Transformerless Power Supply Circuit

One of the major issues while designing  the electronic circuit is, making of low voltage DC supply from AC supply to give the power to  the electronic circuit.  The straight  method is the use of a step down transformer to decrease the high DC voltage to desired low DC voltage. Generally, the low voltage DC is  the low current o/p. The lowest cost and most apt method are using a voltage dropping capacitor in  series with the phase line.

Selecting the right capacitor for the circuit design requires some practical experience and technical knowledge on various electrical components to get the preferred voltage and current. A normal capacitor will not do this task since the circuit will damaged by the rushing current from the mains. So X-rated capacitor is used in AC mains to reduce the AC voltage. This article covers the working of transformerless power supply circuit and its designing steps.

Transformerless Power Supply

The theory of transformerless power supply is prevalent in consumer goods and this kind of power supply is used in low current applications. The basic steps to design a transformerless power supply involves the following steps such as

• X Rated capacitor
• Rectification
• DC Smoothing
• Voltage Regulation
• LED Indicator

X-Rated Capacitor

Before choosing the dropping capacitor, it is essential to know the operation and working principle of the dropping capacitor. The X-rated capacitor is intended for 250, 400, 600 VAC. Higher voltage forms are also available. There are many parameters to be considered before selecting this capacitor such as the mains frequency should be in between 50 to 60Hz, impedance and reactance. These parameters can be calculated using the following formula.

X=1/2πfc

For instance, the reactance of a 0.22 uF capacitor running at the mains frequency 50Hz will be  X = 1 / {2 ¶ x 50 x 0.22 x (1 / 1,000,000) = 14.4 Kilo ohms.

Rectification

The diode is used for rectification, that must have PIV (peak inverse voltage). Here, the PIV voltage is the maximum voltage where a diode can resist when it is reverse biased. The diode IN4001 can endure up to 50 volts and diode IN 4007 has a toleration of 1000 volts. There are the main characteristics of general purpose rectifier diodes. So In4007 diode is an appropriate option, generally a Si diode has a Forward biased voltage drop of 0.6 V. The current rating of rectifier diodes also varies.

DC Smoothing

A Smoothing Capacitor or filter capacitor is used to produce ripple free DC. The main function of this capacitor is to change the output of the half wave or  full wave rectifier into smooth DC. The capacitance and the power rating are two parameters to be considered while selecting the capacitor. The power rating should be superior than the off load o/p voltage of the power supply.The value of the capacitance controls the amount of ripples that perform in the DC output when the load takes current.

Voltage Regulation

Zener diode is used to produce a regulated DC o/p. This diode works  in the reverse bias mode only. If a Si diode is connected in reverse bias, then suddenly its reverse current increases.These diodes are specially designed to exploit the avalanche effect to use in ‘Reference voltage ‘regulators.This diode can be used to make a fixed voltage by passing an incomplete current through it using the series resistor (R). The output voltage of the Zener diode is not affected seriously by Resistor and the o/p remains as a stable reference voltage. But the resistor R is significant, without which the Zener diode will be damaged. The value of resistor R can be calculated using the following formula

R=Vin-Vz/Iz

In the above equation,Vin=input voltage, Vz=o/p voltage and Iz=current through the zener diode.

LED Indicator

The LED indicator is used as a power on indicator. A voltage drop happens across the LED  when it permits forward current. An LED can permit up to 30 to 40mA current without damaging the circuit. A current limiter circuit is used to protect LED from additional current which is flowing through it. The current limiting resistor can be chosen using the following formula

R=V/I

Where, R= Value of the resistor
V= Supply voltage
I= Allowable current

Transformerless Power Supply Circuit Diagram

The circuit diagram of transformerless power supply is shown below. This is a simple transformer less power supply circuit. Here  in the above circuit, 230V AC supply can be dropped by using 225K 400VX rated capacitor. The bleeder resistor R2 is used to remove the stored current from the capacitor when the circuit is detached. Resistor R1 guards the power supply circuit from inrush current  when the power is on.

A full wave rectifier including D1 through D4 is used to resolve the low voltage AC from the C1 capacitor and C2 capacitor eliminates ripples from the DC. By using the above circuit, around 24V at 100 mA current will be existing in the o/p.This 24V DC can be controlled to required o/p voltage using an appropriate 1 watt Zener diode. In the phase line, It is better to add a fuse in the phase line for the purpose of safety.

Thus,this is all about transformerless power supply with circuit diagram.The designing of this kind of power supply is suggested only for who are experienced  or competent in handling AC mains. So do not try this power supply circuit if you are not handled in High voltages. Furthermore, any queries regarding this concept or electronics project kits, please give your comments in the comment section below.Here is a question for you, what are the applications of tranformerless power supplies