HV power supply

[radha]

Hi to all, this is a great site and I am happy to be here ! ;D

I am planning to make a high voltage DC power supply ( 2KV, 1.5A) by using Crockroft-Walton circuit. I have some questions on this :

(i) Will this circuit give this much DC current ?

(ii) What capacitors should I use ? I plan to use high voltage ceramic capacitors . Is this OK ? ( input is 230V, 50Hz )

(iii) Is the VAR rating of the capacitors dependent on the o/p current drawn ?

(iv) What should be the capacitance value ? Low or high ?

Replies will be highly appreciated. Thanks in advance.

[ante]

Hi radha,

Welcome to this forum.

Dangerous things 2000V @ 1.5A, what is it suppose to power? A linear RF amplifier maybe? Do you need a stabilized voltage and ripple free? Can you post the circuit I am not familiar with the Crockroft-Walton circuit?

[audioguru]

Hi Radha,
3,000 Watts is a lot of power for an average person. Are you going to use it for industry? Think about your utility's power bill, if the step-up converter is fairly efficient, at 230V it would be drawing about 17 amps.
How much ripple voltage can you tolerate in its output? With a 50Hz input, the ripple voltage will be enormous unless the capacitors are also enormous. Or use thousands of paralleled ceramic discs.

It might be better to rectify and DC-filter (it would still need enormous capacitors) the 230V mains directly, then use the resulting 325V DC to power an inverter. The inverter would have a small high frequency transformer that steps-up the voltage. The transformer's output would be rectified and filtered with small capacitors to provide 2000V DC.

[radha]

Hi Ante & Audioguru,

Thanks for the reply :).

I think I'll scale down the specs to 1000V, 1A. This is the power supply for a RF amplifier as Ante rightly guessed. I require it for RF plasma research.

Crockroft-Walton circuit is a standard circuit to generate HV DC ( >300kV ). I know that it can generate this voltage in 2 or 3 stages, but I want to be sure of the current. If this do not work out, then I'll look at the convertor-invertor-convertor scheme.

Right now I don't have a jpg/gif of the circuit. I'll post it as soon as I can get one.

I have limited experience of HV, and this is the first time I am doing this. Yeah, and I'm a little wary of the high power involved ! :o

[ante]

radha,

High voltage in itself is not dangerous but when there is some current involved it sure will be. If you are careful when experimenting with this you will live to tell about it! ;D

[Guest Alun]

Why don't you just use an old microwave oven transformer then rectify and filter the output?

Be warned because currents as low as 50mA can be deadly!

If you put 1A through your body and you're a gonner for sure!

How much RF power do you need and at what frequency?

If 2.45GHz @ 600 to 900W is acceptable you could just use the microwave generator (transformer doulbler circuit & magnetron) from an old microwave.

Be warned also about the dangers of high power RF fields they can cook your brains.

[ante]

Alun, radha,

A microwave transformer will do fine if you can settle for less current.

[Guest Alun]

You could always connect two microwave transformers (must be exactly the same type) in paralell.

[ante]

Yes you are correct but you would need three in parallel to reach 1A. If I remember correct the microwave transformers have a 2000V secondary this will give 0.375A from one 750W transformer.

[steven]

:) be carefull the microwave oven transformer core is negative ground

[Guest Alun]

Yes you are correct but you would need three in parallel to reach 1A. If I remember correct the microwave transformers have a 2000V secondary this will give 0.375A from one 750W transformer.

Microwaves are always sold by the power output rating. Since the magnetron is only 50% efficent a transformer from a 750W microwave would be rated at 1500W, and would therefore give you 0.75A at 2kV.

:) be carefull the microwave oven transformer core is negative ground

negative?

Since the transfromer is AC there's no positive or negative. I think you mean that one of end of the secondary is connected to the core which is also grounded.

[radha]

Thanks for all the inputs.

My project is to make a RF source to deliver about 800-1000W. By RF I mean Very Low Frequency RF ( around 15MHz ). Is this frequency harmful ?

So my original idea was this : make a DC power source for 1KV,1A. Use this to power a RF amplifier.

Will the microwave transformer work at this frequency ( 15MHz )? If it does, my work will be greatly simplified. I can skip the power supply part.

The idea of using the microwave oven transformer for the DC power supply section is good. I will look into it. Thanks for the idea.

[radha]

I did some reading and found out the following :

Microwave Oven Transformers ( MOT ) are for generating high DC voltages to power the magnetron. It is the magnetron which produces the RF. So this answers some of my own doubts my previous post. So I cannot use the MOT for RF generatation. ( correct ? ).

Here is a circuit of the Crokcroft-Walton circuit for the generation of high DC voltages.

[ante]

Hi Alun,

I just had a look at my Whirlpool micro, the label says 240VAC 1150W input. The highest setting is 750W so it

[Kevin Weddle]

I don't think you will get the current you desire. Since the current will depend on the capacitors you use, which will be small, you will not get much of it. The peak of the input cycle delivers power to the load because it is where the capacitors will charge from with the diodes in there. The load gets it's current during the peak of the cycle and this current is from the previous capacitor stage. So you do not have a direct link to power. Hence the current will be low and the voltage will drop as soon as you load the circuit.

[radha]

Hi kevin,

I did some calculations and found out that the capacitor values are unpracticably high ( ~ 3000uF ). For less C, the current also will be low ( as you rightly said ). So I am leaving the idea of using the Crockcroft-Walton circuit. I think that the MOT idea will be better, as suggusted by Alun.

[steven]

:)it looks like a high energy radio frenquency device here ,known as the herf gun ive posted the circuits for this in reply to a post somewhere in the forums, before.

[dmdaga]

:Hi Radha,

The circuit you are planning will not work. This circit can not deliver power. This scheme is good for generating high volage only. This ckt. has inharently hi o/p impedance. Voltage regulation will be very very poor.For more details contact me at 09892366363.

D M Daga

[Platonas]

The Crockcroft-Walton is simple to produce high voltages but at very low current values.  The impedance of the cct increases exponentially with the number of stages, hence it is impracticable to be used when you need high current.

Anyway, appart from the microwave oven transformer you may try the Neon Signs transformers.  I am not sure about the power ratings but the certainly have outputs up to 10KV.

[audioguru]

There are some neon sign transformers on the windowsill of a store near me. They are the size of 2 bricks and are rated at 12,000VAC at 20mA. Their mains and HV terminals are exposed bolts, kinda dangerous with kids running around in the store.