Building a Cockcroft-Walton voltage multiplier cascade for DC high voltage generation.

I’m looking to build something capable of at least 300kV.

Overview

The Cockcroft-Walton (CW) multiplier is a voltage multiplier circuit that generates high DC voltages from a lower AC input without requiring a transformer. It uses a ladder network of capacitors and diodes to progressively boost voltage through each stage.

Theory

For an ideal $n$-stage multiplier with peak AC input voltage $V_{peak}$:

\[V_{out} = 2nV_{peak}\]

Under load the output voltage drops due to capacitor discharge. The voltage drop $\Delta V$ for a load current $I$ at frequency $f$ is approximately:

\[\Delta V = \frac{I}{fC} \left( \frac{2n^3}{3} + \frac{n^2}{2} \right)\]

The ripple voltage is given by:

\[V_{ripple} = \frac{I}{fC} \cdot \frac{n(n+1)}{2}\]

Where $C$ is the capacitance per stage (assuming all capacitors are equal).

Design Calculations

Target specifications:

• Output voltage: 300kV DC
• Estimated load current: ~100µA (typical for electrostatic applications)

Stage calculation:

If using a 10kV AC input ($V_{peak} \approx 14.1\text{kV}$):

\[n = \frac{V_{out}}{2V_{peak}} = \frac{300,000}{2 \times 14,100} \approx 11 \text{ stages}\]

Component Selection

Diodes

Each diode needs to handle $2V_{peak}$ plus some safety margin. Fast recovery types are preferred to minimise losses at higher frequencies.

I’m looking at the 2CL2FM (20kV) or 2CL2FP (30kV), cheap, readily available on eBay, and commonly used in CW multipliers. The higher voltage rating of the 2CL2FP gives more headroom which is nice when you’re stacking stages.

Capacitors

Similarly, capacitors will be 20-30kV rated ceramic or film types. Higher capacitance reduces ripple and voltage drop under load, but HV capacitors get expensive quickly so there’s a trade-off there.

Insulation

The plan is to eventually vacuum pot the whole assembly in epoxy, use a vacuum pump to degas the epoxy and eliminate any air bubbles that would become corona sites or flashover paths at these voltages.

For initial testing though, I’ll probably start with oil immersion. Transformer oil or mineral oil gives good insulation and makes it much easier to iterate on the design without committing to a permanent potting. Plus if something fails it’s not encased in a block of epoxy.

Applications

Cockcroft-Walton multipliers appear across a wide range of voltage scales, from small consumer devices to massive scientific instruments, anywhere high voltage DC is required.

I’ve included some photos of smaller circuits that utilise CW multipliers, from $5 bug zappers, eBay HV modules (used for ionisers, electrostatic experiments, etc) all the way up to the largest multipliers used in particle accelerators and nuclear physics. The Cockcroft-Walton generator at the Cavendish Laboratory was used in the first experiment to split the atomic nucleus in 1932.

Build

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