Friday, 7 October 2011

Cockroft-Walton voltage multiplier.

Cockroft-Walton voltage multiplier (CW multiplier) is another type of voltage multiplier that is widely used apart from Marx generator discussed before. It was initially used by scientists John Cockroft and Ernest Walton to perform atom splitting experiment, which earned them the Nobel Prize in Physics in 1951.

I built a simple battery-powered CW multiplier just to get 2mm spark, which enough to lit up a match or a cigarette lighter. As I ve explained long ago, the output from the 555 inverter must be amplified using LM386 before sending to the transformer to get such spark, but what if I didnt have LM386 but instead bunches of diodes and capacitors in my possession? Those diodes and capacitors are enough to substitute the role of LM386, by 'amplifying' the voltage as a CW voltage multiplier.

The function of CW multiplier is very simple. The fed alternating current from the transformer's output will go into the input of the CW multiplier. On first half of the cycle (first half wave), it will charge the first capacitor through the first diode. On second half wave, it will discharge the first capacitor thru the second capacitor and the second diode, so now the voltage stored at the second capacitor will be twice the first voltage stored at the first capacitor. By the end of the whole cycle, second capacitor holds twice the voltage of the supply while the first capacitor holds nothing. This operation is the first stage of the entire operation. Since I have 13 diodes-capacitors pairs, therefore I used about 6 and a half stages of multiplying.

But why the spark is so small ie 2mm length?? That is becuz the voltage generated from the transformer is very low. I measured it was about 180 to 200 V only, comparable to its original inverted operation which is to step down the mains voltage (240V) to about 2-3V ( the transformer is salvaged from battery charger, which was the one i ve been using before). By using CW multiplier, I got 2mm spark which was about 600 V. Though there are about six stages of multiplying I couldnt get six time the voltage from transformer due to diodes internal resistances. Anyway, this is worth experimenting so u know how much u will get by CW-multiplying voltage from a single 9V battery, using just a common battery charger transformer and a 555 inverter.

Below is the circuit I ve used:

Below are the photos taken:

The overall circuit.

The CW multiplier part. Note the white glue tape roller. It was made a former of the spark gap, where the 2mm spark will be observed.

Mains step down transformer salvaged from battery charger

The inverter part. I ve used the 555 inverter circuit.