Voltage Doubler Using 555 IC (Part 11/13)

The important sections of the circuit designed in this project include stepping down AC voltage, converting AC voltage to DC voltage, Smoothing DC voltage, Compensating transient currents, short circuit protection and voltage doubling using 555 IC and capacitors.

First of all, for stepping down the 230 V AC, an 18V-0-18V center tape transformer is taken. The secondary coil of the transformer is connected with a full-bridge rectifier. The full bridge rectifier is built by connecting four 1N4007 diodes to each other designated as D1, D2, D3 and D4 in the schematics. The cathode of D1 and anode of D2 is connected to one of the secondary coil and cathode of D4 and anode of D3 is connected to the center tape of the secondary coil. The cathodes of D2 and D3 are connected from which one terminal is taken out from output of rectifier and anodes of D1 and D4 are connected from which other terminal is taken out from the output from full-wave rectifier.

For regulating supply to 12V level, first, a capacitor of 470 uF (shown as Cin in schematics) is connected between the output terminals of the full-wave rectifier for smoothing purpose. For voltage regulation LM-7812 IC is connected parallel to the smoothing capacitor. The output is drawn from the voltage output terminal of the 7812 IC. A capacitor of 1 uF (shown as Cout in schematics) is connected across the output of the voltage regulator to compensate transient currents. A diode D5 is connected between the Input voltage and output voltage terminals of the voltage regulator IC for short circuit protection. An astable multivibrator circuit on 555 IC is assembled at the output terminal of the voltage regulator with a charging capacitor of 22 nF connected at the output terminals of the supply. The multivibrator circuit charges and discharges capacitor to output double voltage.

The functioning of the circuit can be broken down into following operations –

1. AC to AC Conversion

2. AC to DC Conversion – Full Wave Rectification

3. Smoothing

4. Compensation of Transient Current

5. Voltage Regulation

6. Short Circuit Protection

7. Voltage doubling using astable multivibrator

The voltage of Main Supplies is approximately 220-230V AC which needs to be stepped down to 12V level. To reduce the 220V AC to 12V AC, a step-down transformer with center taping is used. The use of center tap transformer allows to generate both positive and negative voltage at the input, however, only positive voltage will be drawn from the transformer. The circuit takes some drop in the output voltage due to resistive loss. Therefore a transformer of high voltage rating greater than the required 12 V needs to be taken. The transformer should provide 1A current at the output. The most suitable step-down transformer that meets the mentioned voltage and current requirements is 18V-0-18V/2A. This transformer step downs the main line voltage to +/-18V AC, as shown in the below image.

The stepped down AC voltage needs to be converted to DC voltage through rectification. The rectification is the process of converting AC voltage to DC voltage. There are two ways to convert an AC signal to the DC one. One is half wave rectification and another is full wave rectification. In this circuit, a full-wave bridge rectifier is used for converting the 36V AC to 36V DC. The full wave rectification is more efficient than half wave rectification since it provides complete use of both the negative and positive sides of AC signal. In 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, at a time two diodes become forward biased and another two diodes get reverse biased.

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

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

Smoothing is the process of filtering the DC signal by using a capacitor. The output of the full-wave rectifier is not a steady DC voltage. The output of the rectifier has double the frequency of main supplies but contains ripples. Therefore, it needs to be smoothed by connecting a capacitor in parallel to the output of full wave rectifier. The capacitor charges and discharges during a cycle giving a steady DC voltage as the output. So, a capacitor of 470 uF (shown as Cin in the schematics) is connected to the output of rectifier circuit. As the DC which is to be rectified by the rectifier circuit has many AC spikes and unwanted ripples, so to reduce these spikes capacitor is used. The capacitor acts as a filtering capacitor which bypasses all the AC through it to the ground. At the output, the mean DC voltage left is smoother and ripple free.

The 7812 voltage regulator IC is used for obtaining constant 11.5 V from the input voltage. The 7812 IC can have input voltages from 14.5V to 27V and it provides a constant output voltage from 11.4V to 12.6V. The IC has maximum 1A current limit. At the output of LM7812, a potentiometer RV1 is connected. The variable probe of RV1 is connected to the input of 555 IC. This potentiometer helps in providing the variable input voltage to the 555 IC. The range of output voltage by this RV1 is from 5V to 12V. Therefore a variable voltage is obtained at the output.

At the output terminals of the voltage regulator, 1 uF capacitor ( shown as Cout in schematics) is connected in parallel. The capacitor helps in fast response to load transients. Whenever the output load current changes then there is an initial shortage of current, which can be fulfilled by this output capacitor.

The output current variation can be calculated by

Output current ,Iout = C (dV/dt) where

dV = Maximum allowable voltage deviation

dt = Transient response time

Considering dv = 100mV

dt = 100us

In this circuit a capacitor of 10 uF is used so,

C = 1uF

Iout = 1u (0.1/100u)

Iout = 1mA

This way it can be concluded that output capacitor will respond for 1mA current change for a transient response time of 100 us.

The diode D5 is connected between the voltage input and voltage output terminals of 7812 IC, so that it can prevent the external capacitors (Cout in schematics) from discharging  through the IC during an input short circuit. When the input is shorted then the cathode of the diode is at ground potential. The anode terminal of the diode is at high voltage since the capacitor is fully charged. Therefore in such a case, diode is forward biased and all the discharging current from capacitor passes through diode to the ground. This saves the regulator IC from the back current.

The 555 IC in astable multivibrator mode is used at the output of voltage regulator. In astable mode 555 generates a square wave at the output. In astable mode, multivibrator has no stable state and its output keeps oscillating between two unstable states. Therefore at the output, a square wave is obtained by the multivibrator.

The time period or frequency of the square wave is obtained by the resistor-capacitor time constant.

The square wave frequency for astable multivibrator can be determined by the following formula –

F = 1.44/ ( R1+ (2*R2))*C1

By putting all the values,

F = 2.7kHz

In the circuit Pin 3 is the output of the 555 IC hence a square wave of frequency 2.7kHz is available at pin 3 of the IC. At this pin, a wave with 50 percent duty cycle (equal time period for the high and low level) is produced. There are two capacitors(C3 and C4) and two diodes (D6 and D7) connected to pin 3 which help in providing the twice of the input voltage at the output. When pin 3 of the IC is at low voltage then diode D6 gets forward biased and capacitor C3 starts charging via D6 from 0V to adjusted voltage obtained from the potentiometer.

In the second half of the cycle when pin 3 is at high voltage then diode D6 gets reversed biased. This prevents the discharging of capacitor C3 and diode D7 is in forward biased condition. Therefore C4  starts charging via C3 (C3 was fully charged in the previous stage) and through the input supply from the potentiometer of 12V as well. The voltage from C3 capacitor and the input supply adds up and so the charging voltage of C4 is double of the input supply voltage. Therefore twice of the input voltage is obtained at the output.

The following precautions need to be taken while assembling the circuit –

• The input voltage of NE555 should not be exceeded beyond 16V otherwise it can damage the IC.

• The voltage rating of capacitors C3 and C4 must be double as of the input voltage (i.e. maximum voltage from adjustable output).

• The output voltage is not exactly double of the input voltage due to voltage drop in the circuit itself.

• The output diodes (D6 and D7 in schematics) should have a low forward voltage drop, otherwise, it will reduce the output voltage. The Schottky diodes like 1N5819 can be used because they have a low forward voltage drop.

• The voltage rating of a step-down transformer should be between 14.5V to 27V which is the required input voltage for LM-7812. In this range only, 7812 will be able to provide a regulated constant output voltage between 11.4V to 12.6V. This is due to the fact that the LM-7812 itself takes voltage drop of around 2- 3 V.

• The protection diode should always be used while using capacitor after a voltage regulator IC, for preventing the IC from back current while discharging of the capacitor.

• A capacitor should be used at the output of rectifier so that it can handle unwanted mains noise. Similarly, use of a capacitor at the output of the regulator is recommended for handling fast transient changes and noise at the output. The value of output capacitor depends on the deviation in the voltage, current variations and transient response time of the capacitor.

• The capacitors used in the circuit must be of higher voltage rating than the input voltage. Otherwise, the capacitors will start leaking the current due to the excess voltage at their plates and will burst out.

Once the circuit is assembled adjust the input voltage using a potentiometer and measure the output voltage. During testing of the circuit following readings were observed –

So the output voltage is nearly twice the input voltage.

This circuit can be used in Geiger Counter to produce a high voltage for the Geiger-Muller Tube. It can also be used in voltage sensing and the setting reference voltage (especially in Analog to digital converters).