By adding a few more components, efficiency of the power supply can be further increased. Transient protection is achieved in the capacitive power supply by adding an metal oxide varistor (MOV) across the phase and neutral (Fig. 6).
Resistor R2 acts as a bleeder to discharge C1 at power-’off’ and to filter EMI in the line. The MOV connected across the phase and neutral will suppress transients in the lines.
A resistive power supply using two series resistors instead of one (Fig. 7) reduces high-voltage transients and the potential across the resistors. Capacitor C1 and resistor R1 connected across phase and neutral lines act as filters to prevent EMI flowing into the AC lines. MOV also reduces transients.
Capacitive power supply with more power efficiency
An efficient power supply that can deliver 9V DC at 100 mA is shown in Fig. 8. 230V AC is stepped down to 24V AC by capacitor C1. This low-voltage AC is rectified by a bridge rectifier to give 20V DC. Capacitors C2, C3 and C4 eliminate ripples from the DC and store current to stabilise the output. Resistor R2 attenuates EMI generated in the circuit and capacitor C1 discharges through it when power is switched off. MOV gives transient protection.
Instead of a zener diode, a series regulator of the 78XX series can be used to give regulated output as in Fig. 9. Zener diodes ZD1 and ZD2 drop 24V AC to 15 volts, which is then rectified and filtered to get ripple-free 15V DC. IC 7812 regulates the output to 12V stabilised DC. The output current will be around 50 milliamperes, which is sufficient to drive most of the light load circuits.
Capacitive and resistive power supplies are directly powered from 230V AC. So adequate precautionary measures must be taken when building and installing these circuits. An AC-type switch should be placed in series with the phase line to break the power supply. Fixing a fuse will give an additional safety. Power supply section and the electronic circuit must be separated with sufficient spacing while enclosing in the cabinet. Proper shielding and sleeving are required in the AC side of the power supply.
If costly ICs are present in the circuit, do not use a transformerless power supply since these may get damaged in case of a short circuit. Instantaneous power surge in the lines, lightning and short circuits can cause damages in the circuit. Use shockproof cabinets with proper earthing. The circuit can give lethal shock if handled carelessly. So take utmost care during testing and installation. Do not touch any part of the circuit when it is powered.
The author is an associate professor and head of Department of Zoology, Government College for Women, Thiruvananthapuram, Kerala