TM 5-6675-323-14
V o l t a g e b a l a n c e c i r c u i t . T h e X- and Y-plotter motors form inductive loads
am.
across the 34 volt supplies. With the switching action of the motor drivers, it is
possible to generate counter-induced voltages which would drive one or the other of
the 34 volt supplies to a high level. To prevent the possibility of this happening,
a balance circuit has been provided for the 34 volt supplies. Any imbalance
created by the counter-induced voltages is sensed at the junction of resistors R66
and R74 in the balance circuit. With a balanced condition, this point is maintained
at zero volts.
(1) Any voltage at the tie point is sensed at the inverting input of the
comparator US along with the integrated signal from the 20 kHz oscillator. With the
+34 volts high, during the negative half-cycle output from U5, transistor Q27 will
be conducting. This turns on transistor Q18, which in turn gates the FET Q20 on,
d r a w i n g c u r r e n t f r o m t h e + 3 4 v o l t s u p p l y . T h e FET Q20 will pump current into
inductor L3. During the positive half-cycle out of U5, transistors Q26 and Q21 will
b e turned on, gating the FET Q22 on.
The current established in L3 continues to
flow during this half-cycle through FET Q22, acting as a diode, in a direction
opposite the normal flow. This current charges the -34 volt supply, and this
action, combined with the loading of the +34 volt supply, brings the two supplies
into balance. When the supplies are in balance, a n u l l w i l l a g a i n e x i s t a t t h e t i e
point of the resistors R66 and R74.
(2) The comparator U4 and its associated components acts as a safety system
by sensing the average current through the inductor L3 and the resistor R61. If
t h i s current tries to exceed 3 amps, Q24 and Q25 feed back a signal to limit the
current at 3 amps to prevent circuit damage.
(3) During power-up or reset, the balance circuit is disabled by transistor
Q28. This disabling action prevents any attempt at a balancing action before all
voltages have had time to stabilize.
an. Positive 5 volt switching supply.
Switching circuitry is used to develop a
regulated +5 volts from the unregulated +34 volt supply. The +34 volts is applied
W h e n the FET is conducting, a ramp of current is developed in the
to the FET Q15.
inductor L2, which charges the capacitor C15 to the output voltage of +5 volts. The
20 kHz oscillator output is integrated, and the sawtooth is applied to the compara-
tor along with the 5 volt reference from the reference supply, and the +5 volt
s e n s e , which is the +5 volt output from the power supply. The output of U3 switches
If the +5 volt sense
according to the relationship between the two 5 volt levels.
is low, the on time of the FET Q15 will increase, increasing the current flow in L2,
thus increasing the charge on C15.
If the +5 volt sense is high in comparison with
t h e reference, the on time of Q15 will be reduced, allowing the charge on C15 to
bleed down to the proper level.
ao.
Positive 12 volt switching supply. The functioning of the +12 volt switching
supply is essentially identical to that of the +5 volt switching supply. The
This
resistors R14 and R16 at the non-inverting input of U3 form a voltage divider.
divider drops the feedback voltage from the +12 volt output of the supply allowing
the use of the +5 volt reference voltage as the reference in this supply as it is in
the 5 volt switching supply.
ap.
Negative 12 volt supply. The -12 volt supply is a low current, Zener-
r e g u l a t e d e m i t t e r - f o l l o w e r c i r c u i t . The thyristor Q32 has been included for circuit
overload protection.
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