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1-4. Over-current Protection
Over-current protection circuit for 350V line supplying
the power to lamp output detects the voltage drop of the
current detection resistor R107 applied to pin 5 of IC302.
When the load current of 350V line exceeds approx.
0.8A, pin 7 of IC302 develops high and the voltage is
added to pin 6 of IC303 in the same way as described
previously, thus carrying out the protection. Releasing its
operation is carried out in the same way as releasing the
over-voltage protection.
On S6V and +6V lines, the over-current protection is
carried out by detecting the voltage drop owing to the
current flow of L203 at pins 5 and 6 of IC401. When the
total current amount exceeds approx. 8A, pin 7 of IC401
develops high and the voltage biases the base of transis-
tor Q401 through the zener diodes D402 and D401. So,
the voltage is added to pin 6 of IC303 in the same way as
the over-voltage protection described previously, and the
over-current protection is carried out. Releasing its
operation is carried out in the same way as releasing the
over-voltage protection.
The over-current protection on +3.3V, +10V, +13V and
+16.8V lines is carried out by the over-current protection
circuit, which shows over-current protection characteris-
tic, built-in the series regulator IC (IC201, IC202, IC203
and IC204). In this case, the protection is only carried on
the line short-circuited and/or over-loaded, so nothing is
effected on other output lines. Releasing the protection is
carried out by clearing the over-current conditions. When
the overload condition occurs continuously, IC is
overheated and the overheat protection circuit inside the
IC works to shut down its output. In this case, releasing
the protection is carried out by lowering IC from over-
heating with the load current decreased.
1-5. Overheat Protection
As an overheat protection of the power supply, the
temperature of switching FET Q101 of the boost-up-
converter is detected. Positive characteristic thermistor
THP101 for temperature detection is attached on the heat
sink of Q101. When Q101 is overheated owing to the
overload and/or defect of cooling fan, etc., the thermistor
THP101 is also overheated. Then, the resistor value of
THP101 increases abruptly and the transistor Q301 turns
on, the voltage is added to the pin 6 of IC303, and the
overheat protection operation is carried out.
When releasing the overheat protection, stop to supply
the mains power, cool enough after more than approx.
120 seconds, and then re-supply the mains power.
1-2
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