Building a simple 6 kV DC power supply

Warning

So first off this is an article that requires an extensive warning:

• The power supplies described in this article are operating from line power. Even though this being the safer side of these projects working with line power equipment can result in deadly shock. Be sure to disconnect everything before working with line voltages.
• The microwave oven transformers step up voltage from 230V to around 2.1 kV AC. These voltages have spikes up to 3 kV into positive and negative regime - this is high enough to arc over a few centimeters of distance. Keep your hands and any conductive stuff away.
• Arcs are loud, hot and destroy stuff. And they produce an intensive electromagnetic bang. But be never tempted to reach into their vicinity to prevent further danger. Disconnect the circuit instead of touching anything.
• The capacitors that are used in microwave ovens might only have a capacity of around $1 \mu F$ but they do so at a voltage of up to $3 kV$. Since the amount of charges also goes linear with voltage $Q=C*U$ this is a more than deadly amount of charge. Discharge capacitors. They usually do have internal bleeders (around $10 M\Omega$) but do not trust them. First always wait for capacitors to discharge via the internal bleeders after disconnecting the appliance. Then short out the terminals at least using an insulated (remember between 3 and 6 kV depending on configuration) screwdriver - in case the capacitor was charged this might lead to a major bang and a screwdriver that’s welded to the capacitor terminals but at least that’s better than dying from the same discharge though a human body.
• The voltages are high enough to penetrate through thin insulating layers as well as arc over airgaps. Keep enough distance. For 6 kV the minimum safe distance for an electrician is usually considered to be around 3.5m from any life wire while the system is active but that includes a huge amount of safety margin; if possible enclose all your assemblies in an grounded conductive casing. Doing calculations yields a distance of around $4mm$ that cables should be spaced at least when using dry air as insulating medium to be safe for $10 kV$.
• Only do such stuff when you know what you are doing - this is not a beginners project even if it looks simple.
• And as a last remark - do not get routine when working with such voltages. Always follow a plan / checklist. Do power off stuff. Check that you’ve disconnected everything. Wait a given time. Check you really waited for that time. Discharge capacitors, check they’re really discharged by using a multimeter if you have one available. Don’t work with high voltages while you’re tired, in a hurry or not concentrated.

Introduction

So what is this blog article about? Building a cheap power supply out of some scrap or simple buyable components that delivers - depending on the configuration:

• $2.1 kV$ AC at $50 Hz$. This is boring and simple since it would only mean one connects the transformer to the line power side
• $4.2 kV$ pulsed output - this is the configuration also used inside the microwave oven.
• $3 kV$ DC via a full wave rectifier smoothed with a single capacitor.
• $6 kV$ DC via a full wave doubler using two diodes and two capacitors.

The transformer to be used is a standard microwave oven transformer (MOT) as it’s used in many different microwave ovens. Since these ovens use electron tubes - so called Magnetrons - in which electrons are emitted from a hot cathode which is achieved by heating a tungsten filament using around $3V$ and a rather low current (that’s derived by a second secondary winding on the transformer usually) and then accelerated towards an anode which requires a high negative voltage (electrons travel from a negative source to a more positive terminal - and at this voltage reach around 10.8% of the speed of light) since the target is designed to be at ground potential - they produce a somewhat higher voltage of typically around $2.1 kV$. Thus one can use these transformers as source for $2.1 kV$ alternating current. These devices are rather crude and have high losses - around 200W are typically burnt just to magnetize the core.

Side note: Why is the tube called Mangetron? In these devices electrons don’t travel on a straight line but are forced to travel along a curved path by an external magnetic field - these are the two black rings that you can see on the side of the magnetron. The electrons travel towards anodes that form the capacitor plates of an LC resonator and charge them alternatively depending on it’s previous state. That way they drive the resonance frequency of the LC tank circuits that is then also extracted via a single antenna and radiated into the microwave ovens working volume.

Wiring on the transformer

Note that the transformers usually have

• One primary winding with rather thick wire. This is usually visually disconnected from any contact of the secondary side. None of the terminals is grounded - they’re designed to directly attach to phase and neutral of your line power system. Usually one adds an switch as well as some filtering circuit - if it’s somehow possible keep the filtering circuit in there.
• A single secondary winding for $3V$ supply. This is usually used to heat the filament of the thermionic emission cathode and is isolated from any ground potential since the cathode is usually biased to high negative potential.
• The secondary high voltage winding. This usually exposes only one lead. The second end is hard wired to the transformers core and thus the ground potential. So the transformer itself delivers $2.1 kV$ once to the positive and once to the negative relative to ground. This is done for multiple reasons (safety, simple wiring, the case being the target for microwave radiation anyways so it has to be the return path, less insulation on this side of the windings, etc.) and is not changeable. You won’t get a floating HV supply out of these transformer without doing a complete rewinding. It’s also a good idea to add (or leave if it’s present) a fuse to the HV side. This won’t protect the user but the transformer from any shorts.

Please note: Even though the ground wire is connected it won’t protect anyone from contact with the high voltage. The residual current detector won’t trigger since the power supply simply works as an isolated power supply. The only reason the ground wire is connected on the secondary side is to provide a reference potential. One could also operate the device fully open - it’s a little bit harder to assembly since the transformers case and transformer sheets are one of the connectors of the high voltage side but entirely possible. Keep in mind there is no way for an RCD to work when you reach into the transformer - and also no way for a miniature circuit breaker to save anyone from electric shock.

Parts used

Note: All links are amazon affiliate links, this pages author profits from qualified purchases

The following parts are / might be used:

• Any microwave oven transformer - I cannot provide an link to the ones I’m personally using since they’ve been salvaged from old microwave ovens.
• Capacitors 2.1 kV AC with internal 10 MOhm bleeders
• High voltage 12kV 350mA diodes
• 700mA 7kV fuses for the secondary high voltage side

Configurations

Getting 2.1 kV AC

This is pretty simple. One should keep the filtering circuit on the line side if possible. Just attach the two leads of the primary winding to your line voltage and you’re good to go. That’s it.

Getting 4.2 kV half wave AC

Inside the microwave oven a simple trick is used as an AC voltage doubler. The capacitor is attached to one end of the transformer. During one half wave the capacitor is charged via the conductive diode, during the second half wave the whole circuit is basically biased up to 2.1 kV which doubles the available voltage. This is exactly what happens inside microwave ovens.

Note that when dismantling the microwave oven you usually don’t see the grounded side - the 0V side of the high voltage winding of the transformer is directly connected to the transformer casing and thus also to the microwave oven case. The other side is usually directly connected to the capacitor, the diode to the other side of the capacitor and wired directly to the case of the microwave oven again. This is done that way since the whole case also acts as target for electromagnetic radiation and is at the same potential as the target of the electrons which is also ground potential.

A simple 3 kV DC supply

The simplest way is to build a half wave rectifier - just attach the capacitor via the diode on the high side and to ground on the other side. While the transformer is delivering negative output the circuit simply does nothing - during positive output the capacitor is charged up to $3 kV$ that are delivered at maximum by the transformer. The capacitor might be enough to smooth out the ripple of the charging cycle depending on the load.

One can get more stable output when adding a full wave rectifier in front of the single capacitor and get more smooth output by adding capacitors in parallel.

A simple 6 kV DC supply

To get 6 kV DC one can build a full wave doubler circuit. In this case two capacitors are put in series. Their center tap is connected to the grounded side of the transformer. The high voltage side is attached via a diode once in forward and once in reverse direction to the high and low side of the assembly. This way the transformer charges the high side capacitor while delivering the positive half wave and the low side capacitor while delivering the negative half wave. Each capacitor is charged to $3 kV$ separately. Since they’re connected in series the voltages add up to $6 kV$.

This article is tagged: Physics, DIY, Basics, Hardware