Overload Protector

In large installations, status indication circuits may be located far from the equipment being monitored. The power source energizing the status circuits, which are often comprised of power-transistor-driven incandescent indicators, is adversely affected by initially-high surge currents when the lamps are first triggered on. The interconnecting cables and their associated connectors also impose large momentary fault current conditions on the power supply if the cables, upon removal from the connector, are accidentally short circuited while troubleshooting.

To overcome the effects of the current surges a three-terminal current limiter is incorporated in the power supply to increase its overload current protection. The integrated-circuit limiter (component 78M24 in the schematic) has a maximum peak current passthrough of approximately 2 A; the limiter provides about 500 mA under steady-state conditions. The limiter is internally protected by built-in thermal shutdown. The status indicators are sourced from a 28.0-volt supply, but the limiter maximum voltage output is 24 volts. The reference (GND) terminal is biased slightly above ground to permit the limiter to operate at the output voltage requirement.

When the regulator is overloaded, it reduces the output voltage to a safe value via internal current limiting. The output voltage reduction increases the voltage differential across the limiter source/output terminals. The light-emitting diode (LED) connected across the terminals turns on, thus indicating a fault in the circuit being powered. After the fault is corrected the circuit returns to normal operation, and the LED is extinguished.

A separate current-control signal from the monitored circuit also can be used to turn off the power supply. The signal is fed to a pair of operational amplifiers. The amplifiers are connected for complementary output current required by the circuits being powered. The amplifiers, type LM2900 or equivalent, do not have closed-loop feedback to control gain. This provides full open-loop gain to drive the output transistors Q₁ and Q₂. The 1N649 diodes, in series with each of the transistor collector terminals, are used to prevent high transient currents (resulting from driving inductive loads) from destroying the output transistors. The diodes also protect the circuit if it is connected to a source higher than 28 volts.