Simple High-Voltage Capacitor Construction
High-voltage capacitors are very handy for the experimenter. Unfortunately, they can be very difficult to find. Then, if you can find them, they will be quite expensive or even made using potentially dangerous chemicals. For this project, we'll be using several easy-to-find "computer grade" 450 volt electrolytic capacitors wired in series to form a single fairly high-capacity, high-voltage capacitor.
When wired in series, the voltage rating on the capacitors can be added together. The final microfarad value of the capacitor will be equal to their initial values, divided by the number of them in series (try to keep them all equal). For example, using eight series 250 µF, 450 VDC capacitors will lead to a final 32 µF capacitor with a voltage rating of 3,600 VDC. There is one major caveat when constructing such a device, though. When in series, each of the capacitors essentially acts as a voltage divider. Since no two capacitors are truely identical (capacity and leakage current wise), you'll need to add an external resistive voltage divider across each of the capacitors to "even" out the voltage divided across them. For this project, we'll be using 100 kohm, 5 watt metal-oxide resistors for the voltage divider. The series resistor string must also be capable of handling the fairly high heat dissipation, around 20 watts in this case.
Peak Voltage : 4,000 VDC Total Series Resistance : 800,000 Ohms Resistor String Current : 0.005 Amps [ 4,000 Volts / 800,000 Ohms ] Power Dissipation : 20 Watts [ (0.005 Amps ^ 2) * 800,000 Ohms ]
Construction Notes & Pictures
Case overview. The capacitors will be installed in an old ammo box to help protect them and to contain any explosion if the capacitors happen to be overcharged. Heh.
The 9-inch long aluminum bar in the middle will hold the electrolytic capacitors using plastic zip ties. It will then be isolated inside the middle of the case using a piece of 1/4-inch allthread rod and rubber grommets. Two banana jacks will act as the new output terminals.
Electrolytic capacitors used. They are 250 µF / 450 VDC Mallory capacitors. Their maximum "surge" voltage is 525 VDC. They should work up to 500 VDC with no problems. They'll be connected in series using solid #14 copper wire and ring terminals. The zip ties are on the left.
Zip-tying the electrolytic capacitors to the mounting bar. This is a total hack, but it seemed to work out quite well.
All the capacitors wired in series (positive to negative). The output POSITIVE terminal is on the left, the output NEGATIVE terminal is on the right. The series 100 kohm resistors are mounted using separate ring terminals on the capacitor's connecting posts.
Mounting bar hardware. 1/4-inch allthread and coupling nuts.
Final installation inside the case. The threaded rod is secured to the sides of the case (through the rubber grommets) using the couplers and short 1/4-inch bolts. There is a sponge underneath the capacitors to help hold them in place. Art foam lines the sides to prevent any shorting. The wires connecting the output terminals have vinyl tubing to protect them from high-voltage arcing.
Overhead view. Output banana jack terminals are on the left. They are also mounted through rubber grommets to add extra high-voltage protection.
Completed front panel overview. This capacitor bank can now be used for high-voltage power supply filtering. It should NOT be used for high-current pulse discharge applications.