Q & A
■ FIGURE 6
greatly appreciate the help and
I connected an ohmmeter to the
+ and – leads of the motor and it
read 2. 3 ohms.
— Mike Wandishin
AI tested the circuit (Figure 6) in Switcher Cad and it worked great, but I built the circuit and could not
control the speed. I thought the small
transistors were running out of
current gain, so I used five amp
power transistors: TIP42C for Q1 and
MJE13007G for Q2. I think TIP41C
would work for Q2, as well. The
procedure I used was to remove C1
and adjust R1 for a collector voltage
of Q2 in the 0.5 to 1.0 voltage range.
This puts the circuit in the active
range; if either transistor is saturated,
the circuit will be stuck there.
First, I used 0.33 µF for C1 (I
didn’t have a 0.47 µF) and 100K for
R1. The circuit worked okay and had
a wide range of speed. I tried 2. 2 µF
for C1 and had to reduce R1 to 10K.
It still worked but with smaller speed
range. The motor I used was from
Jameco (part number 231917) with a
1.5 to 3 volts DC rating. I did not use
the diodes because the spiking was
The circuit is open loop, so
when the motor is loaded it will slow
down. Since a DC motor is also a
generator, the generated voltage
could be measured during the OFF
time of Q2 and fed back to a
controller. I experimented with this
idea using a CMOS 555 timer but
the IC — even though it works at 1.5
volts — has no drive capability.
Something could be built at three
volts but it gets complicated very
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