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A project log for Pulsed Capacitive Discharge Power Supply

Pulsed-discharge power system suitable for welding, plasma cutting, laser driving, and other high-power equipment.

Jakob WulfkindJakob Wulfkind 06/07/2014 at 18:510 Comments

So apparently the habit that I am actually in is promising myself that I will post a project log every wednesday, and then posting a log a few days late.

Working on this project has taken me fairly far from my existing experience with low-power electronics into the realm of high-current devices. About once every few hours, I get a reminder of this in the form of a jumper wire, diode, or capacitor overheating and going up in a puff of smoke and having to be replaced with a thicker, more robust version. This also seems to be the reason why I haven't already generated anything more powerful than a bright spark -- I was originally using twenty-gauge wire as my discharge electrode leads, and the wires themselves were the source of so much resistance that they significantly reduced the output power. It's an odd change from small-signal electronics to actually have to worry about half an ohm.

Because of the high wattages and discharge currents involved, I've separated the project into three portions: the capacitor bank, with its own dedicated rectifiers, the logic board, with a separately rectified VCC (ground is still common), and the power supply board that provides these rectified voltages from the transformer. The power supply board is fairly straightforward:

D1-D4 are high-amperage discrete rectifier diodes, while B1 is a standard integrated bridge rectifier.

The capacitor board should be separated by a little bit of distance from the other two boards, as a leaky capacitor (or, as shown in the last log, an exploding one) can put the other components at risk. 

With the relay open, cap_charge will increase until it reaches the threshold voltage set by DISCH_VOLTAGE, and will then trigger Q1 to close the relay on the capacitor board and allow power to flow through the electrodes. This will also charge the ELECTRODE_VOLTAGE line, allowing the second comparator to hold the relay closed until it drops below the voltage set by the PULSE_LENGTH potentiometer. This allows a user to set the desired discharge voltage, and the total percentage of the capacitor's power discharged by a single pulse, which will in turn control how long the capacitor bank takes to recharge and initiate another pulse. 

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