Capacitor charger (60-400V)

This is intended to be a short instructable on how to build a capacitor charger since I'll be quite busy until I finish my classes, it might not have as many images as I would like to, but after getting some requests I've decided to take a pair of hours to make it.

First of all, this instructable provides information on how to build a device capable of outputting up to 800V DC, you should have some experience with electronics and follow common sense to avoid getting shocked.

Keep your workspace clear, this reduces the risk a lot.
When testing the circuit never do it with both hands, use only one to activate it, and make modifications with the circuit off, this will avoid discharges through the heart.

Discharge your capacitors when you're not using them, it's easy to forget about them until you lay your hand on top.
Using gloves is a very good idea, standard latex gloves can be useful below 1,000V, I tested this by myself by putting a single layer of a latex glove between a high voltage supply, and it took about 10,000V to make a hole with the contacts touching the surface, I also tried touching both terminals of the capacitor bank when fully charged, no current was able to break through. Obviously you should not try this by yourself.

I don't want to be responsible of any incidents caused by the misuse of the information posted here.

Step 1: Introduction

This capacitor charger is intended to charge small capacitor banks, it's built around the 555 chip, so it can work with an input ranging from 5 to 16 volts, and a charge voltage from 60 to 400 volts. I designed it to work with 12 volts because it is what I think the most common voltage in that range. With that voltage the current draw is about 2 to 4 amps.

It has a comparator with an adjustable voltage divider, this means you can set a trigger by adjusting the potentiometer so the comparator can stop the circuit when the capacitors have reached the desired voltage, indicating this with 2 LEDs.

This design is based on this one by Uzzors2k with some differences:

1- My design uses a different 555 configuration (50% Duty cycle with a potentiometer to adjust the frequency)

This is done to get the optimal operation frequency.

2- My design uses a transformer, instead of an inductor.

This avoids overvoltages, if you run Uzzors2k's circuit without a capacitor load the chances are the circuit gets damaged. The low inductance of the primary helps to reduce voltage peaks, if the capacitors suddenly disconnected the circuit should still work afterwards.

3- I've included a 2N3904 transistor to be controlled by the comparator. I also added a red LED.

It will control the current flow into the pins 4 and 8 of the 555, turning it on or off depending on the comparator. As fas as I know, this isn't really necessary, you could just connect the output directly to the pin 4 with a resistor from Vcc to that pin to keep it on. The red LED acts in conjunction with the Green one, I installed it to check the state of the base of the transistor.

Is there a way to modify this down to do a lower voltage. Say 20v to 400v?

Yes, increase the value of R2 at the Voltage divider at the step 6, this translates into using a 220k or 500k pot instead of a 100k one. However I don't recommend using it to charge capacitors with such a low voltage rating, the charger outputs around 600 Volts or more, all the time, depending on the transformer, using it to charge small capacitors is overkill, and you'll waste energy by doing so, I would rather use another transformer with less and thicker secondary windings to charge capacitors rated for a lower voltage. You also risk damaging them, charging 20 volt capacitors with a 600 volts output is not wise, arcover inside the capacitor could occur specially if the capacitor is small.

Cool!What do you use to draw your circuit diagram?

I used Gimp, following the basic design the 555 circuit diagram previously had. Now I use the free version of Eagle, it's much more convenient and fast.

EDIT: I added a 220uF 16V capacitor across the positive and negative terminals. I did this because my PSU has a small current limit, and by placing a small cap like that I can store a power during the negative half cycle and release it during the positive half cycle. This way I can boost the output a bit more. Do this if you intend to use this with current limited PSUs or batteries with moderate to high internal resistance. In most cases it should add a bit more of power.

Nicely done, Ill try it out someday. :)

Thanks!