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SMALL CIRCUITS COLLECTION

005

Switch for

Switch-free Power Supplies

U. Reiser

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 man-

ually 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 con-

nected to the LED of the semiconductor relay by pressing

pushbutton switch S1. R1 allows a diode current of around 10

mA 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 ener-

gised by T2. Although such a current can

only ﬂow while the pushbutton is pressed,

this possibility must be taken into account

for safety reasons.

When the LED of the semiconduc-

tor relay is energised by the battery cur-

rent, 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 sup-

plies current via R2 to LED D1 and the LED

in IC1. R2 must be matched to the DC volt-

age of the equipment to allow a LED cur-

rent of 10 mA to ﬂow. As long as the push-

button is pressed, two LED currents ﬂow,

and together they should not amount to

more than 20 mA in order to avoid

destroying the LED in IC1.

1

D2

D3

IC2

BT1

1N4001

S201S02

R3

L

S201S02

1

3

2x

LR44

C1

S1

470 µ

T1

2

4

L'

R5

R1

120

D1

BC547

T3

R2

680

T2

R4

10k

BC557

BC547

020024 - 11

2

TR1

B1

mains

switching

unit

Switching off

The voltage drop across D2 and D3 is only

present if a current drawn by the con-

nected equipment ﬂows from the output of

the circuit. If this current is interrupted by

switching off the equipment, T3 and T2 will

L '

020024 - 12

24

Elektor Electronics

12/2002

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SMALL CIRCUITS COLLECTION

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 hav-

ing 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 pushbutton 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 volt-

age 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 equip-

ment 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.

(020024-1)

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