Overvoltage Protection Circuit - Toshiba e-STUDIO170F Service Manual

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Overvoltage Protection Circuit

The overvoltage protection circuit monitors each power voltage (+24V, ±12V, +5V), which is output from

the secondary circuit of the transformer (T1), through the Zener diodes (ZD2-4).

When overvoltage occurs, it stops the oscillation operation of the primary circuit to prevent the overvolt-

age from being applied to each PBA.

When overvoltage occurs, the power voltage exceeds the constant-voltage of the Zener diode, the

cathode current changes, and current flows through the LED of PC3. This causes the light detector of

PC3 to conduct to turn on Q1 so that Q3 will turn off due to a drop in its gate voltage and cut off the cur-

rent in the primary circuit of the transformer. Cutting off the current in the primary circuit causes the

emission power of the secondary circuit to drop and the power voltage to drop, thereby preventing over-

e-STUDIO170F Circuit Description

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