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Simple 2000V Microwave Power Supply

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klugesmith

Mon Sept 28 2009, 10:19PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

GeordieBoy wrote ...

> Whats the reason for this?
Reverse voltage and/or RF from the discharge is likely to fry the rectifier.

Yes. And even a resistive load would continue to conduct as much current as the MOT can deliver, after the capacitor discharge pulse. That bit me in high school when I made a 120V mains voltage doubler to charge a capacitor for Xenon flashlamp.

My cheap yellow multimeter looks practically identical to yours -- will have to go check its resistance.
If you extend the range using a 9 megohm resistor (how did you insert the capital omega character?)
and intend to use it on (say) 2500 VDC maximum, then the meter would indicate 250 volts.
External resistor would be carrying 1/4 mA and dropping 2.25 kV. That's too much power and voltage for
a single 1/4W or 1/2 watt resistor. As others here have posted, you can make up your desired R with several
smaller-valued R's in series to share the voltage and power.

I like the idea posted by Dr. Slack (?) to use an incandescent lamp in series with MOT primary --
to reduce the maximum capacitor-charging voltage. Otherwise you will promptly exceed the 2.5 kV rating.
If you have a Variac and AC ammeter, can quickly find out how much primary voltage
and magnetizing current yields 2.0 kV on the capacitor.
65% of rated output voltage would need much less than 65% of normal magnetizing current.

Aussienitro

Tue Sept 29 2009, 06:57AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
Location:
Posts: 27

To type Î© you can try having Num Lock on and holding ALT then type 234 on number pad then let go of alt, failing that (doesnt work on mine) go into Start, Programs, Accessories, System Tools, Character Map and copy the omega. ALT230 gives Âµ.

Oh Ok, so since all I have on hand is 9 1/4W 1MÎ© resistors that will suffice?

klugesmith

Wed Sept 30 2009, 06:59PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Î˜Ç»Ð¿Ä·ÃŸ for the tip. Along with Î© and Î¼, we can properly state temperatures in Â°C and RLC damping ratio as Î¶.
I'll make a little cut-and-paste reference file of commonly missing symbols.

Your nine 1-megohm resistors would be perfect. Absolutely comfortable at 2500 volts total.
And probably OK for amateur use at 5000 volts total (500 volts and 1/4 watt per resistor) if they aren't packed tightly together.
As part of a permanently connected voltmeter, they would serve to bleed off your capacitor voltage soon after the charger is disconnected.

Rich

Proud Mary

Wed Sept 30 2009, 09:50PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It would help if you could tell us the purpose of this device.

Aussienitro

Wed Dec 23 2009, 07:42PM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
Location:
Posts: 27

Hi Mary, its purpose was simply to charge and discharge the cap, in the process learning how these things work.

Anyways I have yet to make this thing, I only just sourced a suitable 240v HV relay for cheap so now I'm back on board.

But I have been pondering an idea, I bought a dimmer switch wrongly thinking it would let me vary the voltage produced at the transformer. Researching that I found out dimmer switches dont work like that, but essentially change the pulse width of AC current. So what I now have in mind for it is to use it as I originally planned and hopefully have it let me vary the charging speed of the capacitor.. So dim the switch right down so I dont accidentally overcharge the 1uF cap. Is my logic wrong here, is that going to work?

Hazmatt_(The Underdog)

Wed Dec 23 2009, 08:07PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I want to point out something to you because you're new to this and nobody else really caught it.

The output on a MOT is one hot and a case ground.

IF you attempt your schematic there you have some issues to consider:

1. If you remove the core ground you run the risk of the start of the winding arcing to the core.

2. Because the ground is floating on the HV side you may really hurt yourself on either the transformer or the capacitor.

3. You may not be able to case ground the capacitor now because of lekage current. Remeber the cap is not seeing .707Vpk from the transformer now, its seeing 1.414Vpk, so there could be a few hundred volts on the can above ground potential, which could hurt you.

4. If you intend on using this as a HV supply you need to remember that your supply is floating well above ground, probably 1000V or more, so if you try to ground the negative something will give, probably the rectifier.

I know a lot of people float the ground on their MOT's, but they have some experience with it. Just make sure you know what is and what is not at 0V and be extra careful with this stuff. Other then that
I would strongly recommend using the half wave setup and charge the cap as intended, especially if you're new to this, you could really get hurt with these transformers.

Antonio

Wed Dec 23 2009, 10:05PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

"its purpose was simply to charge and discharge the cap, in the process learning how these things work."

Just don't try to discharge a 1 ÂµF capacitor charged to 2800 V over YOU. 3.92 Joules is quite close to the possibly fatal limit for capacitor discharge energy, not counting the MOT connected to it. Don't touch anything while the system is powered up or the capacitor is charged.

Sulaiman

Wed Dec 23 2009, 11:17PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I second that !

4J will leave quite a memory, an MOT will probably kill you.

Aussienitro

Sat Dec 26 2009, 09:24PM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
Location:
Posts: 27

OK, totally over engineering what I first set out to achieve. As a power supply I want to add a variac before the MOT thus allowing a 0-240v AC/DC outlet as well as being able to lower the HV to any rated capacitor. For that I'd like to add a voltage quadrupler after the rectifier. I have 4 0.1A 20kV diodes that can be used but I'm not sure what capacitors would be required for the following schematic

.

So assuming you would want at least 6000v caps what capaccity should they have? Are small ceramic caps suitable? say 6kV at ~1000pF?

Zeus

Sun Dec 27 2009, 06:43AM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

1 nF will get you practically nowhere for 50/60Hz. You need about 0.1 microfarad caps. Also 0.1 A diodes will cook with an MOT as they put out 0.5A.

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