5kV multipurpose power supply

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Maciek Szymanski
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5kV multipurpose power supply

Post by Maciek Szymanski » Sat Feb 20, 2021 7:50 pm

For some time I was building a power supply for my garage fusor and other plasma experiments. This is a demo grade PSU based on the step up transformer from an old breakdown tester. I’ve bought the complete tester quite cheaply and scavenged the trafo, a very nice variable auto transformer and some high power ballast resistors. Around this parts I’ve build a simple 50Hz power supply. The HV transformer is quite big piece of iron and copper with insulated secondary winding and without magnetic shunt. In the tester the current limiting of the transformer was done with the primary side ballast resistors and I decided to copy the solution.

Wiring diagram of the 5kV power supply.

The HV transformer T2 is permanently ballasted with four 270Ω resistors (68Ω effectively). This resistance can be bypassed with the switches W2 and W3 connecting parallel resistors of 33Ω or 2.2Ω resistance for higher output. This is not foolproof - especially the 2.2Ω resistor can be easily overloaded by the secondary side short circuit.
As the HV transformer has a completely insulated secondary winding the rectifier is a full wave bridge build from four д1008 diodes (10kV, 100mA).
The metering circuit is based on two Era MZ-2A analog meters wit 10μA full deflection current. The current measurement is done on the positive side with 200Ω shunt resistor (0.2V per mA). The voltage is measured on the negative (output) side with the help of the divider consisting of the HV 100MΩ resistor and 1MΩ 1% resistor (so output is 10V per kV). The metering signals are brought to two banana sockets on the front panel (GN2 and GN4) for the oscilloscope or chart recorder measurements.
The whole unit fits in the heavy but compact package in the case scavenged from an old programmable function generator. The negative HV voltage is supplied by the custom made HV socket. The metal case is grounded and has two ground binding posts on the back and on the front panel.

Complete supply. On the left side of the case is the variac and ballast resistors on the right side HV transformer, rectifier block and metering circuits.

Detail of the rectifier block and metering circuit. The long object behind the plexiglass plate is the HV 100MΩ resistor of the voltage divider. The big resistor on the plate above the meters is the 200Ω current metering shunt.

Detail of the front panel. In front: power switch and pilot lamp, ballast resistor switches and variac knob.

HV plug and socket. The glow lamp over the ammeter is the metering circuit breakdown indicator.

Today I’ve done some load tests of the supply. I’ve used a set of wire wound high power resistors as the load and measured current-voltage characteristics with different primary ballast resistor sets.

The measurement setup. The set of resistors is mounted on the laboratory stand with a clamp. The left end is grounded with the thick cable going to the binding post on the front panel of the PSU. The PSU itself is grounded on the back side by the connection to the grounding bar visible on the wall below the shelf. The stand is additionally grounded with the white cable. The high voltage is applied to rightmost resistor.

The resistor assembly. The resistors are held together with the textolite spacers and M4 threaded rod inside. The leftmost is 30kΩ the other three are 20kΩ giving together 90kΩ. I’ve used additional 40kΩ resistor to obtain the 130kΩ load.

The measurement results are summarized in the graphs below:

Output voltage vs auto transformer setting. U1 is the voltage of the unloaded auto transformer. Uw is the PSU output voltage without load (R=∞) and with 90kΩ load for 68Ω and 22Ω ballast resistor sets.

Output current vs output voltage. The curves are for 30kΩ, 70kΩ, 90kΩ and 130kΩ load. The continuous curves are for 68Ω ballast resistors and the dashed ones for 22Ω. For the 30kΩ load I’ve not done the measurements with the 22Ω ballast to avoid overload.

The maximum current and voltage vs load resistance. The continuous curves are for 68Ω ballast resistors and the dashed ones for 22Ω.

To summarize - I think it may be a nice “intermediate” power supply with a bit more “stiff” characteristics than the neon sign transformer based one. Those insulation breakdown testers are a common piece of equipment (even in current production: http://www.sigmainstruments.com/high-vo ... -p-18.html).
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Richard Hull
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Re: 5kV multipurpose power supply

Post by Richard Hull » Sat Feb 20, 2021 11:28 pm

Looks like a good healthy transformer for a demo fusor. 5kV with a .1uf capacitor should allow for a 0 to 7kv supply.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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