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

[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.

[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.

[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.

[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?

[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.

[Richard Hull]

I will be very curious to see if any one using this topology ever enters the neutron club. This is a great idea and effort. All the best on this to any and all using it in a fusor. Slick thinking and design.

Richard Hull

[Benjamin Walsh]

Chris, I was wondering if you might be willing to part with a few of your inverters (for a price, of course)? It would be greatly appreciated.

Thanks for the great information and idea regardless of your decision.
-Ben


(Sorry to make a public post, but I lurk too much and don't post enough to have private messaging enabled. Oops.)

[Chris Bradley]

They are boxed up at the moment but in principle I could do that (if I can find them!!). What is the specification/project you are aiming for and how many do you want, where are you?

These days, it might be easier to find a smaller number of higher power lighting or OBIT transformers that are also useful in this arrangement, as they can make use of the 'GFI' type devices.

[Benjamin Walsh]

I was hoping to achieve 30kv at around 15 mA. I was looking at purchasing about 25-30 of your inverters. I live in Southern California (if you want to pm me your email, I can send you my address) about 30 minutes outside Los Angeles.
I hadn't considered an OBIT or similar transformer, and a cursory glance shows that it might end up being a tad more expensive than a CCFL based transformer, depending on how much you are willing to part with your inverters for. But a great option nonetheless!

Thanks,
Ben

[James Hammond]

Chris, how exactly did you go about powering the CCFL inverters you had?