21
POWER
THERMISTOR
The POWER THERMISTOR is a device for
suppressing inrush current to an electric
circuit. Circuits including electric bulbs or
capacitors induce a inrush current more
than 100 times the normal current when the
circuit switch is turned on. The POWER
THERMISTOR in the circuits protects
electric equipments from being damaged
by limiting the inrush current.
MARK
is a NEW POWER THERMISTOR
featuring smaller size and larger energy
capacity to meet severe requirements in
inrush current suppression.
There is the tendency of miniaturization in
low power switching power supply, and
MARK
is effectively applicable for the
needs, though the comparatively larger
sized fixed resistors (ceramic coat) are
used now.
Application
How to use the power thermistor
The power thermistor will suppress inrush current
which is caused by a capacitor, filament for a bulb,
inverter for fluorescent lamp, a heater and etc., also
will control fan motor speed of cooler for electric
circuit.
Especially MARK
was developed to use for power
supply of TV, VCR instead of cement resistor.
Accordingly, suitable thermistors are 6D-22, 5D-18
and 8D-18, and if we consider in the points of small
time constant which means a small size and large
effect for suppressive current which means large
rated zero-power resistance, 8D-18 is the most
suitable one.
Use the following circuit in the power supply for 100V
and 200V.
The most suitable power thermistor for the above
circuit is required to fulfill the following terms and
conditions.
1. The permissible current at ambient temperature of
55 C should be over 2A.
2. The thermistor resistance for suppressive current
which becomes below 30A should be over 4.2
ohm from the under-mentioned formula.
t
SW
VE
IO
CL
Input voltage
: AC 100V 10%
Steady state current
: 2A
Capacitor
: 2000 F
Ambient temperature
: Max.55 C
Suppressive current
: Max.30A at 25 C
SW1
SW2
C2
C1
VE
L
2VE 1.1
30
RC R25
RC
: Internal resistance value in the circuit is 1 ohm
(100V/100A)
R25
: Rated zero-power resistance at 25 C
3. Max. capacitance shall be over 2000 F at AC 100V.
SW2 : ON (200V), OFF (100V)
t
t
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