First, we want to explain how such a controller works and what’s
involved. A bipolar motor has two windings, and thus four leads. Each
winding can carry a positive current, a negative current or no current.
This is indicated in Table 1 by a
‘+’, a ‘–‘ or a blank. A binary counter (IC1) receives clock pulses, in
response to which it counts up or down (corresponding to the motor
turning to the left or the right). The counter increments on the
positive edge of the pulse applied to the clock input if the up/down
input is at the supply level, and it decrements if the up/down input is
at earth level.
The state of the counter is decoded to produce the conditions listed in
Table 2.
Since it must be possible to reverse the direction of the current in
the winding, each winding must be wired into a bridge circuit. This
means that four transistors must be driven for each winding. Only
diagonally opposed transistors may be switched on at any given time,
since otherwise short circuits would occur. At first glance,
Table 2
appears incorrect, since there seem to always be four active intervals.
However, you should consider that a current flows only when a and c are
both active. The proper signals are generated by the logic circuitry,
and each winding can be driven by a bridge circuit consisting of four
BC517 transistors.
![table 1](//2.bp.blogspot.com/_FdGFE8NBDgc/TE6GGCfRofI/AAAAAAAAEGg/oOPHS-gqTP4/s320/Bipolar+Stepper+Motor+Control+T1.gif)
Two
bridge circuits are needed, one for each winding. The disadvantage of
this arrangement is that there is a large voltage drop across the upper
transistors in particular (which are Darlingtons in this case). This
means that there is not much voltage left for the winding, especially
with a 5-V supply. It is thus better to use a different type of bridge
circuit, with PNP transistors in the upper arms. This of course means
that the drive signals for the upper transistors must be reversed. We
thus need an inverted signal in place of 1a. Fortunately, this is
available in the form of 1d.
![table 2](//2.bp.blogspot.com/_FdGFE8NBDgc/TE6GGYhYQHI/AAAAAAAAEGo/4Yw5_NoLRDA/s320/Bipolar+Stepper+Motor+Control+T2.gif)
The
same situation applies to 1b (1c), 2a (2d) and 2b (2c). In this case,
IC4 is not necessary. Stepper motors are often made to work with 12V.
The logic ICs can handle voltages up to 15 to 18 V, so that using a
supply voltage of 12 V or a bit higher will not cause any problems. With
a supply voltage at this level, the losses in the bridge circuits are
also not as significant. However, you should increase the resistor
values (to 22 kΩ, for example). You should preferably use the same power
supply for the motor and the controller logic. This is because all
branches of the bridge circuit will conduct at the same time in the
absence of control signals, which yields short-circuits.
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