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Clamping Circuit If a clamp is required, a power zener diode can be used as shown
in the figure. The maximum zener clamp voltage must not
exceed 75 volts.
If a clamp is required, the transformer secondary voltage must
be re-checked to insure that the minimum clamp voltage is not
exceeded under high line and low load conditions when there is
no regeneration. Otherwise, the zener might overheat and fail.
To determine the required diode power rating, start with a 5W
device and monitor the zener current with a current probe. Power
(in watts) is the average current (in amps) times the zener voltage.
Estimate the average current from the oscilloscope trace and
compute the power. Select a zener rated slightly higher than the
measured power.
If the average power is too high to be conveniently dissipated in a
zener diode, the active voltage clamp circuit shown below can be
used instead. Power is dissipated in the 25Ω, 30W resistor if the
Motor Power Supply voltage exceeds 75 volts.
Active clamp circuit
49.9K
4.02K
64.9K
0.01 uF
100V
4.02K
0.1 uF
LM336BZ
6415 Installation & Hardware Reference Manual - Rev E
Caution
+12
4.02K
8
2
3
4
2200PF
499K
MOTOR POWER SUPPLY
IN4002
7
1
22Ω
LM311
1K
0.1 uF
C - 9
25
Ω
30W
IRF633
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