Hello everyone, I'm in the process of building a power supply to power a flash joule heating reactor. The equipment was selected from a set up that I saw that was successful but was vague in describing the circuitry involved.

The power supply I'm trying to build will be sending a ~237VDC pulse from a capacitor bank through a SCR diode into a flash joule reactor. The material inside the flash joule reactor will be compressed to ~2 ohms.

The details of the power supply are as follows:

1. A 0-140VAC Variac will allow the input voltage to be adjusted. The Variac will connect to the primary side of a "MOT" or "Microwave Oven Transformer".

2. The MOT will supply voltage to "two rectifiers for a half wave rectification". This is where I'm having trouble, understanding the use of two rectifiers for half wave rectification.

3. The rectifiers provide a pulse of DC into the Capacitor Bank. The Capacitor Bank consists of a total of 6 250VDC 27000uf capacitors. 3 capacitors are wired in parallel with each other giving 81000uf. The other 3 capacitors are wired in parallel also, but in series with first 3, reducing the capacitance by 1/2 which gives a total capacitance of 40500uf.

4. The Capacitor Bank has a Dump Resistor to bleed away the voltage at a rate of ~1VDC per second.

5. The Capacitor Bank is then wired to a SCR Diode. The SCR Diode is fired with a gate voltage of ~3VDC provided by 2 AA batteries and a push button switch. The Variac is used to increase the input voltage until the voltage is ~240VDC. The dump resistor bleeds the voltage down slowly and the SCR Diode is fired at exactly 237VDC.

This charge is what is used to heat the compressed material inside the reactor to ~3500K. The goal is to apply 10800 Joules to the material.

I was hoping that someone may be able to help me with the rectification portion of this? I'm unclear on how I would use two rectifiers to get half wave rectification?

Any help would be greatly appreciated. I'm in the process of building this power supply and I have hit a wall on this part of it.

Just a disclaimer, I'm a chemical technician now but I have years of prior experience in the industrial electrical field working with high voltages and currents. So I understand the dangers involved with building such a power supply and I always put safety first. Unfortunately most of my experience was with motors and power distribution.

I'm going to be using this project to make flash graphene from different carbon sources and eventually plan to scale it a bit.

I hope I included enough detail? If not please let me know? I have drawn out a schematic minus the rectification portion of the circuit. I can include that if it's helpful?

Thanks for reading my massive post.

 

Just wanted to give a quick clarification on my above post:

To sum up my question, when rectifying the voltage coming out of the "MOT" transformer from AC to DC, in what scenario would I use two rectifiers for half wave rectification?

The only thing I can come up with is: "dual half wave rectification". Unless I am misunderstanding what the person who built the original is describing?

In essence the system is just sending an arc across the material sandwiched between the two electrodes. Which in turn heats the material to ~3500K in that moment. Hopefully this helps.

I've also contacted the person who built this set up to see if they can offer any insight. I'll update this post as I progress and post a schematic once I figure it out for anyone else interested in building a flash joule heating system.

I'll start a new post for the build and commissioning of it when I get to that point. I have most everything on hand or on the way.

 

If you only need 237 Volts, why the Microwave transformer?
They are typically 2200 Volts, that's a 10:1 mismatch?

 

If you only need 237 Volts, why the Microwave transformer?
They are typically 2200 Volts, that's a 10:1 mismatch?

Convenience mostly, while it does have much more capability than I need, they're cheap, functional and abundant. Using the Variac will allow me to dial that in each time I fire the circuit. That's 237VDC which will be the rectified voltage. The AC voltage coming from the MOT secondary feeding into the half wave rectifier will be different. I'll have to post that once I test it.

That's not to say I won't switch to something else down the road in place of the MOT. I could foresee the MOT output voltage being "touchy" to reliably control with the Variac. I'll have to test it and see if it's going to have issues.

I also finally got in touch with the person who built the original system. They did verify it's half wave rectification with one rectifier, not two.

Anyway I will post a schematic later today of what I intend for the circuit to look like. I'm just getting started so I'm sure there will be more kinks to work out.

 

You are aware that almost any point in your circuit will be at a LETHAL potential?

 

Yes, very much so. A very good reason why I'm trying to have a full grasp on the circuitry before I even begin assembly.

I fortunately have a lot of experience working with deadly voltage/current, i.e. 480VAC 3PH, 4160VAC, etc. So I'm very careful and overly cautious. No expense will be spared on insulating and/or isolating any live components.

I appreciate you saying it though, it's a good thing to look out.

 

Basic idea.
The electrical would come out of a mixture of goals. Safety first as mentioned.
To be scaled for commercial use efficiently convert 230VAC 3 phase to rectified low voltage DC,
To store and release current in rapid pulses.
Mechanism for mix and load ground powder material..

evaluate which step down converter works best for this.
Design large capacitors for storage
design the microcontroller to facilitate the capacitor charging and another controller to
release the vaporizing pulses.

 

For starters, a microwave oven transformer usually has one end of the high voltage output tied to the frame. The "two diodes half wave rectifier" would be a half-wave voltage doubler, which is an actual legitimate sort of circuit. That can easily provide you with 4000 volts DC. The capacitor groups described in post #1will quickly fail at when 4000 volts are applied.
Really, the only transformer needed, aside from the Variac would be an isolation transformer. A doubler supply from 120 volt mains can easily provide the 237 volts.

The really bad news is that it comes across to me as a fake you-tube presentation.
The chemical process may be valid but the described hardware sounds very lethal.

 

Thanks for all of the replies, all appreciated. So here's where I am currently with this.

@MisterBill2, Assuming that the efficiency of the half wave rectifier component is: 40.5% then in order to get a half wave rectified voltage of: 240V DC then I'll need to apply: ~592.6V AC, since it is a half wave rectification the MOT xfmr should provide that voltage with care taken not exceed that voltage value.

I will be using a 1000V power rectifier diode for rectifying the AC to DC. I'm still in testing phase for now. The secondary coil of the MOT xfmr is connected to a voltmeter that will give me the voltage going into the rectifier so I can watch it as I charge the capacitor bank with a second voltmeter reading the DC voltage.

With the 6 250V DC rated capacitors wired up as 2 parallel sets of 3 in series, that should give a voltage rating of 500V DC. The maximum I should ever encounter is ~240V DC across the bank. Unless I decide to whip the dial on my Variac and destroy my expensive caps, which I'm not ever going to do.

The science is sound and has been repeated elsewhere. There is a YouTube presentation on this but I don't think that necessarily means it's fake? The person who built it has answered my question about the whole "two diode" scenario and it was an error indeed.

I'll be starting to test and commission things soon. My old Oscilloscope has given up the ghost so I am waiting on my new one to come in before I begin. I am hoping to use that to observe and possibly tune the DC waveform if I need to.

I'm still in the process of sourcing a disconnect switch to go between the rectifier and capacitor bank. I may end up installing a flyback diode as well. I am working on trying to devise a way to monitor the AC voltage and wire in an interlock that will protect the equipment from overvoltage.

Still have a lot of work ahead. Once I test the Variac and MOT xfmr through the diode I will have a better idea of what I'm going to be dealing with before I ever hook up my caps.

I'll hopefully have some data to work with soon. I'm getting there slowly but surely. I'm currently in the middle of rebuilding my vacuum pump, an Edwards E2M12 and that's been a messy, time consuming undertaking but I just about have it wrapped up.

 

Reading the thread again, the whole concept of electric arc heating is disturbing. Rocket lighters and explosive detonators use a similar scheme of a high voltage source supplying enough current to vaporize a section of wire, producing both a shock wave and a ball of very hot metal vapor that might be defined as plasma. Rather a hazardous mixture at best.