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Rare HV to LV converter

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Antonio

Sat Apr 05 2014, 12:23AM

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

A simple rule about charging a capacitor through a resistor from a fixed voltage is that the resistor dissipates the same energy stored in the capacitor. It's possible to use an inductor and switches/diodes instead of the resistor, greatly improving the efficiency.

Microwatt

Wed Apr 09 2014, 08:59PM

Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
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howabout using a tesla coil backward? is it not posible to get 200 ac out and use a smps to get it down to 12vdc?

Antonio

Thu Apr 10 2014, 12:06AM

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

Microwatt wrote ...

howabout using a tesla coil backward? is it not posible to get 200 ac out and use a smps to get it down to 12vdc?

This would work, but what to use as switch from the high-voltage supply to the top terminal of the Tesla coil? The switch would have to block twice the full voltage. A spark switch would waste half the energy or more. An inverted flyback converter is less bad, but again there is need of a switch blocking the full voltage.

Microwatt

Thu Apr 10 2014, 04:33PM

Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
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Posts: 224

well the power wasted will make up for the high costs of SiC IGBT's you will need to use.

GammaRay

Fri Apr 11 2014, 02:25AM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
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Posts: 104

UPDATE:

I found a Buck converter I built years ago. I modified it with a gas filled spark arrestor (500v) on the input for S1, then connected the buck converter, through a 1M resistor to cap C2 charged to 1200v (the 1M input resistor not shown on the schematic). It worked, it down converted 1200v to around 5 volts on a load. However, I assume using a spark gap as a switch and a 1M input resistor is amazingly inefficient.

1397182762 5323 FT162126 Buck Schematic 20

Now that this approach shows some promise (at least at 1200v input), the question in my mind is exactly what switch to use as a replacement for the gas filled arrestor for use with this particular circuit considering that the switch could see voltages as high as 35kV. I remain appreciative of helpful suggestions.

Sulaiman

Fri Apr 11 2014, 07:35AM

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

GammaRay, it looks to me that your circuit charges up C2 then transfers the energy to C1
... a current source / current limiting device would need to be used for repetitive operation
... bringing the problem back to charging a capacitor from the eht stage voltage
unfortunately the energy loss in a resistor charging a capacitor is equal to the capacitor stored energy
(never realised this before..neat)
applies only if the capacitor voltage is allowed to reach the charging voltage
which in this case would defeat the purpose.
So the energy dissipated in a series charging resistor
will be more than the energy transferred to the capacitor,
just didn't want a useful relationship misleading estimations.
I can't think of any practical converter, I'll be reading this thread with interest
only power grid eht dc / ac converter technology comes to mind.

Ash Small

Fri Apr 11 2014, 01:16PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Ok, as resistors are out, how about an inductor, under oil, to limit current? The problem of the 'HV switch' still remains, and there will also be insulation issues to deal with.

Sulaiman

Fri Apr 11 2014, 05:00PM

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

something like resonant charging could work efficiently
but the resonant capacitor per stage would be twice stage voltage
and I cant imagine how large an rsg for the whole system would be
- certainly lots of rotational energy!
Then there's the problem of the main capacitor voltage falling during discharge.

Ash Small

Fri Apr 11 2014, 05:04PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

something like resonant charging could work efficiently
but the resonant capacitor per stage would be twice stage voltage
and I cant imagine how large an rsg for the whole system would be
- certainly lots of rotational energy!
Then there's the problem of the main capacitor voltage falling during discharge.

So how about either a series mosfet string, or HV triode, and a variable inductor to deal with falling voltage. Just move the core out of the winding to reduce inductance?

EDIT: or place the inductor between the main bank and a much smaller capacitor, and then discharge the small one into your LV bank using an scr or similar. The inductance (if it were large enough) would prevent the main HV bank from discharging (I know this would need a BIG inductance, so maybe the above idea would be better)

Artlav

Sat Apr 12 2014, 01:04PM

Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94

Meanwhile, i kept wondering about the idea from my first reply - using a couple of HV triodes in a regular push-pull converter, and decided to try it out.

Somewhat unsurprisingly, it didn't work at all.
There was only 0.5V at the output, instead of expected 20V, with 1000V input.
Not sure if it will be a good idea to try it with 10-20KV.
The triodes are 6Ð¡20Ð¡ i mentioned above, rated up to 25KV, and 1 mA.

So, what am a doing wrong here?

Here is the schematic - a 555 PWM with an inverter driving a couple of triodes between -12 and +6, with a tapped transformer attached:

And the construction:

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