The Cockcroft-Walton multiplier (AKA voltage doubler, voltage multiplier)

[ruxton.stanislaw]

The Cockcroft-Walton multiplier, also known as a voltage multiplier circuit, is a clever design using capacitors and diodes to achieve a high output voltage from a relatively low voltage AC or pulsating DC input. Here's a breakdown of how it works:

Basic Principle:

• It utilises a series of capacitors and diodes arranged in a ladder network.
• Each stage in the ladder network progressively increases the voltage.
• The output voltage is the sum of the individual voltage increases across each stage.

Components:

1. Input: The circuit requires a low voltage AC or pulsating DC input. An isolation transformer is often used at the beginning of the system to isolate the multiplier circuit from the main power grid.
2. Voltage Multiplier Stages: These stages consist of capacitors (C1, C2, etc.) and diodes (D1, D2, etc.) connected in a specific pattern. The number of stages determines the final voltage multiplication factor.
3. Output: The final high voltage DC output is taken from a specific point in the ladder network.

Operation:

Imagine the input voltage alternates between positive and negative. Here's what happens during one cycle (positive half cycle):

1. Charging the First Capacitor (C1): The positive voltage from the input pushes current through Diode D1 (forward biased) and charges Capacitor C1 to the peak value of the input voltage.
2. Voltage Addition at Node B: Since the bottom plate of C1 is connected to the output (point B), the voltage at point B becomes the input voltage (positive) plus the voltage stored across C1.
3. Charging Subsequent Stages: During this time, Diode D2 is reverse biased and blocks current flow. As the voltage at point B is now higher than the input voltage, the next capacitor (C2) charges through Diode D3 (forward biased) to this higher voltage.
4. Progressive Voltage Increase: With each stage, the charging process repeats, with the voltage at the previous stage adding to the input voltage to charge the next capacitor. This creates a stepping-up effect, progressively increasing the voltage across each capacitor in the ladder.

Negative Half Cycle:

During the negative half cycle of the input voltage, the process works in reverse, with the capacitors discharging through different diodes to maintain the overall voltage across the output.

Key Points:

• The final output voltage is approximately equal to the number of stages in the multiplier ladder multiplied by the peak voltage of the input.
• Cockcroft-Walton multipliers are relatively simple and inexpensive to build.
• They are well-suited for applications requiring high voltage but relatively low current, such as powering photomultiplier tubes or electrostatic lenses.

Limitations:

• The output voltage can be susceptible to voltage ripple depending on the design and load current.
• They are less efficient than transformer-based voltage converters, especially at higher output voltages.

Here are some resources for further learning:

• Circuit Cellar - Cockcroft-Walton Voltage Multiplier: https://en.wikipedia.org/wiki/Cockcroft–Walton_generator
• Wikipedia - Cockcroft–Walton generator: https://de.wikipedia.org/wiki/Cockcroft-Walton-Beschleuniger