Mains Manager


Very often we forget to
switch off the peripherals like monitor, scanner, and printer while
switching off our PC. The problem is that there are separate power
switches to turn the peripherals off. Normally, the peripherals are
connected to a single of those four-way trailing sockets that are
plugged into a single wall socket. If that socket is accessible, all the
devices could be switched off from there and none of the equipment used
will require any modification. Here is a mains manager circuit that
allows you to turn all the equipment on or off by just operating the
switch on any one of the devices; for example, when you switch off your
PC, the monitor as well as other equipment will get powered down
automatically.

You may choose the main equipment to control other gadgets. The main
equipment is to be directly plugged into the master socket, while all
other equipment are to be connected via the slave socket. The mains
supply from the wall socket is to be connected to the input of the mains
manager circuit. The unit operates by sensing the current drawn by the
control equipment/load from the master socket. On sensing that the
control equipment is on, it powers up the other (slave) sockets. The
load on the master socket can be anywhere between 20 VA and 500 VA,
while the load on the slave sockets can be 60 VA to 1200 VA. During the
positive half cycle of the mains AC supply, diodes D4, D5, and D6 have a
voltage drop of about 1.8 volts when current is drawn from the master
socket.

Diode D7 carries the current during negative half cycles. Capacitor
C3, in series with diode D3, is connected across the diode combination
of D4 through D6, in addition to diode D7 as well as resistor R10. Thus
current pulses during positive half-cycles, charge up the capacitor to
1.8 volts via diode D3. This voltage is sufficient to hold transistor T2
in forward biased condition for about 200 ms even after the controlling
load on the master socket is switched off. When transistor T2 is ‘on’,
transistor T1 gets forward biased and is switched on. This, in turn,
triggers Triac 1, which then powers the slave loads. Capacitor C4 and
resistor R9 form a snubber network to ensure that the triac turns off
cleanly with an inductive load.

Circuit diagram:

Mains Manager Circuit

Mains Manager Circuit Diagram

LED1 indicates that the unit is operating. Capacitor C1 and zener
ZD1 are effectively in series across the mains. The resulting 15V pulses
across ZD1 are rectified by diode D2 and smoothened by capacitor C2 to
provide the necessary DC supply for the circuit around transistors T1
and T2. Resistor R3 is used to limit the switching-on surge current,
while resistor R1 serves as a bleeder for rapidly discharging capacitor
C1 when the unit is unplugged. LED1 glows whenever the unit is plugged
into the mains. Diode D1, in anti-parallel to LED1, carries the current
during the opposite half cycles. Don’t plug anything into the master or
slave sockets without testing the unit.

If possible, plug the unit into the mains via an earth leakage
circuit breaker. The mains LED1 should glow and the slave LED2 should
remain off. Now connect a table lamp to the master socket and switch it
‘on’. The lamp should operate as usual. The slave LED
should turn ‘on’ whenever the lamp plugged into slave socket is
switched on. Both lamps should be at full brightness without any
flicker. If so, the unit is working correctly and can be put into use.

Note:
The device connected to the master socket must have its power switch on
the primary side of the internal transformer. Some electronic equipment
have the power switch on the secondary side and hence these devices
continue to draw a small current from the mains even when switched off.
Thus such devices, if connected as the master, will not control the
slave units correctly.

Though this unit removes the power from the equipment being
controlled, it doesn’t provide isolation from the mains. So, before
working inside any equipment connected to this unit, it must be
unplugged from the socket.


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