Description
This circuit is particularly useful in controlling a load from
several locations where the load may be switched on from one location
and switched off from another. Any number of momentary (N/O) switches or
push buttons may be connected in parallel.
The circuit uses a N-channel power MOSFET to control the load and
can supply fairly large currents depending on the MOSFET used. The
IRFZ44 is a 50 amp device available at electronic stores.
Circuit Diagram
The combination (10K, 10uF and diode) on the left side of the
schematic insures the circuit powers on with the MOSFET turned off and
the NPN transistor conducting. These components can be omitted if the
initial power-on condition is not a concern. In this initial state
(MOSFET off), the voltage at the gate of the MOSFET will be near zero
and the voltage on the 1uF capacitor connected to the switches will also
be near zero.
When a switch is closed, the 1uF capacitor is connected to the
junction of the 220 ohm and 470K resistors causing the voltage to fall
to near zero turning off the NPN transistor. As the transistor turns
off, the collector voltage rises and turns on the MOSFET when the
voltage climbs above about 3 volts. The drain terminal (D) of the MOSFET
now moves close to ground preventing the NPN transistor from turning
back on. When the switch is opened, the 1uF cap will charge through the
1M and 10K resistors to the full supply voltage. When a switch is again
closed, the 1uF capacitor will cause the NPN transistor to turn back on
due to the positive voltage on the capacitor applied to the junction of
the two resistors (470K, 220). The MOSFET will now turn off and the
drain voltage will rise to the supply voltage which in turn keeps the
NPN transistor conducting with a positive voltage on the base. The
circuit has now returned to the initial turn-on state.
The small (0.1uF) capacitor connected from the transistor base
to ground functions to filter out noise that could cause false
triggering if the switches are located far away from the circuit using
long wires. If false triggering becomes a problem, either the capacitor
value (0.1) or the 220 ohm resistor value can be increased to provide
better filtering. Increasing these values however will increase the
switching times of the MOSFET (rise and fall times) generating more heat
when the MOSFET changes state. This is probably not a problem with
small loads of a couple amps or less, but may be a problem at higher
load currents. The circuit was tested at 1.5 amps using the IRF510 and 6
amps using the IRFZ44.
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