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.

Step 2: Parts List

555 timer chip (I would buy a bunch of these since they come quite handy)
LM311 comparator chip
8pin chip sockets (so you can replace the chips)
Green and Red LEDs
3x 15k resistors
2x 1k resistors
1Mohm resistor
1x 100R resistor
1x 680R resistor
1x 6k8 resistor
10k potentiometer
100k potentiometer
2x UF4007 or BA159 fast recovery diodes
2N3904 (or similar) transistor
MOSFET (I used an IRF540N)
100nF ceramic capacitor
10nF ceramic capacitor
Transformer (obtained from CFL)
Enameled copper wire

You might also want to buy a terminal block with 2 outputs to make a solid connection with the capacitor bank.

Step 3: How It Works

If you are familiar with high voltages, you'll know the 555 is a very used IC to create a series of DC pulses to be fed into a MOSFET or transistor in order to increase the current in those pulses to drive a transformer and step up the voltage. This circuit does just that, but it also has an LM311, a comparator that stops the charge operation when a certain voltage in the capacitor bank is reached.

As their name indicates, comparators compare a fixed voltage, also called reference voltage, with a variable voltage, in this case the voltage of the capacitors. Obviously, if we fer 400V directly into the comparator it would be destroyed instantly, that's why we need to use voltage dividers, that are basically two resistors in series, the potential at the top and the values of the resistors will determine the voltage in between the two resistors assuming there's no output current, like our case. If that voltage is higher than the reference voltage the charger will stop. The LM311 is a bit special, since the output can only sink current, however this is not a problem, since we can use a 2.2k resistor from Vcc to the output to ensure a HIGH until the LM311 sinks the current coming from it creating a LOW.

You have the schematic below, you can start mounting everything in it's place.

Step 4: Making the Transformer

The transformer requires to be winded in a special way in order for it to work properly.

To do this we'll use a transformer from a CFL light bulb, this is a medium sized relative to all the ones I've collected over the time. In order to rewind it, we first need to take it apart, this is done by removing the adhesive surrounding the ferrite core, then, with a lighter, we heat it evenly until the glue used to keep the pieces joined together melts, and the two "E" parts of the core can be extracted. Don't breath the fumes since the glue used to hold it together is usually cheap Chinese stuff made out of who knows what...

Next we take out the copper wire, we cut one end and pull from it until it's all out, in theory this wire could be reused, but many transformers are dipped in that sort of glue or resin, coating the wire and making it difficult to use it again.

Once we have the plastic part and the two "E" parts of the core, we can start winding, it's recommendable to wash them first to clean any residue, don't drop the core since it's quite fragile and it will probably break.

I coiled about 10 to 20 turns of ~0.5mm copper wire I took from a PSU toroid, this will be the primary. More turns means more inductance, therefore less current and more frequency to keep it out of the saturation range. Less turns means less frequency, but a higher current demand.Too much turns at the primary could damage the MOSFET due the inductive kick.

For the secondary I usually wind the rest of the transformer up to 2/3 of it's volume with a wire of around ~0.3mm.

The thickness is not a crucial point, but the primary must be always thicker than the secondary, thick enough so it doesn't heats up too much when current flows through it. The secondary can't be too thin, or it could arc, also, if we use too many turns the current would be too low to charge the capacitors at an acceptable rate.

When using flybacks you must identify the output with a positive current flow. That's the output where the diode goes, if you're not sure how to do this, connect the multimeter at both output terminals in the milliamp scale, you will measure a few milliamps, the sign of this value will tell you the direction of the current.

Step 5: The Diodes: Important Note

As I said before, the diodes must be placed at the positive current terminal of the transformer, the other one will be the negative, and will be also connected to ground.

The diodes will hold the charge in the capacitors. Even though the UF4007 are rated for 1 Amp and 30 Amps peak I don't really trust them, so I used two just in case. You can use to220 diodes with more current rating if you are skeptical about this.

Step 6:

After everything it's soldered in place the charger should works as intended. To calibrate the maximum charge voltage you need to follow this steps:

Connect the capacitor bank you want to charge
Set the voltage divider potentiometer to the highest resistance
Connect a multimeter to the capacitor bank (1000 volt setting)
Start the circuit and wait until it turns off automatically
If the voltage is below the voltage desired increase the resistance, the voltage should rise until stopping again at a higher voltage.
Repeat step 5 until reaching the desired voltage
Discharge the capacitor bank and charge it again, check if the charge voltage is the correct, if it exceeds the limit decrease the resistance a bit.

If you want this charger to be used with a single type of capacitors using the same charge voltage all the time you can use the formula below to calculate the resistor R2 to substitute both the 15k and the 100k pot. Remember, U1 will be the constant voltage of the capacitors you want to charge. U2 will be Vin/2, in this case, since we're using 12V to power the circuit U2 will be equal to 6V.

To adjust the 555 timer frequency you can vary the resistance of the potentiometer of the 555. Less resistance means more frequency, and vice versa. My way to set it correctly is to use a big capacitor bank and modify the frequency until I get the fastest charge times, it's a bit rudimentary, but it works.

I wrote this instructable in just a few hours, so if you see something wrong or you want me to add anything, please let me know.