Voltage Doubler Using 555 IC (Part 11/13)

Required components –

Fig. 1: List of components required for 555 IC based voltage doubler

Block diagram –

Fig. 2: Block diagram of voltage doubler based on 555 IC

Firstly, to step down the 230V AC, a 18V-0-18V center strip transformer is used. The secondary coil of the transformer is connected to a full bridge 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 the anode of D2 are connected to one of the secondary coils and the cathode of D4 and the anode of D3 are connected to the central ribbon of the secondary coil. The cathodes of D2 and D3 are connected, of which one terminal is taken from the output of the rectifier and the anodes of D1 and D4 are connected, of which another terminal is taken from the output of the full-wave rectifier.

To regulate the supply to the 12 V level, first, a 470 uF capacitor (shown as Cin in the schematic) is connected across the output terminals of the full-wave rectifier for smoothing purposes. For voltage regulation, IC LM-7812 is connected in parallel with the smoothing capacitor. The output is taken from the voltage output terminal of the 7812 IC. A 1 uF capacitor (shown as Cout in the schematic) is connected to 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 in the 555 IC is mounted at the output terminal of the voltage regulator with a 22 nF load capacitor connected to the output terminals of the source. The multivibrator circuit charges and discharges the capacitor to produce double voltage.

The working of the circuit can be divided into the following operations –

1. AC to AC Conversion

2. AC to DC Conversion – Full Wave Rectification

3. Smoothing

4. Transient Current Compensation

5. Voltage regulation

6. Short circuit protection

7. Voltage doubling using astable multivibrator

AC to AC Conversion

The voltage of the main sources is approximately 220-230 Vac, which needs to be reduced to the 12 V level. To reduce 220 Vac to 12 Vac, a step-down transformer with a center strip is used. The use of the center tap transformer allows generating positive and negative voltages at the input, however, only the positive voltage will be extracted from the transformer. The circuit experiences some drop in output voltage due to resistive loss. Therefore, a transformer with a high voltage rating greater than the required 12V needs to be used. The transformer must 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 reduces the main line voltage to +/- 18 Vac, as shown in the image below.

Fig. 3: 18-0-18V Transformer Circuit Diagram

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 36V AC to 36V DC. 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. 4: 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. 5: Circuit Diagram showing the positive cycle of the Full Wave Rectifier

Figure 6: Circuit diagram showing the negative cycle of the full-wave rectifier

1N4007 diodes are chosen to build the full wave rectifier because they have maximum (average) forward current of 1A and in reverse bias condition can sustain peak reverse voltage 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 470 uF capacitor (shown as Cin 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.

Voltage regulation

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

Compensating Transient Currents

At the output terminals of the voltage regulator, a uF capacitor (shown as Cout in the schematic) is connected in parallel. The capacitor helps in quick response to load transients. Whenever the output load current changes, there is an initial shortage of current, which can be met 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

A 10 uF capacitor is used in this circuit, so

C = 1uF

Iout = 1u (0.1/100u)

Iout = 1mA

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

Fig. 8: Transeint Current Compensator Circuit Diagram

Short circuit protection

Diode D5 is connected between the voltage input and voltage output terminals of the 7812 IC, to prevent the external capacitors (Cout in the schematics) from being discharged through the IC during an input short circuit. When the input is shorted, the cathode of the diode is at ground potential. The anode terminal of the diode is at high voltage as the capacitor is fully charged. Therefore, in this case, the diode is forward biased and all the capacitor discharge current passes through the diode to ground. This saves the feedback current regulator IC.

Fig. 9: Short circuit protection diode circuit diagram

Multivibrator Circuit

IC 555 in astable multivibrator mode is used at the voltage regulator output. In astable mode, the 555 generates a square wave at the output. In astable mode, the multivibrator does not have a stable state and its output continues to oscillate 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

Putting all the values,

F=2.7kHz

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

In the second half of the cycle, when pin 3 is at high voltage, diode D6 becomes reverse biased. This prevents capacitor C3 from discharging and diode D7 remains in a forward bias condition. Therefore C4 starts charging via C3 (C3 was fully charged in the previous stage) and also via the 12V potentiometer input power. The voltage of capacitor C3 and the input source add together and therefore the load voltage of C4 is twice the input supply voltage. Therefore, twice the input voltage is obtained at the output.

The following precautions must be taken while assembling the circuit –

• The input voltage of NE555 must not be exceeded beyond 16V, otherwise it may damage the IC.

• The nominal voltage of capacitors C3 and C4 must be twice the input voltage (i.e. maximum adjustable output voltage).

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

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

• The nominal voltage of a step-down transformer must be between 14.5 V and 27 V, which is the required input voltage for the LM-7812. Only in this range will the 7812 be able to provide a constant output voltage regulated between 11.4V and 12.6V. This is due to the fact that the LM-7812 itself experiences a voltage drop of around 2 to 3 v.

• Protection diode should always be used when using the capacitor after a voltage regulator IC, to prevent the IC from countercurrent during capacitor discharge.

• A capacitor must be used at the output of the rectifier so that it can deal with unwanted noise from the mains. Likewise, the use of a capacitor at the output of the regulator is recommended to deal with rapid transient changes and noise at the output. The value of the output capacitor depends on the voltage deviation, current variations and the transient response time of the capacitor.

• Capacitors used in the circuit must have a higher voltage rating than the input voltage. Otherwise, the capacitors will start leaking current due to excess voltage on their plates and will explode.

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

Fig. 11: Table listing the output characteristics of the 555 IC based voltage doubler

Therefore, the output voltage is almost twice the input voltage.

This circuit can be used in Geiger Counter to produce high voltage for the Geiger-Muller Tube. It can also be used in voltage detection and reference voltage adjustment (especially in analog-to-digital converters).

Circuit Diagram-555-IC-Based-Voltage-Doubler