Low Voltage, High Current Time Delay Circuit

Low Voltage, High Current Time Delay Circuit

Circuit Description

In this circuit a LM339 quad voltage comparator is used to generate a
time delay and control a high current output at low voltage.
Approximatey 5 amps of current can be obtained using a couple fresh
alkaline D batteries. Three of the comparators are wired in parallel to
drive a medium power PNP transistor (2N2905 or similar) which in turn
drives a high current NPN transistor (TIP35 or similar). The 4th
comparator is used to generate a time delay after the normally closed
switch is opened. Two resistors (36K and 62K) are used as a voltage
divider which applies about two-thirds of the battery voltage to the (+)
comparator input, or about 2 volts. The delay time after the switch is
opened will be around one time constant using a 50uF capacitor and 100K
variable resistor, or about (50u * 100K) = 5 seconds. The time can be
reduced by adjusting the resistor to a lower value or using a smaller
capacitor. Longer times can be obtained with a larger resistor or
capacitor. To operate the circuit on higher voltages, the 10 ohm
resistor should be increased proportionally, (4.5 volts = 15 ohms).

Circuit diagram

Circuit

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