A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

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Chris Bradley
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A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

This is the first article describing practical outcomes of my epicyclotron project, and possibly the most useful one for future fusor applications. This is the power supply I developed for the project (thread reference; viewtopic.php?f=8&t=2029#p9250 epicyclotron project #5). I've been using variations of this circuit for 3 years, and have applied for a UK patent for it.

With this topology, it is possible to use many feeble HV AC power sources, and add their collective power and voltages together into a substantial power supply.

I have a small degree of reservation in putting forward this construction because, in the normal scheme of things, the voltages and powers that amateurs have access to are usually limited by availability and cost so the progression to high volts and power is slowed and there is time to acquire the native skill and cautions for HV work (and the consequent X-ray spectrum they may generate between electrodes in a vacuum). However, as you will see, this topology means that one can buy readily available parts, brand new, and move straight on to having one's own high power, high voltage circuit for just a few $10's. All the risks of any multi 10's kV x multi-mA supply go along with this, so please excuse me to just briefly flag up the usual cautions.

The history/origin of this circuit:- Part of the construction of the circuits in my epicyclotron involve applying a pulse on top of an HVDC voltage. It dawned on me that because one can capacitively isolate *any* time varying signal with a capacitor and a diode in a charge-pump arrangement, and 'stick' it on top of an HV level, then a natural consequence of this is that you can simply keep adding HV stages on top of other HV stages providing you give each supply its own capacitive 'Greinacher' type isolation and that they share a common reference potential.

So I arrived at the circuit which is, generically, shown below. I have also attached a copy of the patent application which shows, and describes in detail and practice, other configurations of this circuit.

...
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Prototyping the circuit:

Initially, I wasn't entirely sure this circuit would live up to the job, so I ran a 4 stage device built from 4 x 4W 12V (car) neon light supplies for several months before being satisfied it was a robust design. I bought inverters from this site;

http://shop.wiltec.info/product_info.ph ... -40mm.html

Unfortunately, the price has gone up since I bought 100 at E1.25 each. However, they are able to put out a lot more power than I was expecting, so I didn't need as many as I thought and six of these is fine for a 30kV supply capable of delivering 80W at those voltages. (6 of these inverters will put out higher powers for a short time but will overheat, so for higher currents you'd need to parallel up more such supplies.)

With a nominal rating of 4W each I figured I'd be needing dozens. In fact, performance testing showed they were happy enough operating continuously up to 20V and 1A (at the same time). At 20V they are putting out ~2kV RMS unloaded. (They can be run to 'military' power to 25V and 1.25A, but anything higher they overheat and burn out.) They operate at around 65% efficiency, across a wide range of output voltages/currents. So I rate one unit can reliably deliver 13W, whilst dissipating 7W of heat, though they may not perform that well in an unventilated enclosure so I avoid running them that hard continuously.

The sum total of the electronics parts here that I paid, which are still commonly available on the internet, was 1xinverter ($1.50), 1x100pF 30kV cap ($0.45), 1x10nF 6kV cap ($0.30), 2x8kV 10mA diode ($0.25) = $2.50 per stage. 6 stages = $15. Box, terminal block, nuts and bolts, plus heat shrink were an extra $10. Total for this 30kV 80W device, built from all-new, readily available parts = $25.

Below I have attached an example of one of the 6-stage 30kV supplies I have made with these little 12V inverters. (This is one of the two PSU's shown in the photo in viewtopic.php?f=8&t=2029#p9250 .) In the demonstration shown here, the DVM is reading 1/200th of the output voltage (it is a 1M meter in series with 200M), so it is showing 20kV, which is across a gang of 100M resistors totalling 5MOhm at 4mA (80W output, about its max continuous power rating).

Clearly, the circuits I'm presenting here can be used with any transformer type. Even mixed types. Preferably, you'd want to use a high frequency type to keep cap values low. The following is the important element of implementing this type of circuit with common lighting inverters: These high frequency inverter type transformers usually have a shunt capacitor in at least one of the output lines. It is NOT possible simply to attach the linking capacitors (caps C1, C2 & C3 in the diagram) on to these outputs, because the built-in caps will take on too high a voltage when they isolate the stage they are feeding. In the case of these little inverters I've used, when I got them I was somewhat frustrated that they are fully potted! Ach! Initially that seemed to make circuit modifications difficult. Fortunately, only one output of the output transformer within it had a capacitive shunt, and the other wasn't earthed. So what I have done is drill through the case at the right point, then screw in a bolt which shorts the output [that used to have the capacitive shunt] to the case of the unit. The original shunt capacitor is now eliminated from the circuit. Then I put the high voltage 'link' capacitor onto the other line, which is therefore then directly connected to the output transformer within.

So the characteristics of the driving inverter you will need for this circuit is that one line is a shunt capacitor that you can remove from the circuit, and then you need to be able to set up an earth on one of the output transformer's lines, and add your own high voltage capacitor to the other.
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Circuit Variations:

If you have more powerful inverter units, then you can use this capacitive linking from each supply into a number of the stages, rather than just one per supply (see Figure 8 of the 'HV_psu_pat' attachment to explain). If you end up with just one supply feeding all the stages through serial caps (to increase their isolation voltage), then you end up back with a regular CW multiplier (as shown in Figure 7(ii))!! You can see how this circuit comes 'full circle' back to the conventional design.

A very useful outcome I have realised, even for a regular multiplier, is that capacitors in a CW multiplier do two distinct jobs. They don't do the same work. The caps in direct series with the output load need to be high values to smooth the load output. However, the other caps, the 'link' capacitors, can be significantly smaller than those. This is because the caps connected to the AC supply get pulled up and down in each cycle anyway, and it doesn't matter if their voltage varies more widely that the 'storage' caps in series with the load as they are only serving to follow the input voltage. It's surprising how small these 'link' caps can be – I use two serial 100pF 30kV caps to isolate my 60kV supply, and they feed 'storage' caps of 10nF, that's 200 times smaller capacitance value for the 'link' caps than the 'storage' caps yet without detriment to the output ripple!

The other particular thing to note (which you might have noticed in the previous picture already) is that with this capacitively isolated PSU design, it is also possible to capacitively isolate the ground itself! This is a bit bizarre at first sight, because it seems that the output stack is not coupled to anything, so how can it generate a potential!? I think this is a **Really Big Deal** for this invention – it provides a means to construct a supply with a fully DC isolated output. It is not 'floating', but it does mean you can freely connect ground to either end (or a middle) output. It also means you can stack these supplies in series to add up their output voltages with other such supplies – providing the link capacitors can give enough isolation. I think this works because capacitively coupling multiple nodes of the stack to the ground causes each stage to 'push' against each other. Collectively, they can then drive power through this completely capacitive decoupling. I figure that particular feature, and together with the use of multiple small supplies on one stack, is useful and patentable.

Another great thing about this topology is that power control can be achieved by simply turning each supply on or off. This is not a continuous control, but provides a degree of discrete power/voltage levels that may be sufficient control for some applications.

Incidentally, if you have doubts that such 'weedy' looking inverters can do this collectively, I can confirm that the voltage reading showing is accurate. Firstly, I can test that DVM arrangement with my calibrated 30kV high meg tester and it reads accurately (similarly, the resistor values have been similarly checked). Secondly, this thing reeks of ozone when it is operating, and the noise of coronal discharge is usual as I have not sought to particularly reduce the localised breakdown of the thin internal wires. I dare say I am losing a mA or so to coronal discharges themselves! Thirdly, it generates copious detectable x-rays across electrodes in vacuo, as expected for the measured voltage.

On that last point, please may I just note that if you use this topology and gang up a total of a few 100W worth of lower-power cold cathode inverters, which are pretty easy to get a hold of these days, then you WILL end up with a few 100W of HV power. I speak from experience in saying that you get a bit blasé about handling these lower powered units on their own, but the HV and X-ray hazards when summed like this are as real as any other high power HV supply.

If you have an interest in trying to build one of these, then the attachments will provide you with a fuller description of what's involved. Remember that your HV safety is your own concern, it's real easy to get into trouble with lots of HV power around, both direct and indirect contact and from X-rays, and this topology makes 'lots of HV power' readily accessible on a budget.

There are some weaknesses to the design, the main one being that if the stage connected to the potential furthest from ground is shorted to ground, then the capacitor jumps through the full voltage which causes the full electrical stress across that inverter's output transformer. However, this is generally OK unless you are deliberately using it to make big fat sparks directly to ground. Just avoid 'sparking' as best you can, and the occasional arcing should be tolerated. I've stress-tested the PSU shown at its 'full' nominal value (36kV), and generally the 'top' one, or two, inverters blow up and 'protect' the rest of the stack. Broken inverters are easily swapped out. Sometimes you may not even notice there are faulty inverters because the stack will still keep operating even if one, or a few, of the stages is no longer producing any power. If you have ballast in series then the circuit generally survives and protects the inverters. I've described this issue along with some further mitigations and circuit variations in the patent application documents, as attached above.
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Just a bit more eye candy for you here, so the first pic is showing how seriously risky such a build can get!!! HV wires everywhere! This dodgy looking layout sat under my main work-bench for several months before I got around to building the enclosed modules to keep it all safe and tidy. These 4 inverters gave me a working envelope of 24kV/10mA, up to 60W or so, and performed well whilst I 'stress tested' them during regular experimental operation.

Second pic is showing the testing of two modules, six inverters each, one with 30kV of capacitive isolation in series with a second with 60kV isolation. I was testing total output to beyond 60kV here, but there were certain difficulties, such as all the LCD displays with a meter turning black so I got no readings from them, not having suitable test loads, and the copious production of ozone which became a bit unbearable quite quickly!

Third pic is a typical installation, all neat and tidy once again, during an experiment. (This is a close up of item #5 in the photo in the link at the top of this thread).
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Donald McKinley »

Wow Chris,

Thanks for posting. Beautiful work. I can't wait for the rest of your project details. It's very inspiring. It looks like this design could be made ultra compact.

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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by nicolas leboucher »

hello chris,

I ve bought some of the same inverters for my diy scanning electron microscope, in order to build the hv power supply for accelerating column. Can you please give more precise explanation for shorting the output capacitor with the screww that can be seen on your pictures ? thanks
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Hello Nicolas,

Sure. I have some nice x-ray images of the internal parts of these inverters to show how to do this, but, sorry, can't get to them right now. Should be able to get them later in the week and I'll explain in more detail.

There is a certain hit-and-miss with getting the screw in the right place, but, usefully, there is a label on top which is very useful to guide where to put the hole! I've lost about one in 20 to getting the hole in the wrong place ... :}

One thing to note is that I got two different types from this supplier, and the potting in the second batch was a bit variable. I guess a cost-reduction design change. ~ a third had no potting above the board, ~ a third were mostly potted and the rest fully potted. A bit variable. Quality versus price, I'm afraid! But this can be dealt with easily enough because if the potting is bad you can remove the aluminium cover and solder direct to the transformer. There's usually a way where there is a will!...

When you get your inverters, please post a photo of the black potted bottom, and of the end with the white wires, of one of the inverters in your batch then I'll know which type you've been sent so I can advise you appropriately.

(PS - thanks for your intro post and the efforts you've posted on your website. Hope your projects go well.)
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by nicolas leboucher »

chris,

thanks for your reply and i ll send pictures of the inverters as soon as i ll get them.

I m on the way to build th HV power supply for the accelerating column of my homemade scanning electron microscope, starting from your design. I m also working on the new design for my vacuum system around an Edwards diffstak 63 diffusion pump and a couple of inficon gauges and pneumatic valves, the whole being controlled by a labjack and labview software.

Concerning my website i will had within the next days some pages about homemade cyclotrons and electrostatic accelerators, going back on my very first intend which was to give an overview in french of the more significant amateur projects in physics.
I m also inserting english translation within all the pages, realizing that the amateur particles physics enthousiasts community is mainly from US and UK.

I d like to point out the great job you ve done on your epicyclotron. Building a cyclotron was my very first intend, but i give up last year due to the lack of used electromagnets on the french market. You create an alternative way and i m really thinking of building one after my vacuum system completion.
One question : does your patent prevent anybody from using the same design without your permission ? I ve translated all the posts on your project for my own, and i also wonder if i can publish a page about your project on my site.

Thanks a lot for all the shared inforamtion !!!
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

My patent has no effect on anyone outside UK and US. I'll update that thread with a quick comment about it, just in case a reader were to want to try it.

But please let me restate to ensure I do not mislead anyone in any way; my epicyclotron is an experimental design and I have yet to figure out its exact behaviour, even for myself yet!
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by nicolas leboucher »

hello chris,

if think there is really no confusion about that, your project just offers an opportunity to try a new concept, without any garantee of success. the main interest in amateur physics is to build uncommon things while learning, and the most exciting thing is probably to open new research paths with limited means.

I have two practical questions:

what is the value of the resistors placed in serie with the linking capacitors? You mention a value of ten times the impedance of the inverters, but what is that value?

the inverters are powered in series with a DC power supply, what are the characteristics of the one that you are using?
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Rich Feldman »

Nice work there Chris, on a great idea. I'm looking forward to seeing more.
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Here's pictures that will help you make sense of the 'bolt' for shorting the shunt capacitor out of the circuit, that I've described and that appears in the photos of my prototypes above.

First 4 image samples show;
- an inverter from the top,
- one that has had the aluminium stripped off a sample that has had a poor and incomplete potting (this is easily done by hand, just cut off the connectors at the wire ends and bend the can off), and in this example I have soldered a wire onto the pad that needs to be shorted to ground,
- an inverter with a hole drilled 18mm to the left of the end with the output (white) wires, and 5.5mm up from the bottom, per orientation shown,
- an x-ray of an inverter, the shunt capacitor being shown in the lower right of the picture (which is what we need to short out).

So the steps I followed to modify my inverters are;
1) Drill a 2.5mm hole 18mm from the right and 5.5mm up, with the output side to the right. If the label is stuck generally central, then this is just between, and slightly high above, the 'g' and 'e' of 'verlangern'.
i) drill so as to just puncture through the Al case,
ii) stop and inspect the hole - if black material has come out with the drilling, there's potting there, go to step 1(iii). If not, inspect with a torch and probe, e.g. with a fine screwdriver - is there any resistance to putting in the screwdriver in (note - you may come across a piece of insulating paper in there), or can you reach down with it to, or see, a soldered pad in the hole? If so, you may prefer to strip the Al case off, as shown in the photos, and solder to the pad directly. If you don't want to do it that way, go to step 2.
iii) keep drilling further down, the drill will cut easily through the potting until you feel some resistance - which should be the board and the solder pad. Drill just an extra mm beyond that, and a little bit of metal [solder] should come out of the hole by the drill flutes. Stop there!
2) Tap the inverter on the workbench, upside down, so debris drops out of the hole. Take a 3mm machine bolt. It will self-tap into the 2.5mm hole if you hold it carefully and perpendicular as it bites the Al. Screw it in until you feel resistance as it bottoms out in the hole. Don't over tighten, it'll easily strip the self-tapped thread in that Al case.
3) Measure the resistance between the case and each of the white output lines. One should now read ~146 Ohms (the upper lead, as you look at these photos). The other will be DC isolated but you might be able to read the 50pF cap on it - cut that lead off.
4) If you don't read ~146Ohms on either lead then tighten the bolt up a little further. Keep tightening the bolt up a little until you read ~146Ohms. It should take around a half dozen turns of the screw to reach the solder point where you've shorted that connection to ground.
5) If (4) doesn't work out for you, maybe you needed to have drilled a little further. It should have been around 6~8mm deep. You could try drilling a bit deeper, though you've probably missed the spot due to some internal manufacturing anomaly (or it isn't like the ones I've done!).

If these steps have worked out, you should now have an inverter that looks like the next set of 4 photos, with one white lead with 146Ohms to ground (because the other side of the transformer is shorted to the case by the bolt) and the other lead is cut off.

If you have stripped the case off because there was no potting where you drilled, then solder to the point shown in the previous photos, and that should then be subsequently connected to the same connection as the black lead (input) when you rig up the circuit.

There is always the option that you just strip the case off the inverter in the first step and then drill or cut the potting away from the solder pad (if it is potted properly) and solder on to it. Your choice. But the Al package has seemed useful for screwing the inverter in place, and seems to help a little with LF shielding.

Please let me know if that is a good description for you, or if there are any more questions on preparing these inverters.

Also to say, once you have your inverters and know the V/I requirements you need for your supply, then let me know and I'll work with you to figure the best arrangement to achieve the output requirements. Max input requirements are 2-20V at N Amps (N = no of inverters) if you run all the input leads in parallel.

In the prototype you see above, I have arranged the six inverters in 3 pairs so that they can be connected all in parallel (20V x 6A) or in series (60V x 2A), which suits my bench supplies. You might have spotted a plastic divider between each pair - that's so the cases don't short to each other when they are wired in series.

...
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Ed Meserve »

Chris, really nice work, and thank-you for sharing. After seeing what you've done here, we are going to use your method for our power supply.

Did you take the x-rays of the inverter yourself?

regards,
Ed
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by nicolas leboucher »

hello chris,

I ve received this week-end the inverters from

http://shop.wiltec.info/product_info.ph ... -40mm.html

and the model seems to be quiet different as yours.

as you can see on the first ans second pictures, the white wires now come out from the upper part of the alu pot side.

as you can see on the third picture, i stripped off the aluminium on a sample that has had a poor and incomplete potting.

white wire F is connected to pad A
white wire E is connected to pad B
a capacitor 50 pf have been measured between C and D

so for this model ( I ve got 10 of it) I will give within a few days new length measurements to drill the hole over pad D which is about 11mm from the right side ( without alu thickness) and 4mm from the bottom side (without alu thickness).
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Hi Nicolas,

You're OK with these. I think the current batch has probably undergone a process modification to save a second (..?!) on manufacturing time by not giving any stress-relief routing to the output wires - they come straight out now, whereas on the ones I described above there is a loop that I presume was there originally to help avoid the wires getting pulled of the pads if the wire is strained. (Maybe - just a guess why that was like that in the first batch.)

So if you look at the comparison below, I think this is like yours and you have much the same, just that the wires are routed differently. That's all.

In regards the pad arrangement, if you take a look at the photo above where I have soldered a wire to a pad, that group of 4 solder points are the pads numbered 1,2,4&5 in the x-ray below, which are all electrically connected and are on the transformer side of the shunt cap. I chose to aim for pad 4, but any will do the task.

But looking at your inverter does look like there may be further differences to the board's pads. I can't quite tell. Looks like they have cut down the number of solder pads.

As long as you are measuring ~146Ohm (the ohmic resistance of the secondary) between your wire F and pad D then pad D would be as good as any to use. All you're after with this topology, or with any inverter, is to take one output of the transformer HV secondary to ground and the other you add your own shunt capacitor to (with a value high enough to isolate the output). Whatever way you do that, that's all it's about.

Well, in any case, I trust I've given the detail you need to determine which two pads are the capacitor's solders, and you can see now to short the transformer side of that cap to ground. Whether you do it with the can pulled off, or drill through as I've done, I think you'll be good to figure out for your own build now.

Good luck!

So, what output are you actually after, Nicholas? For your application of an SEM you should never experience a discharge, so you won't need those internal resistors I've shown in my prototype. Instead, all you'd need is a big-value ballast resistor just as a 'safety resistor', just in case of a fault mode. But if you want to ask further let me know what you want your outputs to be/do, and I'd be happy to advise what I think is the easiest arrangement to accomplish it. [If you don't need a big current (which should be the case for an SEM) then you can use one inverter per 2 or 3 stages instead... ] Or just feel free just to experiment for yourself!

...
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Hi Ed, these were taken on a professional real-time x-ray microscope. I've pondered whether to attempt an x-ray with these power supplies, but it was just a 'pondering' at this stage. X-ray generation needs to be from a very small target for image focus, which should not be too difficult, but I've no idea on how to image the x-rays afterwards. I guess I'd need a 6" wide CCD!?
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

I had some issues with one of my supplies this weekend. I've ran the two prototype units you see above for a year with few problems. One I have had to replace a broken inverter on one occasion, and the other one twice when I was deliberately testing out sparking discharge robustness.

So, generally reliable in practice but occasionally the weakness I've described above shows up if it is subjected to a hard arcing load.

This weekend I was running some fairly routine tests but I had installed a 68k ballast resistor internally to the unit. I guess the current through it reached a level where some voltage potential had been reached and I heard a couple of arcs internally and found it was under-performing after that. The set-up was as per a negative polarity output, with the +ve end grounded, so the inverter at the -ve end took the most hammering. I replaced that but then the new one broke straight after too, which was odd (maybe a duff inverter - we're not talking 'expensive quality' here!)

Anyhow, I decided to implement one of the solutions presented in the patent application documents, above. You can see how a 'charge pump' arrangement can be set up in Figure 12. The photo below shows the practical implementation of this, on the lower stage (only).

I'm uncomfortable with the RC times in this, as these HV diodes have a trr of around 150ns. I'm going to try a range of higher shunt resistor values and I think they should be more comparable with the impedance (@40kHz) of the capacitors. I think it is better that they are in the order of a few 10's k. It is a drop of a couple of 100V at 5mA, but the stack itself doesn't suffer from insufficient volts - it is the current and arcing protection it needs. I think it is probably better to sacrifice more volts for safer tolerance to adverse loads. I'll do a few more experiments to see if there is some amount of ohmic shunt resistance that, essentially, fully (or mostly) protects it from arc discharging.

... If anyone has got any other good ideas on circuit implementations that can protect the inverters from external arc discharges, then I'd be interested to hear them ....


edit ... I've just now upped the shunt resistors to 68k and I can now strike repetitive 15kV sparks and no damage seems to have yet occurred. I'll test under continuous load to check that this amount of shunt resistance is not too great, but as the inverters themselves cannot really put much more than 5mA out, so I don't expect a continuous load to overheat the (3W) resistors.
...
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by ab0032 »

You have two sources for the current in the sparks,
1) discharge of the charge stored in the caps
2) current pumped in at D1 in your diagram.

Your 68k is quite big, I see two alternatives,

1) instead of putting one large resistor in at the end, you could try putting smaller resistors, perhaps 1ks into the diode chain between every two diodes and a final one that is smaller than 68k at the end of it.
That way the caps at the end will discharge quicker, but their charge is limited and after that the charge in the caps further down the cascade will discharge, but slower.

2) Ignore the charge in the caps, the energy stored in them is pretty small anyways (less than 1 Joule?), and simply limit the current going into D1 way at the bottom. This could pose a problem though, if you do not let new charge in but keep pumping, the lower end at D2 might reach a high potential too, in your case a high positive potential, as you keep pumping the electrons out the other end. Since the circuit is not made for this, it could damage something at the lower end of the voltage multiplier cascade. But because of this positive charge pumping could also break down or at least slow down.

I would be really interested to see how alternative 1) performs but also curious about the second.
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Chris Bradley
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Hi Alex. The issue isn't the current in the spark, per se - the output caps have no dv/dt issue. The issue is that a big dv/dt on the coupling caps will cause a sudden voltage rise at the inverters. I was thinking whether there is some suitable inductance to increase the impedance in a short rise time event, but as there may be a variety of external ballasting then I'm not sure if I can pick an inductance value that doesn't interfere or resonate with the coupling caps but does achieve the necessary impedance in a high dv/dt event.

So an ohmic impedance seems to be the easiest and best 'catch-all'.

68k is a drop of 340V at 5mA (a loss of 1.7W per resistor at max current). This means I'd be losing something under 10% of output power (at max power) to that much ohmic resistance, but it seems the safest and simplest way. It's really not much because there is an excess of volts from the inverters (they can pump 36kV, while the coupling caps are 30kV rated - I have 6kV I can 'lose' without impact of volts!) Bear in mind the 50pF coupling cap already presents an impedance of 80k at 40kHz, the issue is that their impedance for a high dv/dt drops substantially and therefore pulls the inverter around excessively (if you want to look at the maths of the electronics that way).

Also need to look at RC times - With 68k, the RC time is now around 3us, so as the voltage ramps up towards, say, 20kV on the end of the inverter then that protection diode (150ns trr) should be opening before it gets much above 2kV.
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by ab0032 »

Ahh you are one step ahead of me, I didnt think of the inverters. True.

The 68k could burn out if it is not 30kV proof which means you could get an arc there and hence also the current after the 68k is destroyed, so I think it would be better to use multiple resistors distributed around the circuit that can actually take the voltages they receive, eg between the diodes for example.

The thought of putting some mHenries into the output line also crossed my head, but I really dont know if that is such a good idea in the end... It would stop current from rising too quickly, but it would also keep it going when you might not want it any more leading to potential peaks in excess of the 30kV you might want.
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

The resistors might, indeed, see 30kV across them. But they are in series with a relatively paltry ~50pF, so they'll never have 30kV across them for very long.

This is the issue; for example, imagine there is an arc from the -HV (let's say it is at 30kV) to ground. The coupling capacitor at the lowest end will have one end oscillating (AC) around ground, and the other end will be being held at -30kV but as it is shorted so it leaps to 0V so the other end jumps up to 30kV. If the 68k resistor is in the way, then as that terminal jumps to 0V so it is the resistor that will have this 30kV across it, in that instant.

However it only has it momentarily because the current through it starts charging up the 50pF link capacitor. For a us, there will, indeed, be a ~half an amp running through that 68k resistor giving it an instantaneous power of ~0.5^2 x 68000 = ~17kW. The 3W resistor survives because 17kW for one us is 17mJ. The resistor can handle a 17mJ impulse. In doing so, it slows the ramp-rate of the coupling capacitor's connection to the inverter, thus protects it from receiving a sudden voltage sufficient to cause the inverter internal damage.
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by ab0032 »

You can buy the inverters cheaper at a place like mouser.com

Here is just one example of the price for 40 TDK brand inverters that output 2x 6mA at 31.16 Euros, they have a huge selection of inverters, dim-able and whatever. Most likely you will have to add something for shipping. Maybe they only sell to businesses, I dont know. Pick the country closest to you from mouser...

http://de.mouser.com/ProductDetail/TDK/ ... Ed3HXyQA6O

The dimmer function may be interesting for alternative circuit designs.

> I bought inverters from this site;
>
> http://shop.wiltec.info/product_info.ph ... -40mm.html
>
> Unfortunately, the price has gone up since I bought 100 at E1.25 each.
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Re: A new topology for building a low-cost DIY HV power supply ... 30kV/80W for $25.

Post by Chris Bradley »

Hi Alex.

When I said 'the price has gone up', it hasn't gone up much! You can still buy 50 of the Wiltec items at 1.39 each (including taxes). So the whole purchase price of these is cheaper than a quarter of the additional VAT (alone) on the one you mentioned!

But you mention something I should have emphasised - most cheap inverters are intrinsically 'dimmable' because they simply pass the applied volts to the transformer within it, and so the output is 'analogue'. Whether the driver electronics work at variable volts may be a different question.

The ones I mentioned from Wiltec operate from just a volt or two up - I guess this is just the VBE of the internal bipolars that drive the simple oscillator circuit within. So you adjust the input volts to get control of the output power.

The only figures of merit worth worrying about are therefore the VBE and VCEO of the internal transistors that determine the lower and upper supply voltage limits. I'd hazard a guess that the ones in the Wiltec units are around 1.5V and 30V so volts inputs are OK but I think I have blown them up before in 'performance testing' due to excess current beyond their ICM.

It'd, obviously, be much less useful if you were stuck to one output power if your inverters could only operate at one given voltage, so you don't want ones like that if you are trying to build this circuit for a variable supply, but I don't think you'll find many like that in the really cheap varieties.
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Re: A new topology for building a low-cost DIY HV power supp

Post by Deiter Hanbicki »

Hi Chris,
The power inverters on Wiltech dont seem to exist anymore and I can't seem to find any that arent >$30, am I searching for the wrong thing or do cheap inverters not exist anymore?
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Re: A new topology for building a low-cost DIY HV power supp

Post by Chris Bradley »

Unfortunately, I can imagine a time when CCFL inverters will, indeed, no longer be available.

I guess wiltech have moved over to LEDs only.

For now you can still get CCFL inverters for 'old' computer monitors, but these are moving to LEDs too, so that supply will die out.
see... http://www.ebay.co.uk/itm/310952320305

At some stage you will have to go looking for ferrite transformers yourself and build your own oscillator. This is really not a very big challenge, but finding the transformers might be. (I've always been keen to get a hold of piezo-transformers, which would really suit this topology.)

Lots of alternative low-voltage technologies have been taking over high-voltage solutions since transistors took over from thermionic valves. That trend will continue I am sure, and loss of a cheap supply of CCFL inverters will eventually become loss of a supply of any CCFL inverters. Folks late to amateur plasma experiments will find, and are finding, that a supply of HV components, from resistors to capacitors and diodes, are simply drying up.

Things can only get more expensive, if available at all, as any real industrial demand drops away to zero. It seems these are twilight years for any sort of mass-produced high voltage parts.
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