High Voltage Detector Amanda Potter Can you think of an application for a circuit that turns on a load at a specified voltage? How about an electric heater or water pump could be turned on when there's excess power. Or a drain back valve of a passive water heating system could be operated at low temperatures. We'd like to turn a fan on at high voltages. This will disperse the hydrogen being produced by charging batteries. With very few modifications, Chris Graecen's short circuit regulator in issue 28 can be used to do this (see figure 1). Detect the Voltage The LM723 detects the voltage. It turns on when the input voltage rises above 7 Volts. The 3.0Kę / 3.3Kę voltage divider lowers the voltages to within this range. The 1Kę potentiometer can be varied to set the high voltage point. Ensure Against Oscillation The NE555 ensures that oscillation does not occur. The LM723 turns on the load when the voltage rises above the setpoint. Without the NE555, a large enough load will decrease the battery voltage enough to turn off the LM723. The result is that the LM723 and the load oscillate on and off. The NE555 stops this by turning on for a duration of time specified by the capacitor and resistor at pins 6 and 7 (time = 1.1RC). In our circuit, the fan turns on for a minute at a time. The output of the NE555 (pin 3) goes high when there is a falling pulse at the input (pin 2). The 2N2222A ensures that a rising pulse at the output of the LM723 produces a falling pulse at the input of the NE555. Turn on the Load For small DC loads, such as the fan, an N channel MOSFET can be used to turn on the load. A 2222A npn transistor cannot be used because it does not have a large enough gain. A high signal at pin 3 of the NE555 turns the MOSFET on allowing current to flow from the battery through the load and MOSFET to ground. The 1N4001 on pin 3 prevents the NE555 from sinking current. The 18V and 22V zener diodes protect the MOSFET; gate voltages higher than 20Volts and drain collector voltages higher than 60 Volts will blow up the FET. The circuit could also power a relay that drives an ac load. This can be done by replacing the load with a relay. Dark lines in the schematic indicate high current carrying wire. Sixteen gauge or large should be used, depending on the load. A diode and capacitor should be put in parallel with an inductive load or a relay. The capacitor keeps the voltage and current in phase. The diode stops the inductor from reverse biasing the FET by providing a path for current when the load is turned off. Cicuit Indication and Control The LEDs tell us what the circuit is doing. The green LED indicates that the circuit is on. The yellow LED indicates that the load is on. There are two switches in the circuit. The switch on the left side disconnects the circuit from the battery. The switch on the right side turns on the MOSFET, which turns on the load, independent of voltage.