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Design Note 26
Issue 2 January 1996
DN26 - 1
D.C. Motor Speed Control using the ZTX450
Introduction
The function of the following circuit is to
improve the load/speed regulation of a
d.c. machine. One of the main reasons
why the speed of a permanent magnet
field d.c. motor varies with load, is that
a voltage drop is current and hence load
dependent. The circuit given in Figure 1
stabilises the speed of the motor by
cancelling out the effect of the motor
rotor resistance using a bridge circuit.
Theory
If the bridge and variable resistor are
analysed on their own (Figure 2), a proof
can be derived which shows that the
control voltage (i.e. the speed) is not
directly dependent on the motor current.
(Please refer to Appendix).
The value of r
a
varies from motor to
motor, so the bridge must be balanced
to suit the motor employed.
M
Q2,3 - ZTX214C
Q2
Q3
R2
680
RV1
220
R1
270
U1 ZRA125
C2
0.1µF
Q1
ZTX450
C1
220pF
D1 1N4000
C3
100
µ
F
R3
22
1
ra
100
330
+6V
0V
Figure 1
DC Motor Speed Control Circuit.
