enough can be adjusted to the correct millivolt range
required by the shunt.
For instance, a meter will a full-scale current rating
of 50 microamperes and an internal resistance of 600
ohms can be adjusted to 50 millivolts full-scale by
adding the right amount of series resistance.
As an example, if the shunt is rated at 100 amperes
and has a parallel voltage of 50 millivolts, then the
resistance in series with the meter that is across the
shunt is calculated as:
(0.05 volts divided by 0.000050 amperes) = 1000
ohms minus the 600 ohms meter internal resistance =
A “Kelvin” configuration (shunt) resistor features
four leads rather than just the two connection points
shown in Figure 8. These four terminal resistors enable a
current to be applied through two opposite leads and a
sensing voltage to be measured across the other two
leads. The Kelvin configuration effectively eliminates the
resistance and temperature coefficient of the leads.
A Kelvin connection is essential for accurate sensing.
Current measurement using a shunt resistor and voltmeter
is particularly well-suited for applications involving
particularly large magnitudes of current. Refer to Figure 9.
In such applications, the shunt resistor’s resistance will
be in the order of milliohms or micro-ohms, so that only a
modest amount of voltage will be dropped at full current.
Resistance this low is comparable to wire connection
resistance, which means voltage measured across such a
shunt must be done in such a way as to avoid detecting
voltage dropped across the current-carrying wire
connections which may result in magnetically induced
When high voltage is involved, it is never a good idea
to put the current meter on the high voltage side of the
power supply. This could put thousands of volts inside the
meter on a front panel and can be very dangerous — even
lethal. Figure 10 shows the correct way to make the
When creating your own shunt out of
individual resistors, make sure that they are several
times oversized in wattage to prevent heating and
also to prevent possible failure. Always use several
resistors in parallel since one resistor could fail and
open up putting the full high voltage on the panel
If a full-scale meter reading of five amperes is
required and the full-scale voltage across the shunt
resistor is set to five volts, then a shunt resistance of
one ohm is required with a wattage rating of at
least five watts.
In this case, a good solution is to use ten 10
ohm/one watt resistors in parallel to produce the
one ohm resistance required. The meter is actually reading
zero to five volts, but the scale on the meter would read
zero to five amperes as each volt of deflection represents
One slight disadvantage when using this method to
measure the anode current of a tetrode or pentode
vacuum tube in a power amplifier is that the measurement
will show the combined currents of the anode and the
screen grid since the measurement is being made on the
cathode side of the circuit.
Measuring current, voltage, and resistance are
combined in an analog volt-ohm-milliammeter (VOM). An
example from the 1970s is the RadioShack Micronta
model shown in Figure 11. Several ranges of DC and AC
voltages and currents are selectable by a multi-position
Resistance is measured by first shorting the two test
leads and adjusting the needle to a full-scale reading,
which represents zero resistance. This is done to adjust
out the resistance of the test leads and is especially
FIGURE 9. Kelvin current.
October 2017 47