Hi Lash,
You need -40Kv direct voltage at 20 mA direct current which equals 800 Watts of power.
And you want to generate that power with a transformer with a primary winding of 120V RMS at 60Hz (because you live in the USA).
Assuming that you use a bridge rectifier, the rectification efficiency would be around 70% and the transformer efficiency would be around 60% as a guess.
This means that the overall efficiency would be 42%, so the input power required would be 1905W.
Assuming you could tolerate a ripple voltage of 1% (40V) means that you would need a reservoir capacitor of 0.42 uF minimum with a working voltage of 44KV minimum.
You would also need 4 of 44 KV minimum 20mA minimum rectifiers.
The transformer turns ratio would be 236 to 1 approximately.
The transformer turns per volt would probably be 4 so the primary winding would be 120V * 4 = 480 turns
And the secondary turns would be 480 * 236 = 113,280 turns.
You would need to pay particular attention to the design of the secondary winding to avoid flash over and the transformer would probably need to be varnish impregnated.
My advice would be to wind the secondary in layers with each layer insulated from the other layers.
This transformer would not only be massive but it would also be a massive undertaking.
I hope the above gives you a good feel for what you are proposing.
spec
No probs LashThank you for your response.
That's good because it will be large.I knew the transformer would be large.
The transformer will not weigh as much as 200 lb. Off the top of my head, probably the weight would be around 12 lbs and the transformer would measure, 9" x 9" x 9"The only type of transformers that I know of that are actually able to supply this current and voltage are x ray transformers (and most of the old transformers are at least in 200 lb. steel tanks).
Yes. I would guess a center core of around 2.5" x 2.5".As for size, is there any way to calculate the core size needed.
No but a suitable core will be available. One approach is to canablise an existing high-power transformeror do you know of any cores large enough to do this job?
Not sure at this stage. Do you mean an E and I laminated core.Also I take it the core would probably be in an EE style.
There are special transformer materials: insulating sheets and tapes for the job.With regards to insulating each layer on the secondary, what would I use to insulate it?
No probs Lash
Are you dead set on a transformer approach?
spec
No, that is not true; a voltage multiplier can be used at any frequency, but the higher the frequency the smaller the components can be. For example a reservoir capacitor may need to be 0.42uf at 120Hz (twice 60Hz) but, at 600Hz, the reservoir capacitor would only need to be 0.042uF for the same ripple voltage. The mass of the transformer is also inversely proportional to frequency, for a perfect transformer, that is.Well, I was just thinking it would be the simplest. The voltage multipliers using capacitors, from my understanding, require high frequency.
Very wise to take safety into account but, providing the HV generator is designed to comply with national safety standards (not difficult), there will not be a hazard.Also, I'm not sure that I want to have all those HV capacitors around (I'm not the largest fan of the possibility of lethal amount of power being stored even when I'm not powering the unit).
One approach would be to make a 10KV mains transformer and multiply that by a four voltage doubler stages. Eight diodes and eight capacitors would be required and the whole voltage multiplier stage could be put in an earthed metal case for safety.I would love a cost effective and simple way to get that amount voltage and current, but the options seem to limited to either using a transformer, or using voltage multipliers. It is not as cut and dried as you think.
Not only would the transformer be a pain to wind, but there would be a high risk that the transformer would not perform as required.I will admit though, the thought of making a transformer with a coil of 113,260 turns does seem a bit daunting.
One approach would be to make a 10KV mains transformer and multiply that by a four voltage doubler stages. Eight diodes and eight capacitors would be required and the whole voltage multiplier stage could be put in an earthed metal case for safety.
spec
Good move. In general, it is best with any new project to consider all approaches, never mind how unlikely, and do paper designs to highlight any benefits/major problems. This applies particularly to challenging projects.Hmm. These are some good points. I think I shall do a bit more research into using multipliers then.
Excellent- a Rolls Royce approach. Grounding conductors, tags, wire etc, should always be substantial to handle any fault currents.1. I am glad you brought up grounding, because this was something else I was wondering about. I plan on using a "star grounding" method. I was planning on doing this by having the chassis of the power supply, the steel vacuum chamber, and all other pumps and electronics grounded (about equidistant) to a screw tapped into a metal plate, that would have a wire then going to the a wall outlet ground. Does this seem acceptable?
You can have any polarity you like- it is not an issue. But the high voltage will mean that a large and expensive double pole change-over switch/relay may be required to change polarity. The polarity changing method can be sorted later, when you decide what approach to use and a few trial designs have been considered.2. With voltage multipliers the polarity of the DC current will just depend on which way the diodes and capacitors are wired? (If so I'll probably ask in a different post if it is possible to add a switch to reverse the polarity).
That is correct, and not a big issue.3. I take it since I am planning having this connected to a Variac that is then connected to a wall outlet, that I will need to add a fuse and some other measures before the transformer to keep it from drawing too much current? (I know this may seem trivial but I find that when working with specific things like this, sometimes things can get a bit odd or finicky, so I would rather ask than not ask and destroy a breaker box).
That is correct. But it would be best to consider having the voltage measuring device built-in. Meters are available quite cheaply ($5US).4. Also, I know that when measuring the voltage you would take measure it from the main output (by adding something like a 100 Mega ohm resistor and using a regular voltmeter and then have it hooked to the ground).
If you are talking about the current consumed by the load, yes. Measuring the current at the earth end would be a very wise approach.But with current you would take it from where the PSU is grounded correct?
My pleasure Lash. As has been said before on ETO, it is nice to talk to someone who is informative, explicit, prompt and answers our questions.Anyways, thank you very much for responding and being very specific and informative with your answers. I greatly appreciate it!
That's what we have have been taking about Eric.hi,
You should consider a Cockroft Walton voltage multiplier.
https://www.google.co.uk/search?q=c...eb-47RAhVHOFAKHZ0QAu8QsAQITw&biw=1393&bih=924
E
Yes everything gets difficult, and expensive, with extra high tension (EHT).Another thing to consider is a way to discharge the energy stores in the capacitors when the power source is switched off. Even constructing a suitable resistor chain for that voltage would need a lot of care. Another possibility would be to construct a special switch that would arc before it finaly closed.
Les.
Thank you for your response. I knew the transformer would be large. The only type of transformers that I know of that are actually able to supply this current and voltage are x ray transformers (and most of the old transformers are at least in 200 lb. steel tanks)
Good move. In general, it is best with any new project to consider all approaches, never mind how unlikely, and do paper designs to highlight any benefits/major problems. This applies particularly to challenging projects.
spec
That is correct. But it would be best to consider having the voltage measuring device built-in. Meters are available quite cheaply ($5US).
If you are talking about the current consumed by the load, yes. Measuring the current at the earth end would be a very wise approach.
Again, best to consider having the current monitoring built in. (about $5US again).
Just one question, what accuracy do you need for the voltage and current measurement. Also do you want digital or analog meters.
spec
An important parameter that controls the design is the voltage regulation, which means the amount that the output voltage will drop from a zero current load to a 20mA load. Incidentally the ripple voltage will increase from practically zero at zero current load, to maximum at 20mA current load.
Can you specify the values that you need for both regulation and ripple voltage.
spec
Another thing to consider is a way to discharge the energy stores in the capacitors when the power source is switched off. Even constructing a suitable resistor chain for that voltage would need a lot of care. Another possibility would be to construct a special switch that would arc before it finaly closed.
Les.
Can you describe how this unit will be used, because there may be some safety issues there.
spec
hi L,
As you say, you have no experience in designing or working with HV circuits, I would advise you not to do attempt to construct a 40kV 20mA.
Any mistake you make could prove lethal to you or others.
Contact the Electrical department of a local College or University and ask them for guidance/assistance.
Asking for advice on a on-line Forum is IMHO , is not the way to go.
Eric
As I see it, you are taking a very considered approach and gathering all the information you can about the various issues, including safety. This is commendable.Of Course! I always like to keep an open mind. The transformer project does still sound interesting to me, but it does sound like a large project on its own maybe to be attempted at a later time (the power supply is part of a much larger project, and not necessarily the main focus itself).
I would recommend buying a couple of digital panel meters, and a small mains to 15V power supply. The 15v power supply line would power all your auxiliary functions, including ditital meters, and the two digital panel meters would indicate voltage and current respectively. Yes, you would use high value resistors to divide the high voltage down. Now that we know what you want, this area is not really a technical or cost driver and can be detailed further down the line.Buying a self-powered digital voltmeter was the plan, but almost no voltmeter that I know of (besides very specialized multimeters), can handle that voltage to be measured (hence why I would attach the resistor before the voltmeter). As for current, current is the trickiest part to me. Apparently you can do some things with resistors to get the voltmeter to read 1 volt per milliamp, but I am wondering if there is a better way. The plan with the measurement is simply to get the meters to read 1 volt per KV and for current 1 volt per mA.
Would a full wave rectified, 40Kv peak signal be acceptable? If so the reservoir capacitor can be eliminated.For what it is being used for I do not believe ripple is that large of an issue.
NotedIf I do use any capacitors I would like to have some bleeder resistors added. Having all that stored energy around is not something I am too comfortable with, for my safety and others.
Thanks for information- sounds like interesting research.The unit is going to be used to supply power to a grid within a high vacuum chamber to create a large enough potential difference between the grid and the chamber to Ionize the gas. I am wanting to look at and research some of the effects of the plasmas of different gases at different voltages and pressures. I understand in order to get a plasma much lower voltages than -40KV are needed. However, I do plan on using this supply and chamber for eventual experiments with such thing as IEC. This power supply also would be something can be used in most other projects I plan to do with HV.
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