Simple 4 kVDC Power Supply |
Overview
This is a simple high-voltage (4 to 5 kilovolt) power supply you can build using parts salvaged from old microwave ovens and other swap fest goodies. The main power supply component is a Microwave Oven Transformer (MOT). Try to find a transformer which is physically small so it is easier to mount and work around. These transformers usually have a secondary voltage of around 1,800 to 1,900 VAC (RMS) with the standard 120 VAC input. Since microwave oven transformers are literally made as cheap as possible, they will have a fairly high input current. This is due to the fact that they don't have enough windings on the primary side. The transformer will also have metal "shunts" between the primary and secondary windings to limit the transformer's secondary output current. This power supply isn't really designed to be operated for a long period of time without saturating or overheating. Try to find a real high-voltage transformer if you need a very reliable setup. Otherwise, there are several tricks we can use to overcome those limitations.
On the 120 VAC primary input, you can add a series NTC resistor from an old computer switching power supply. Look for a green "blob" component in series with the AC input line in just about any computer power supply. It might even look like a big ceramic capacitor and should have a PCB marking of "NTC" or "RT" or something similar. NTC resistors are used to limit the input surge current when first powering the switching power supply. The NTC resistor will have a small initial value of around 10 ohms or so, and its value will lower as it "heats up" (current flows). This component is not a requirement, but should help eliminate the transformer from buzzing on start up.
Another modification we'll have to do to the transformer is isolate the secondary winding from the transformer's core. Normally, one side of the transformer's high-voltage secondary is tied to Earth ground, which the core is at. We'll need to disconnect and isolate this connection so we can have a power supply which is completely isolated from any ground reference. This is so we can easily power negative voltage projects, like a magnetron. Since the stock transformer only outputs around 1,800 VAC, we'll also be using a "voltage doubler" diode network on the transformer's secondary output to reach a final peak output of around 5,000 VDC.
The biggest, and probably the most costly, part of this power supply is the high-voltage ripple capacitor right after the voltage doubler network. If you can find a good 6+ kV, 20 µF or better capacitor - use it. If not, you'll have to make your own. If you do need to make your own, search swap fests (or eBay) for a dozen 450 VDC or better "computer grade" electrolytic capacitors. Try to make sure the capacitors are all the same. The final µF value doesn't really matter, but aim for at least a final total of 20 µF or so. Equalization/bleeder resistors will be added across each of the capacitors to even out their voltage load. Remember that capacitors in series increase their voltage handling capabilties, while decreasing their overall capacitance value.
The final DC output will be via isolated banana jacks mounted in heavy rubber grommets and secured using plumbing washers and nylon nuts. All the required high-voltage isolation can be a real pain, and the power supply shown here starts to "crackle" after a while, so it's mostly just a starting point for your own design.
To discharge the high-voltage capacitor bank if you need to work around it, write "Kevin" on one finger and "Rose" on the other. Touch each of these fingers to the postive and negative terminals of the capacitor bank and wait for a second or two. That's it!
Construction Notes & Pictures

Capacitor bank parts overview.
Twelve Mallory 650 µF / 450 VDC electrolytic capacitors will be mounted to a piece of wood using double-side foam tape. Four rubber feet under the wood base will provide vibration protection for the final capacitor bank assembly.

Arrange the capacitors as shown.
The capacitor's final positive is marked with a (+) and the negative with a (-). The red dots indicate the postive terminals on the rest of the capacitors. Note how they are arranged for the shortest possible interconnection. Also note the pieces of art foam attached to the sides of the capacitors to further isolate and protect them from vibration.

Completed capacitor bank.
All the capacitors are wired in series (+ to -) using crimped ring terminals and short pieces of #18 gauge solid wire. The final measured capacitance value was 57.5 µF. Try to mount the 100 kohm equalization resistors "in the air" so they can dissipate heat more efficiently.

Parts overview for the rest of the power supply.
On the left are the parts for the 120 VAC input. An IEC power connector, a fuse holder, a SPST switch, a AC line filter, and a neon power indicator lamp with an internal dropping resistor. Next to that is a small terminal block which will connect to the secondary windings on the transformer. In the middle are the output banana jack terminals and the rubber grommets used to provide high-voltage isolation. Above them, is an optional power resistor which can be used in series with the high-voltage output to limit the dangerous output current.
On the right-hand side is the microwave oven transfomer, two high-voltage diodes, and a 0.86 µF high-voltage capacitor. Those three things all can be salvaged from old microwave ovens.
Everything will be mounted inside an old ammo box.

High-voltage capacitor mounting hardware.
It is mounted to a piece of wood using two metal brackets and assorted hardware. Secure and isolate the capacitor with pieces of art foam.

Internal view of the 120 VAC input connections.
The terminal block to the right will be for easy connection to the transformer's 1,800 VAC secondary output.

Close up of the microwave oven transformer.
The 120 VAC primary input is on the left, the 3.3 VAC and 1,800 VAC secondaries are on the right. The 3.3 VAC filament winding wires can be removed.

You may wish to knock out the magnetic shunts between the transformer's windings. You can see where they used to be in the above picture. Use a pin punch and a rubber mallet to gently loosen them. Try not to damage any of the transformer's windings.

You'll also need to isolate the transformer's secondary winding from the transformer's core. You can see this connection circled in the above picture.

Here you can see how the transformer's windings where isolated and soldered to two stand-off insulators epoxied to the transformer. This gives the transformer a final 1,800 VAC (RMS) output which is isolated from Earth ground.

Mount the transformer as so. There is a transient snubber circuit across the transformer's primary input. The high-voltage secondary output goes to the black terminal block.
From here on, you'll need to excercise standard high-voltage construction practices and cautions.

Install the ripple capacitor bank into the ammo box and mount the output and divider banana jack terminals making sure they are not shorted. Use additional rubber grommets or washers if needed.
The ammo box shown here in this project was slightly too small, so maybe try using something else. You'll also want to mount the high-voltage series capacitor at this time, again making sure its body is not shorted against anything. Double and triple check the polarity connections on everything, then connect up the voltage doubler diodes to the high-voltage series capacitor and ripple capacitor bank.

High-voltage divider resistor.
Nine 499 kohm resistors are wired in series and placed inside a piece of 3/8 inch I.D. vinyl tubing. Secure the ends of the tubing with plastic caps. This series-resistor network along with one more 499 kohm resistor make a "divide-by-10" voltage divider. This is so you can measure the power supply's final output without the need for a special volt meter.

Completed power supply internal view.
Long high-voltage connections are placed in vinyl tubing.

"I love it when a plan comes together..."
The theoretical final voltage output should be the transformer's 1,800 VAC output multiplied by 2.8 (5,040 VDC). The measured value from the divide-by-10 network is 504 VDC. This corresponds to an output voltage of 5,040 VDC. The analog meter is reading just under 5,000 VDC.

Completed front panel overview.
120 VAC input is on the lower right-hand side. The power switch has a protective rubber boot. The protection bars are four inch brass drawer handles.

Completed rear panel overview.
The high-voltage output is on the left-hand side, and the divide-by-10 reference output is on the right-hand side.
