46 October 2017
the meter to what could be a lethal potential. The meter is
connected (through a series resistor) at the bottom resistor
in the chain. If the voltage across the bottom resistor is
greater than a few hundred volts, then an additional small-value resistor can be placed in series at the ground side of
the network to limit this to some reasonable level.
As an example: If the voltage to be measured is 3,000
volts and the high voltage filter capacitance is 30
microfarads, the bleeder-resistor should discharge the
power supply to near zero in five time constants:
One time constant 5 x TC = R x C, then R = 5 x TC / C.
R = 30 / 30 x 10E - 6 = 1,000,000 (one megaohm).
If the bleeder-resistor network consists of 10 x 100K
resistors to make up the required one megaohm total,
then the bottom resistor will have 300 volts across it
The dropping resistor (Rs) in series with the meter
(added to the meter series resistance) is now in parallel
with the bottom resistor in the chain and — depending on
the sensitivity of the meter — these must be taken into
account if an accurate voltage reading is to be realized.
This problem can be mitigated by reducing the value of
series resistance (Rs) and adding a series potentiometer
(R1) to adjust for the correct voltage reading.
In this example, the meter is actually
reading 300 volts, which represents one
tenth of the actual voltage being
measured. The scale on the meter face
could show a voltage range of 0 to 5,000
volts, and R1 would then be set to a 3,000
volt deflection which would be 3/5 of full
To configure a meter to measure
current, a resistance (called a shunt) is
placed in parallel with it. Full-scale current measurements
ranging from microamperes to tens of thousands of
amperes are possible depending on the full-scale current
range of the meter.
For higher current measurements, three shunt
standard full-scale measurements are typically used: 50,
75, and 100 millivolts. The current and millivolt rating of
the shunt is usually marked on the side of the connection
The meter being used in parallel must be sensitive
enough to accommodate the millivolt range of the shunt.
Many panel meters are already designed to connect
directly to the shunt, and the full-scale of the meter in
millivolts is often shown in small letters on the meter face.
Refer to Figure 7.
The scale on the meter face is then scaled to conform
to the shunt current rating. A meter with a five milliampere
full-scale reading and an internal resistance of 10 ohms
will have a full-scale rating of 50 millivolts. Likewise, a
meter with a one milliampere full-scale reading and an
internal resistance of 50 ohms will also have a full-scale
rating of 50 millivolts.
For high currents, the resistance of the shunt will be
so much lower than the internal resistance of the meter (in
parallel) that the actual current measurement is affected
High current shunts quite often have one or
more parallel flat pieces of copper or brass in
parallel between the connecting posts as shown in
Figure 8. The width and thickness of the flat pieces
determine the resistance and hence the current
Several flat pieces are used in parallel to
prevent the shunt from heating due to the high
current, as heating will increase the resistance and
distort the meter reading. Used shunts will
sometimes have one or more small notches in the
side of one or more of the flat pieces.
These cuts or notches are usually made at the
factory, slightly increasing the parallel resistance of
the shunt to accurately adjust the meter reading.
The full-scale range of any meter that is sensitive
A 500 amp