Swith For diagram

Switch For Switch Free Power Supplies

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 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.