Load switches with controlled limited turn-on times, launched from several peer points on two wires, can significantly save energy.
Load switches with controlled limited turn-on times, launched from several peer points on two wires, can significantly save energy. Such switches can be used at the entrance to the premises, passage through corridors and halls, etc.
The previously known load switch-timer used a sequence of normally open buttons in its operation, which limited the maximum possible load current .
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The advantage of the switches described below is that you can control their operation from several points using just two wires and a few normally open buttons connected in parallel.
The device (Figure 1) works as follows. LED1 indicates the presence of the supply voltage and the readiness of the device for operation. Capacitor C1, energy storage, is charged through the diode VD1 to the supply voltage. Both transistors VT1 and VT2 are closed. A minimum current of about 5 mA flows through the load (lamp EL1 or relay K1).
Figures 1 Circuit of a load-timer switch with control from several points with low-voltage power supply.
Pressing either of the SB control buttons opens both transistors, The capacitor C1 begins to discharge through transistor VT1, potentiometer R3 and resistor R4. Transistor VT2 opens and connects the load to the power source for the duration of the discharge of capacitor C1. This time is determined by the RC-constant discharge chain C1-R3-R4 and is adjustable from 5 to 40 seconds. Then the load is disconnected, the device is ready for the next start.
The second device (Figure 2) is slightly different from the previous one. In it, the BJT is replaced by a MOSFET (VT1). The turn-on time on time of this device is from 6 to 60 seconds.
Figures 2 MOSFET variant of the load-timer switch scheme with control from several points with low-voltage power supply.
The third device (Figure 3) is designed to operate on 120 V 60 Hz AC power. Its turn-on time is also determined by the constant of the RC-chain C3-R5-R6 and ranges from 10 to 80 seconds.
Figures 3 Diagram of a load-timer switch with control from several points with network power.
When the power supply voltage is applied to the devices for the first time (Figure 1-3), the load is briefly connected to the power source, then disconnected, and the devices are ready for operation.
This article was originally published on EDN.