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Varistors Introduction
TECHNICAL NOTE
Technical Note
www.vishay.com
Vishay BCcomponents
Revision: 04-Sep-13
10
Document Number: 29079
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Switched-mode power supply protection
Protection of a thyristor bridge in a washing machine
BEHAVIOUR OF THE CIRCUIT WITHOUT
VARISTOR PROTECTION
The measured peak current through the pump motor when
S is closed is 1 A (see protection of a thyristor bridge in a
washing machine drawing). The energy expended in
establishing the electromagnetic field in the inductance of
the motor is therefore:
Without varistor protection, an initial current of 1 A will flow
through the thyristor bridge when S is opened, and a voltage
sufficient to damage or destroy the thyristors will be
developed. Arching will occur across the opening contacts
of the switch.
BEHAVIOUR OF THE CIRCUIT WITH
VARISTOR VDRS07H250 INSERTED
On opening switch S, the peak voltage developed across
the varistor is: V = C
max.
x I

= 600 V
The thyristors in the bridge can withstand this voltage
without damage.
The total energy returned to the circuit is 200 mJ. Of this
200 mJ. 15.1 mJ is dissipated in the heater, and 184.3 mJ is
dissipated in the varistor. The varistor can withstand more
than 10
5
transients containing this amount of energy.
For suppression of internally generated spikes in electronic
circuits, see Varistor used across a transistor or coil in a
television circuit and Varistor used across a switch or coil
drawings.
In both examples shown in the drawings Varistor used
across a transistor or coil in a television circuit and Varistor
used across a switch or coil, type VDRS05 should be used
for up to approximately 50 A, and type VDRS07 up to
approximately 120 A.
Varistor used across a transistor or coil in a television circuit
Varistor used across a switch or coil
Influence of a transient on the mains voltage
fuse
U
POWER
SUPPLY
to drum motor
heater
PUMP
MOTOR
U
S
220 V
50 Hz
R
H
= 24 Ω
R
p
33 Ω
L
0.4 H
back
e.m.f.
I
2
x
L
2
---
0.4
2
--------
200 mJ==
U
U
V
AB
clamping
voltage
mains
safe voltage
(with VDR)
dangerous voltage
(without VDR)
short
circuit
fuse
opens
V
AB
U
fuse
1.5 A
short
circuit
M
washing machine motor
220 W
220 V
mains
I
line
inductance
line
capacitance
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