HV capacitor question (Rolling my own) / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

HV capacitor question (Rolling my own)

first previous 2 3 4 5 next

Move Thread

LAN_403

Ash Small

Sat May 17 2014, 09:09PM

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

I've just ordered five 50ml tubes from these people:

for Â£15.62 with free delivery. It should be here by midweek

Newton Brawn

Sat May 17 2014, 09:35PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi Ash !

Let me know the maximum working peak voltage to be applied in your capacitor.

Netwon

Ash Small

Sat May 17 2014, 11:21PM

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

Newton Brawn wrote ...

Hi Ash !

Let me know the maximum working peak voltage to be applied in your capacitor.

Netwon

I'm planning to establish that by 'destructive testing'. This is an experiment to test this particular dielectric.

Can you give me a theoretical figure for 155 microns of polythylene?

Newton Brawn

Mon May 19 2014, 02:11AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Ash;

For polyethylene: the dielectric strengh is about 20kV per mm, the dielectric constant is 2,2 ~2.3.

Case #1: No air inside the insulation.
If the insulation is solid without ANY air inside the 0.155 mm polyethylene insulation max voltage could be calculated as 0.155x20kV = 3.1kV
Applying a safety factor of 2, the max working peak voltage in such capacitor could be 1.5kV.

Case #2: The insulation is 0.155mm polyetylene an some 0.05mm air pockets inside.
The ratio (solid dielectric thickness)/(dielectric constant of solid insulation) is 0.155/2.2 = 0.07.
Ploting these numbers to the graphic we can see that the starting corona voltage is 1.5 kV.
Applying a safety factor of 2, the max working peak voltage in such capacitor could be 750V.

Now you disclose the maximun working peak voltage for such cap.

Ash Small

Sun May 25 2014, 01:32PM

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

As I pointed out earlier in this thread, there is a whole range of values for the dielectric strength of polythene. 20kV/mm is at the lower end of this range.

I have calculated that if I use four layers of polythene, and roll a lower value capacitor, using the polythene I have, capacitance works out at ~45nF, and max voltage is around four times that for one layer. 45nF does sound like quite a useful value for a primary tank capacitor. Also, the dielectric grease will go a lot further. It should also be a lot easier to actually roll.

Newton Brawn

Wed May 28 2014, 01:30AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Your capacitor :

Using 3 layers of polyethylene 0.155mm,
Assuming some 0.05mm thick air bubles between the polyethylene and between the polyethilene and aluminum foil.

The the ratio solid dielectric thicknress / dielectric constant of the solid dielectric is
4 x 0.155 /2.2 = 0.28mm.

Looking the graphic, the crossing of line 0.05mm with the 0.28mm curve shows that the starting corona voltage is about 4.2kV peak;

Applying a safety factor of 2, the max working peak voltage in such capacitor could be 2100V.

Testing:
Coil a capacitor using 4 layers of 0.155mm dielectric, the capacitor to be designed for 200pF,
a 200uH coil, a 100k resistor, a variable voltage souce, all in series connection.
and a small portable radio placed near the coil.
The radio will indicate when the corona starts....

Ash Small

Wed May 28 2014, 07:52AM

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

Newton Brawn wrote ...

Testing:
Coil a capacitor using 4 layers of 0.155mm dielectric, the capacitor to be designed for 200pF,
a 200uH coil, a 100k resistor, a variable voltage souce, all in series connection.
and a small portable radio placed near the coil.
The radio will indicate when the corona starts....

This is very interesting. It's certainly worth testing to see when corona starts.

I'm anticipating a value of around 45nF for this capacitor. Will the circuit you describe be suitable for this value?

I assume you need an AC variable voltage source?

I doubt if I'll have time to roll the capacitor for another week or so. I'll need to get a small radio as well. I don't think I have one that's not digital. Does it make any difference whether it's AM or FM?

Sulaiman

Wed May 28 2014, 12:37PM

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

FM will be >80 MHz so go for an AM radio, preferably with Long Wave as that's closest to TC frequencies.
e.g. 45nF with 14 uH = 200 kHz

Ash Small

Wed May 28 2014, 04:27PM

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

Thanks Sulaiman, and does the voltage supply also have to be variable frequency, to match the resonant frequency of the LC circuit, or can I use a DC supply?

Sulaiman

Wed May 28 2014, 05:47PM

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

The idea is to operate the inductance and capacitance as a resonant circuit
the radio should be quiet at the resonant frequency
because it is a near constant sine wave .. a carrier wave in communications parlance.

When the voltage peaks exceed the capacitor safe peak voltage
in this case when corona or flashover starts
it will effectively amplitude modulate the carrier
and will be heard as 'hash' / noise on an am radio.

How to drive the L.C circuit ?
you could drive it as a parallel tuned circuit, which requires a high voltage signal generator
or as a series tuned circuit which requires a high current, this is how I'd do it.
(theoretically, I don't use this method)

The peak output voltage of the signal generator needs to be
(peak capacitor voltage) divided by (resonant circuit Q)
the resonant circuit q will be just a little less than the inductor Q
(the Capacitor will have a much higher Q than most inductors)
e.g. For +/- 5kV peak across the capacitor, using an inductor with a Q of 100
the signal generator needs to put out +/- 5000/100 = +/- 50 V peak.
A good air-core 14 uH inductor would probably have a Q of around 250.

Using the example of 14 uH and 45 nF, with resonant frequency of 200 kHz
the impedance of the capacitor (and inductor) would be
2.pi.F.L = 2.pi.200E3.14E-6 = 17.6 Ohm
so the peak signal generator output current needs to be 5000/17.6 = 284 Amps peak !

So if you are hoping for a 5 kV 45 nF capacitor
this method would require a signal generator of +/- 50 Vpk @ +/- 284 Apk !!!!!!!!!

It does not need to be a sinewave source, a hard-switched 1/2 bridge would do,
the series resonant circuit would only draw a sinewave current when driven with a squarewave.

If you have such a 1/2 bridge then no problem ;)
and you would be testing the capacitor for voltage and current stress.

A more practical approach would be to use the d.i.y. capacitor in the circuit that you intend it for
(sgtc, drsstc etc.)
and slowly wind up the drive voltage/power.
Of course the radio would need to tune to the operating frequency.
--------------------------------------- ----------------------------------------------

I tested my d.i.y. capacitors using a dc flyback, more controlable.
You could also use an NST or OBIT powered via a Variac.
(I have a 40 kV 'scope probe to monitor voltage)
These methods would not easily be 'tested' using a radio,
observing in the dark was my technique.

P.S. In retrospect, I GUESS that my capacitors failed because
I thought that the peak voltage as determined by corona would be the working peak voltage,
but for commercial capacitors the rms voltage is usually around 40% of peak dc voltage
(something to do with dielectric adsorbtion I think)
which corresponds to about 56% peak voltage !
This may be where the 'rule of thumb' using a capacitor of peak voltage rating 2x operating peak voltage comes from.

first previous 2 3 4 5 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob