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TN22
3/7
This thyristor has been designed for use as a fluo-
rescent tube starter switch.
An electronic starter circuit provides :
I
A pre-heating period during which a heating
current is applied to the cathode heaters.
I
One or several high voltage striking pulses
across the lamp.
TN22
R
S
STARTER CIRCUIT
CONTROLLER
(TIMER)
INDUCTANCE
BALLAST
FLUORESCENT
TUBE
AC
VOLTAGE
BASIC APPLICATION DIAGRAM
PRINCIPLE OF OPERATION
1/ Pre-heating
At rest the switch S is opened and when the mains
voltage is applied across the circuit a full wave rec-
tified current flows through the resistor R and the
TN22 gate : at every half-cycle when this current
reaches the gate triggering current (I
GT
) the thyris-
tor turns on.
When the device is turned on the heating current,
limited by the ballast choke, flows through the tube
heaters.
The pre-heating time is typically 2 or 3 seconds.
2/ Pulsing
At the end of the pre-heating phase the switch S is
turned on. At this moment :
If the current through the devices is higher than the
holding current (I
H
) the thyristor remains on until
the current falls below I
H
. Then the thyristor turns
off.
If the current is equal or lower than the holding cur-
rent the thyristor turns off instantaneously.
When the thyristor turns off the current flowing
through the ballast choke generates a high voltage
pulse. This overvoltage is clamped by the thyristor
avalanche characteristic (V
BR
).
Ifthelampisnotstruckafterthefirstpulse,thesys-
tem starts a new ignition sequence again.
3/ Steady state
When the lamp is on the running voltage is about
150V and the starter switch is in the off-state.
IMPLEMENTATION
The resistor R must be chosen to ensure a proper
triggering in the worst case (minimum operating
temperature) according to the specified gate trig-
gering current and the peak line voltage.
Switch S : This function can be realized with a gate
sensitive SCR type : P0130AA 1EA3
This component is a low voltage device (< 50V)
and the maximum current sunk through this switch
can reach the level of the thyristor holding current.
The pre-heating period can be determined by the
time
constant
of
a
capacitor-resistor
charged by the voltage drop of diodes used in se-
ries in the thyristor cathode.
circuit