0.1Ω series sense resistor and an
oscilloscope (the capacitor
remained in the circuit). The
results are shown in Figure 3.
Without the inductor, there is over
1A of peak-to-peak ripple current
(one vertical division in Figure 3
corresponds to 1A). As it’s clear
from this figure, the inductor
makes the whole load circuit
behave more like a variable
resistor from the ‘supply under
test’ INPUT standpoint (remember
that inductors tend to ‘oppose’
sudden current changes). The
measured input current is very
much a DC signal without
appreciable ripple, which is our goal.
It’s important to note that — unlike in the traditional
approach — most of the heat in this circuit is dissipated in
the incandescent lamps instead of the MOSFET. Since the
MOSFET is either turned OFF (close to infinite resistance)
or turned ON (close to zero resistance), the power
dissipated in the device is much lower than with the
traditional circuit. The incandescent lamps do the heavy
lifting here and dissipate most of the heat. Plus — unlike
the MOSFETs — incandescent light bulbs do not need
Furthermore, you get visual indication that the current
is flowing through the lamps, which I find is satisfying and
advantageous user feedback in this sort of test equipment.
42 July 2016
■ FIGURE 4.
■ FIGURE 3. Load current with and without inductor.
■ FIGURE 5. Schematic diagram.