220V Remote Load Monitor

How to monitor the working status of AC electrical appliances at a distance is a problem that many electricians often encounter in their work. The AC 220V remote load monitor introduced below can solve this problem simply and effectively.

How it works

The monitor circuit is shown in the dotted box in the figure above, and the circuit outside the box is a controlled remote load circuit diagram drawn for the convenience of explaining the principle.

When the power switch SA is closed, the load is powered on, wherein the load current is equal to the current flowing through the crystal diodes VD1 to VD4.

Since the forward conduction voltage drop of each crystal diode is about 0.7V, an alternating positive and negative AC voltage drop of about 1.4V can be generated between points a and b. The AC voltage triggers the bidirectional thyristor VS to conduct through the current limiting resistor R, so that the controlled indicator lamp H is powered on and illuminated. Once a circuit breaker fault occurs in the load or power transmission line, the voltage drop on VD1~VD4 will disappear immediately. VS will be cut off when the AC power passes through zero due to the lack of trigger voltage, and H will be powered off and stop emitting light. The working condition of the remote load can be judged by whether the lamp H is emitting light or not.

Component Selection

VS uses 1A, 400~600V ordinary small plastic-encapsulated bidirectional thyristors. Such as MAC97A6, BCR1AM-6, Z0105, etc. VD1~VD4 uses 1N5400 or 2CZ series silicon rectifier diodes. The maximum reverse working voltage URM does not need to be considered. The maximum rectifier current Ipm is determined according to the power of the controlled load. For example, when a 1N5400 silicon rectifier diode is selected as shown in the figure above, Ipm=3A, then the maximum load power can be 1000W (referring to pure resistive load).

H uses a 220V low-wattage tungsten filament light bulb, and R uses an RTX-1/4W metal film resistor.

Production and use

The figure below is the wiring diagram of the printed circuit board. The actual size of the printed circuit board is only 40mm×25mm.

The soldered circuit board can be placed in an insulated and sealed small box of suitable size. The lamp holder of the indicator lamp H is fixed on the box panel, and three external wires a, b, and c are led out from the side of the box. The circuit can also be directly installed on the distribution board of the AC 220V remote load.

The assembled 220V remote load monitor can be put into use without any debugging as long as the quality of circuit components is guaranteed and the welding is correct.