Transformer without AC to DC power supply (Part 10/13)

The capacitors used to reduce AC voltage are non-polarized X-rated capacitors. These X-rated capacitors are designed to pass high voltage AC according to their voltage rating (e.g. 250V, 400V, 600V). Any normal electrolytic capacitor will not work for this purpose. The electrolytic capacitor has a fixed polarity on its plates. Therefore, when AC is applied to its plates, the capacitor plates will get two opposite polarities by the positive and negative half of the AC. Therefore, in any of the AC cycles, the capacitor plates will get reverse polarity generating holes in the capacitor dielectric. Therefore, a normal electrolytic capacitor will break down due to the AC signal passing through the capacitor. The X-rated non-polarized capacitor can be placed into the mains to reduce 230 Vac. These capacitors need to be connected line to line and are built to be used in high voltage AC circuits.

The power supply circuit designed in this project takes 220-230V AC from the mains and steps it down to 5.67V DC. The circuit has a maximum current limit of 7.2 mA at the output.

Required components –

Component name	Specification	Amount
Capacitor C1	0.1uF, 400V ceramic	1
Capacitor C2	2200uF 25V	1
Capacitor C3	0.47uF 25V	1
Resistor R1	200k 1W	1
Resistor R2	100 ohms 1W	1
Diode D1-D4	1N4007	4
Zener diode D5	5.6V 500mW	1

Block diagram –

Fig. 1: Block diagram of transformer without AC to DC power supply

Circuit Connections –

The mains phase line is connected to a 0.1 uF 400V ceramic capacitor and a bleeder resistor (shown as R1 in the schematics) is connected in parallel to the capacitor. This RC circuit is connected in series to a full wave rectifier. The full bridge rectifier is constructed by connecting four 1N4007 diodes together designated as D1, D2, D3 and D4 in the schematics. The cathode of D1 and anode of D2 are connected to the phase line via RC circuit and the cathode of D4 and anode of D3 are connected to the neutral wire. The cathodes of D2 and D3 are connected, of which one terminal is taken for rectifier output and the anodes of D1 and D4 are connected, of which another terminal is taken for full wave rectifier output.

A 2200 uF capacitor is connected to the output terminals of the full-bridge rectifier for smoothing the DC pulse and a 5.6 V zener diode is connected to the output terminals of the circuit for voltage regulation.

The operation of the circuit can be interrupted by the following operations –

1. AC to AC Conversion

2. AC to DC Conversion – Full Wave Rectification

3. Smoothing

4. Voltage regulation

AC to AC Conversion

The mains voltage is approximately 220-230 V, which must be reduced to 5.6 V AC before being converted to DC at the output. To reduce the 220 Vac, a 0.1 uF 400 V X-rated capacitor is used in series with the phase line. Resistor R1 of 200 k ohms and nominal power of 1 W is connected in parallel to the capacitor. Resistor R1 is a bleed resistor used for safety purposes. The bleed resistor discharges the capacitor when the circuit is disconnected and prevents any electrical shock. The voltage drop across the capacitor depends on the value of the charge at the output and the reactance of the capacitor.

Capacitor Reactance

X= 1/ (2*π*f*C);

Taking π=3.14

AC power frequency (f) = 50 Hz

Capacitance (C) = 0.1 uF

Reactance (X) = 31.8 k ohm

The maximum current supplied by the source is derived as follows –

I(theoretical) = V/X (V = 230V AC)

I(theoretical) = 230/31800

I(theoretical) = 7.2 mA

Fig. 2: Circuit diagram of AC voltage reducer based on X-rated capacitor and bleeder resistor

AC to DC Conversion – Full Wave Rectification

The reduced AC voltage needs to be converted to DC voltage through rectification. Rectification is the process of converting AC voltage to DC voltage. There are two ways to convert an AC signal to DC. One is half-wave rectification and the other is full-wave rectification. In this circuit, a full-wave bridge rectifier is used to convert AC voltage to DC voltage. Full-wave rectification is more efficient than half-wave rectification as it provides full use of both the negative and positive sides of the AC signal. In the full-wave bridge rectifier configuration, four diodes are connected in such a way that current flows through them in only one direction, resulting in a DC signal at the output. During full-wave rectification, two diodes are forward biased and two other diodes are reverse biased.

Fig. 3: Full Wave Rectifier Circuit Diagram

During the positive half cycle of the supply, diodes D2 and D4 conduct in series while diodes D1 and D3 are reverse biased and current flows through the output terminal passing through D2, output terminal and D4. During the negative half cycle of the supply, diodes D1 and D3 conduct in series, but diodes D1 and D2 are reverse biased and current flows through D3, output terminal and D1. The direction of current in both directions through the output terminal in both conditions remains the same.

Fig. 4: Circuit diagram showing positive cycle of full wave rectifier

Fig. 5: Circuit diagram showing negative cycle of full wave rectifier

The 1N4007 diodes are chosen to build the full-wave rectifier because they have a maximum (average) forward current of 1A and, in reverse bias condition, can sustain peak reverse voltage of up to 1000V. This is why 1N4007 diodes are used in this design for full wave rectification.

Smoothing

Smoothing is the process of filtering the DC signal using a capacitor. The output of the full wave rectifier is not a constant DC voltage. The rectifier output has twice the frequency of the main sources, but contains ripples. Therefore, it needs to be smoothed out by connecting a capacitor in parallel to the output of the full-wave rectifier. The capacitor charges and discharges during a cycle, providing a constant DC voltage as output. Therefore, a 2200 uF capacitor (shown as C2 in the schematic) is connected to the output of the rectifier circuit. As the DC that has to be rectified by the rectifier circuit has many AC spikes and unwanted ripples, to reduce these spikes a capacitor is used. The capacitor acts as a filtering capacitor that shunts all the AC through it to ground. At the output, the remaining average DC voltage is smoother and ripple-free.

Capacitor C2 is of high value and a capacitor C3 of small value is connected parallel to it, so that capacitor C3 decreases the equivalent impedance of capacitor C2.

Fig. 6: Circuit diagram of smoothing capacitor for transformer without AC to DC power supply

Voltage regulation

To provide a regulated 5.6 V at the output, a 5.6 V 500 mV zener diode is connected in series to resistance R2. The diode provides regulated and stabilized voltage at the output, regardless of fluctuation in input voltage and variation in load current. The zener diode provides 7.2 mA current at the output. If a current level other than 7.2 mA is required in the charging circuit, the capacitor C1 with a different value can be replaced with the existing one.

Transformerless sources can only be used for low current loads. To draw the current in amps, a higher value of X-rated capacitor needs to be used so that the impedance of the circuit can be reduced. But during capacitor discharge, current will flow through the bleed resistor. Therefore, a very high watt resistor is required as the bleed resistor and a resistor with such a high watt rating is not viable. This is why this type of power supply can only be designed for circuits with low current demand.

Fig. 7: Circuit diagram of Zener diode based voltage regulator

Tests and precautions –

The following precautions must be taken while assembling the circuit –

 To reduce the AC voltage level, only an X-rated non-polarized ceramic capacitor should be used.

 The current rating of a bridge diode must be greater than or equal to the current required at the output. Otherwise, it will not be able to provide the required current at the output.

 Always use a capacitor after the rectifier circuit so that it can handle mains noise.

 The capacitor used in the circuit must have a higher rated voltage than the input supply voltage. Otherwise, the capacitors will start leaking current due to excess voltage on their plates and will explode.

 Always use a high-watt resistor according to the power dissipation of the circuit.

 The nominal power of the zener diode must be greater than or equal to the power dissipated by the circuit.

 The zener diode starts to heat up and gets damaged if the current flowing through it is greater than its current limit value (Iz = Pz/Vz = 0.5/5.67 = 88mA).

Once the circuit has been assembled, connect it to the electrical network and use a multimeter to take readings at the output. During the test, the voltage across the zener diode was measured 5.67 V and the voltage across the resistance R2 was measured 0.69 V. The current through the zener diode and the resistance R2 is calculated as follows –

I = V/R2

I = 0.69/100

I = 7mA

The power dissipation through the zener diode is calculated as follows –

P = Vz*I

P = 5.67*0.007

PL = 39mW

The circuit designed in this project can be used to supply power to low current loads and appliances such as electronic toys, LED lamps, and electronic appliances.

Circuit Diagram-Transformer-Less-AC-DC-Power Supply