Transformer Less Power Supply

                                               

          If we want to convert a high voltage AC to a low voltage AC, then the conventional method is to use a step-down transformer. But as we all know transformers are bulky, heavy and costly. So in this section we are using an alternative method in stepping down the voltage rather than using a transformer. This is done by the use of a voltage dropping capacitor in series with the phase line. Thus this type of power supply is called a transformer less power supply.

        The above figure shows a typical transformer less power supply which can convert a 230v AC to a lower voltage DC. Selection of the Dropping capacitor and the circuit design requires some technical knowledge and practical experience to get the desired voltage and current. An ordinary capacitor will not do the job since the device will be destroyed by the rushing current from the mains. Mains spikes will create holes in the dielectric and the capacitor will fail to work. X-rated Capacitor specified for the use in AC mains is required for reducing AC voltage

         Before selecting the dropping capacitor, it is necessary to understand the working principle and operation of the dropping capacitor. The X rated capacitor is designed for 250,400, 600 VAC. Higher voltage versions are also available. The Effective Impedance ( Z ),Reactance ( X ) and the Mains frequency ( 50 – 60 Hz ) are the important parameters to be considered while selecting the capacitor. The reactance (X ) of the capacitor ( C )in the mains frequency ( f ) can be calculated using the formula 𝑋 = 1 / (2 𝜋 𝑓 𝐶 ) For example the reactance of a 0.22 uF capacitor running in the mains frequency 50Hz will be X = 1 / {2 π x 50 x 0.22 x ( 1 / 1,000,000) } = 14475.976 Ohms 0r 14.4 Kilo ohms. Reactance of the capacitor 0.22 µF is calculated as 𝑋 = 1 / (2 𝜋 𝑓 𝐶 ).Where f is the 50 Hz frequency of mains and C is the value of capacitor in Farads. Therefore the rectance of the capacitor appears as 14475.97 Ohms or 14.4 K Ohms. To get current just divide mains Volt by the reactance in kilo ohm. That is 230 / 14.4 = 15.9 mA. Effective impedance (Z) of the capacitor is determined by taking the load resistance ( R ) as an important parameter. Impedance can be calculated using the formula Z = v R + X Suppose the current in the circuit is I and Mains voltage is V then the equation appears like I = V / X The final equation thus becomes I = 230 V / 14. 4 = 15.9 mA. Therefore, if a 0.22 µF capacitor rated for 230 V is used, it can deliver around 15 mA current to the circuit. But this is not sufficient for many circuits. So it is recommended to use a 470 nF capacitor rated for 400 V for such circuits to give required current. Table showing the X rated capacitor types and the output voltage and current without load. The table below shows the X rated capacitor types and the output voltage and current without load.

       Diodes used for rectification should have sufficient Peak inverse voltage (PIV). The peak inverse voltage is the maximum voltage a diode can withstand when it is reverse biased. 1N 4001 diode can withstand up to 50 Volts and 1N 4007 has a toleration of 1000 Volts. So a suitable option is a rectifier diode 1N4007. Usually a silicon diode has a forward voltage drop of 0.6 V. The current rating (Forward current) of rectifier diodes also vary. Most of the general purpose rectifier diodes in the 1N series have 1 ampere current rating. A Smoothing Capacitor is used to generate ripple free DC. Smoothing capacitor is also called Filter capacitor and its function is to convert half wave / full wave output of the rectifier into smooth DC. The power rating and the capacitance are two important aspects to be considered while selecting the smoothing capacitor. The power rating must be greater than the off load output voltage of the power supply. The capacitance value determines the amount of ripples that appear in the DC output when the load takes current. For example, a full wave rectified DC output obtained from 50Hz AC mains operating a circuit that is drawing 100 mA current will have a ripple of 700 mV peak-to-peak in the filter capacitor rated 1000 µF. The ripple that appears in the capacitor is directly proportional to the load current and is inversely proportional to the capacitance value. It is better to keep the ripple below 1.5 V peak-to-peaks under full load condition. So a high value capacitor (1000 µF or 2200 µF) rated 25 volts or more must be used to get a ripple free DC output. If ripple is excess, it will affect the functioning of the circuit especially RF and IR circuits. Zener diode is used to generate a regulated DC output. A Zener diode is designed to operate in the reverse breakdown region. If a silicon diode is reverse biased, a point reached where its reverse current suddenly increases. The voltage at which this occurs is known as “Avalanche or Zener “value of the diode. Zener diodes are specially made to exploit the avalanche effect for use in ‘Reference voltage ‘regulators. A Zener diode can be used to generate a fixed voltage by passing a limited current through it using the series resistor (R). The Zener output voltage is not seriously affected by R and the output remains as a stable reference voltage. But the limiting resistor R is important, without which the Zener diode will be destroyed. Even if the supply voltage varies, R will take up any excess voltage. The value of R can be calculated using the formula R = Vin – Vz / Iz Where Vin is the input voltage, Vz output voltage and Iz current through the Zener In most circuits, Iz is kept as low as 5mA. If the supply voltage is 18V, the voltage that is to be dropped across R to get 12V output is 6volts. If the maximum Zener current allowed is 100 mA, then R will pass the maximum desired output current plus 5 mA . So the value of R appears as R = 18 – 12 / 105 mA = 6 / 105 x 1000 = 57 ohms Power rating of the Zener is also an important factor to be considered while selecting the Zener diode. According to the formula P = IV. P is the power in watts, I current in Amps and V, the voltage. So the maximum power dissipation that can be allowed in a Zener is the Zener voltage multiplied by the current flowing through it. For example, if a 12V Zener passes 12 V DC and 100 mA current, its power dissipation will be 1.2 Watts. So a Zener diode rated 1.3W should be used. As this type of circuit is compact and light weighted we can use it in many applications. One of those is to make and LED table lamp using a plastic bottle. Here the output of the circuit is given to a series of LED bulbs and the body of the table lamp is made of plastic bottles.

 

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