Voltage Multiplier
Voltage multiplier definition
The voltage multiplier is an electronic circuit that delivers the output voltage whose amplitude (peak value) is two, three, or more times greater than the amplitude (peak value) of the input voltage.
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The voltage multiplier is an electronic circuit that converts the low AC voltage into high DC voltage.
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The voltage multiplier is an AC-to-DC converter, made up of diodes and capacitors that produce a high voltage DC output from a low voltage AC input.
What is voltage multiplier?
Voltage multiplier power supplies have been used for many years. Walton and Cockroft built an 800 kV supply for an ion accelerator in 1932. Since that time the voltage multiplier has been used primarily when high voltages and low currents are required. The use of voltage multiplier circuits reduces the size of the high voltage transformer and, in some cases, makes it possible to eliminate the transformer.
The recent technological developments have made it possible to design a voltage multiplier that efficiently converts the low AC voltage into high DC voltage comparable to that of the more conventional transformer-rectifier-filter-circuit.
The voltage multiplier is made up of capacitors and diodes that are connected in different configurations. Voltage multiplier has different stages. Each stage is made up of one diode and one capacitor. These arrangements of diodes and capacitors make it possible to produce rectified and filtered output voltage whose amplitude (peak value) is larger than the input AC voltage.
Types of voltage multipliers
Voltage multipliers are classified into four types:
- Half-wave voltage doubler
- Full-wave voltage doubler
- Voltage tripler
- Voltage quadrupler
Half-wave voltage doubler
As its name suggests, a half-wave voltage doubler is a voltage multiplier circuit whose output voltage amplitude is twice that of the input voltage amplitude. A half-wave voltage doubler drives the voltage to the output during either positive or negative half cycle. The half-wave voltage doubler circuit consists of two diodes, two capacitors, and AC input voltage source.
During positive half cycle:
The circuit diagram of the half-wave voltage doubler is shown in the below figure. During the positive half cycle, diode D1 is forward biased. So it allows electric current through it. This current will flows to the capacitor C1 and charges it to the peak value of input voltage I.e. Vm.
However, current does not flow to the capacitor C2 because the diode D2 is reverse biased. So the diode D2 blocks the electric current flowing towards the capacitor C2. Therefore, during the positive half cycle, capacitor C1 is charged whereas capacitor C2 is uncharged.
During negative half cycle:
During the negative half cycle, diode D1 is reverse biased. So the diode D1 will not allow electric current through it. Therefore, during the negative half cycle, the capacitor C1 will not be charged. However, the charge (Vm) stored in the capacitor C1 is discharged (released).
On the other hand, the diode D2 is forward biased during the negative half cycle. So the diode D2 allows electric current through it. This current will flows to the capacitor C2 and charges it. The capacitor C2 charges to a value 2Vm because the input voltage Vm and capacitor C1 voltage Vm is added to the capacitor C2. Hence, during the negative half cycle, the capacitor C2 is charged by both input supply voltage Vm and capacitor C1 voltage Vm. Therefore, the capacitor C2 is charged to 2Vm.
If a load is connected to the circuit at the output side, the charge (2Vm) stored in the capacitor C2 is discharged and flows to the output.
During the next positive half cycle, diode D1 is forward biased and diode D2 is reverse biased. So the capacitor C1 charges to Vm whereas capacitor C2 will not be charged. However, the charge (2Vm) stored in the capacitor C2 will be discharged and flows to the output load. Thus, the half-wave voltage doubler drives a voltage of 2Vm to the output load.
The capacitor C2 gets charged again in the next half cycle.
The voltage (2Vm) obtained at the output side is twice that of the input voltage (Vm).
The capacitors C1 and C2 in half wave-voltage doubler charges in alternate half cycles.
The output waveform of the half-wave voltage doubler is almost similar to the half wave rectifier with filter. The only difference is the output voltage amplitude of the half-wave voltage doubler is twice that of the input voltage amplitude but in half wave rectifier with filter, the output voltage amplitude is same as the input voltage amplitude.
The half-wave voltage doubler supplies the voltage to the output load in one cycle (either positive or negative half cycle). In our case, the half-wave voltage doubler supplies the voltage to the output load during positive half cycles. Therefore, the output signal regulation of the half-wave voltage doubler is poor.
Advantages of half-wave voltage doubler
High voltages are produced from the low input voltage source without using the expensive high voltage transformers.
Disadvantages of half-wave voltage doubler
Large ripples (unwanted fluctuations) are present in the output signal.
Full-wave voltage doubler
The full-wave voltage doubler consists of two diodes, two capacitors, and input AC voltage source.
During positive half cycle:
During the positive half cycle of the input AC signal, diode D1 is forward biased. So the diode D1 allows electric current through it. This current will flows to the capacitor C1 and charges it to the peak value of input voltage I.e Vm.
On the other hand, diode D2 is reverse biased during the positive half cycle. So the diode D2 does not allow electric current through it. Therefore, the capacitor C2 is uncharged.
During negative half cycle:
During the negative half cycle of the input AC signal, the diode D2 is forward biased. So the diode D2 allows electric current through it. This current will flows to the capacitor C2 and charges it to the peak value of the input voltage I.e. Vm.
On the other hand, diode D1 is reverse biased during the negative half cycle. So the diode D1 does not allow electric current through it.
Thus, the capacitor C1 and capacitor C2 are charged during alternate half cycles.
The output voltage is taken across the two series connected capacitors C1 and C2.
If no load is connected, the output voltage is equal to the sum of capacitor C1 voltage and capacitor C2 voltage I.e. C1 + C2 = Vm + Vm = 2Vm. When a load is connected to the output terminals, the output voltage Vo will be somewhat less than 2Vm.
The circuit is called full-wave voltage doubler because one of the output capacitors is being charged during each half cycle of the input voltage.
Voltage tripler
The voltage tripler can be obtained by adding one more diode-capacitor stage to the half-wave voltage doubler circuit.
During first positive half cycle:
During the first positive half cycle of the input AC signal, the diode D1 is forward biased whereas diodes D2 and D3 are reverse biased. Hence, the diode D1 allows electric current through it. This current will flows to the capacitor C1 and charges it to the peak value of the input voltage I.e. Vm.
During negative half cycle:
During the negative half cycle, diode D2 is forward biased whereas diodes D1 and D3 are reverse biased. Hence, the diode D2 allows electric current through it. This current will flows to the capacitor C2 and charges it. The capacitor C2 is charged to twice the peak voltage of the input signal (2Vm). This is because the charge (Vm) stored in the capacitor C1 is discharged during the negative half cycle.
Therefore, the capacitor C1 voltage (Vm) and the input voltage (Vm) is added to the capacitor C2 I.e Capacitor voltage + input voltage = Vm + Vm = 2Vm. As a result, the capacitor C2 charges to 2Vm.
During second positive half cycle:
During the second positive half cycle, the diode D3 is forward biased whereas diodes D1 and D2 are reverse biased. Diode D1 is reverse biased because the voltage at X is negative due to charged voltage Vm, across C1 and diode D2 is reverse biased because of its orientation. As a result, the voltage (2Vm) across capacitor C2 is discharged. This charge will flow to the capacitor C3 and charges it to the same voltage 2Vm.
The capacitors C1 and C3 are in series and the output voltage is taken across the two series connected capacitors C1 and C3. The voltage across capacitor C1 is Vm and capacitor C3 is 2Vm. So the total output voltage is equal to the sum of capacitor C1 voltage and capacitor C3 voltage I.e. C1 + C3 = Vm + 2Vm = 3Vm.
Therefore, the total output voltage obtained in voltage tripler is 3Vm which is three times more than the applied input voltage.
Voltage quadrupler
The voltage quadrupler can be obtained by adding one more diode-capacitor stage to the voltage tripler circuit.
During first positive half cycle:
During the first positive half cycle of the input AC signal, the diode D1 is forward biased whereas diodes D2, D3 and D4 are reverse biased. Hence, the diode D1 allows electric current through it. This current will flows to the capacitor C1 and charges it to the peak value of the input voltage I.e. Vm.
During first negative half cycle:
During the first negative half cycle, diode D2 is forward biased and diodes D1, D3 and D4 are reverse biased. Hence, the diode D2 allows electric current through it. This current will flows to the capacitor C2 and charges it. The capacitor C2 is charged to twice the peak voltage of the input signal (2Vm). This is because the charge (Vm) stored in the capacitor C1 is discharged during the negative half cycle.
Therefore, the capacitor C1 voltage (Vm) and the input voltage (Vm) is added to the capacitor C2 I.e Capacitor voltage + input voltage = Vm + Vm = 2Vm. As a result, the capacitor C2 charges to 2Vm.
During second positive half cycle:
During the second positive half cycle, the diode D3 is forward biased and diodes D1, D2 and D4 are reverse biased. Diode D1 is reverse biased because the voltage at X is negative due to charged voltage Vm, across C1 and, diode D2 and D4 are reverse biased because of their orientation. As a result, the voltage (2Vm) across capacitor C2 is discharged. This charge will flow to the capacitor C3 and charges it to the same voltage 2Vm.
During second negative half cycle:
During the second negative half cycle, diodes D2 and D4 are forward biased whereas diodes D1 and D3 are reverse biased. As a result, the charge (2Vm) stored in the capacitor C3 is discharged. This charge will flow to the capacitor C4 and charges it to the same voltage (2Vm).
The capacitors C2 and C4 are in series and the output voltage is taken across the two series connected capacitors C2 and C4. The voltage across capacitor C2 is 2Vm and capacitor C4 is 2Vm. So the total output voltage is equal to the sum of capacitor C2 voltage and capacitor C4 voltage I.e. C2 + C4 = 2Vm + 2Vm = 4Vm.
Therefore, the total output voltage obtained in voltage quadrupler is 4Vm which is four times more than the applied input voltage.
Applications of voltage multipliers
Voltage multipliers are used in:
- Cathode Ray Tubes (CRTs)
- Traveling wave tubes
- Laser systems
- X-ray systems
- LCD backlighting
- hv power supplies
- Power supplies
- Oscilloscopes
- Particle accelerators
- Ion pumps
- Copy machines
