Toshiba 4560 Service Manual page 311

18.11.2 +5V power circuit
This is a circuit which generates DC +5V voltage supplied to CPUs, logic ICs, etc. DC +24V supplied
from CN5 is dropped to +5V by a 3-terminal regulator IC9. In this circuit, R65 and R64 are protective
resistors which compensate for the power loss occurring due to the generation of +5V. When current
consumption for the +5V line is approx. 80 mA or less, the power loss is compensated for by R65 and IC9
but when the consumption current value exceeds 80 mA, the input voltage at IC9-pin 1 decreases due to
voltage drop in R65. When the emitter voltage of Q13 becomes 0.6V lower than its base voltage, Q13
starts conducting and by making excess current flow through R64, shares a part of the power loss.
In this way, by varying the protective resistance value with the magnitude of consumption current, the
heat caused by the power loss is distributed and dissipated.
Nov. 1997 © TOSHIBA 18 - 53 4560/4570 RADF