to the meter, a resistor of at least 300
milliwatts is required.
This would exceed both the voltage rating and the
wattage rating of a 1/4 watt resistor.
Most high voltage power supplies used in tube-type
amplifiers have a built-in bleeder-resistor network
consisting of one or several high wattage resistors. This
network is used to discharge the high voltage capacitor
bank when the amplifier is turned off to prevent possible
electrocution when the covers are removed.
The resistance value of the bleeder-resistor network is
usually a compromise between the amount of time that is
required to completely discharge the high voltage
capacitor (at least five time constants, typically 30
seconds) and the power draw of the bleeder-resistor given
off as heat when the amplifier is in operation.
The meter shown in Figure 6 can be configured to
accurately measure the high voltage without subjecting
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