Switch For Switch-Free Power Supplies

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Portable radios,
CD-players and cassette recorders that can also be operated from mains
power often do not have a mains power switch, but instead are switched
off on the ‘DC side’. This means that the power supply is permanently
connected to the mains network if the mains cable is not unplugged. The
situation with equipment requiring a mains adapter is similar. This is
not ideal for the environment or for your pocketbook. The following
circuit allows power to be switched on manually and switched off
automatically, directly at the equipment. Optoelectronic isolation
between the mains voltage and the switching stage ensures continued
compliance with safety regulations.

Switching on:

The circuit consists of the switch-on stage T1 and the hold-on and
switch-off stage T2/T3. Both stages drive the power switch, which is
implemented using a semiconductor relay (IC1). The voltage from the two
button cells (2–3 V) is connected to the LED
of the semiconductor relay by pressing push-button switch S1. R1 allows a
diode current of around 10mA to flow. At the same time, T1 prevents a
‘charging current’ from flowing into the batteries when the semiconductor
relay that switches the mains voltage is energized by T2. Although such
a current can only flow while the push-button is pressed, this
possibility must be taken into account for safety reasons.

Circuit

Circuit diagram

When the LED of the semiconductor relay is
energized by the battery current, the triac connects the mains voltage
to the transformer of the power supply. The DC voltage provided to the
load is twice reduced by 0.65 V by diodes D2 and D3. This threshold
voltage, smoothed by C1, provides a base current for T3, which drives T2
into conduction. T2 in turn supplies current via R2 to LED D1 and the LED in IC1. R2 must be matched to the DC voltage of the equipment to allow a LED current of 10mA to flow. As long as the push-button is pressed, two LED currents flow, and together they should not amount to more than 20mA in order to avoid destroying the LED in IC1.

Switching off:

The voltage drop across D2 and D3 is only present if a current drawn
by the connected equipment flows from the output of the circuit. If this
current is interrupted by switching off the equipment, T3 and T2 will
be cut off. The semiconductor relay will then open, and the mains
voltage will be switched off. This switch-off process is delayed by
capacitor C1, so that (for example) you can exchange an audio cassette
without causing the recorder to be disconnected from the mains. For the
semiconductor relay, you should select a type having a zero-crossing
switch. This means that the triac will only switch on at the zero point
of the mains voltage, regardless of when the push-button is pressed.

Almost no current will thus flow at the instant when the triac
switches, which prevents inductive switching spikes and associated
interference. The S201S01 semiconductor relay used here can switch
currents up to 8 A (continuous) or 80 A (single-cycle peak). Figure 2
shows how to connect the circuit between the power supply and the
charging capacitor. When laying out the circuit board, ensure that all
components carrying mains voltage are separated from each other by at
least 3 mm and from the low-voltage area by at least 6 mm. Naturally,
the same considerations apply to fitting the circuit board into the
equipment to be switched. If there is not sufficient space inside the
equipment, the circuit can be fitted between the equipment and the mains
adapter as a sort of cable switch.