HV Capacitors in the UK / General Chatting / Forums

Forums

4hv.org :: Forums :: General Chatting

« Previous topic | Next topic »

HV Capacitors in the UK

1 2 next

Move Thread

LAN_403

Ryan Parle

Tue Mar 31 2009, 11:37AM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

I've been looking for some capacitors to build a MMC for my first tesla coil project, but i've not been having much luck

Several member on this forum suggested the Cornell Dubilier 940c & 942c series of capacitors, but there dont seem to be many places in the uk i can get these, Farnell have a 15ukp charge for any of these capacitors as they are US stock and digikey have a minimum order quantity of 50 units.

Can anyone suggest where i might be able to find some suitable capacitors? I'm prepared to use brands other than Cornell Dubilier if it is cost effective.

The value for my capacitor needs to be 0.0188uf and suitable for use with a 10kv NST, my calculations show 14140v so i suppose it would be a good idea to round this up to either 15kv or 16kv

Can anyone in the uk suggest where to look?

I'm also hapy to consider buying maybe up to 4 big high voltage capacitors from a surplus supplier if anyone can suggest a supplier.

Sulaiman

Tue Mar 31 2009, 08:05PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

These

work well.

High voltage, polypropylene, foil (or film/foil) are the classics
Modern (Wima etc.) MMKT types seem to be in use successfully.
eBay is your best resource.

If you get stuck PM me and I'll see if I can help (my stock etc.)

Don't forget, you may want something like this

across each MMC 'row'

Ryan Parle

Wed Apr 01 2009, 09:47AM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

Just spotted this on ebay:

at 14kv it is a bit of a close call, do you think these will be suitable for use with a 10vk 30ma NST?

Steve Conner

Wed Apr 01 2009, 09:39PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Probably not, the 14kV RMS rating is for 50/60Hz, the high frequencies found in Tesla coils are much more stressful to the capacitor.

The yellow ones from RS that Sulaiman linked to are probably the best bet. They are similar to the CDE 942 series, except made in Europe.

I used the 0.47uF 1kV versions of these in my DRSSTC, and they've performed well:

Sulaiman

Wed Apr 01 2009, 09:51PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

They are some kind of phase-correction capacitor I think,
notice the ratings;
14 kV rms, which is much more than 10 kV rms so OK
50/60 Hz which is much less than an sgtc.
These capacitors may work in an sgtc, probably would be ok for a while
may be quite brief life though as the current bursts will be much above specs.
connections may fail.
At hf and hv the dielectric may deteriorate and/or fail,
oil dielectric will heat up and may cause can rupture/explosion etc.
But - they may be ok.
The cost of finding out is similar to the cost of a dependable pp foil/film mmc.
_____________________________________________ _________________
EVERY part of an sgtc can significantly affect performance,
consider the overall costs of your sgtc and then think how far you want to economize one one of the three most important components.
(Spark gap and power source being more important)

Ryan Parle

Thu Apr 02 2009, 02:33PM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

Ok Thanks for the advice guys, I've managed to find a combination using the type of caps suggested by Sulaiman. 3 strings of 10 x 100nf 1.5kv to give me a total value of 30nf 15kv

I've gone for bigger total capacitance as i am now planning on using a 10kv 50ma NST instead of the of the 10kv 30ma i had originally planned on.

I'm also wondering if it is possible to extend the strings to 11 long and having a tap point at the 10x3 point to give 30nf 15kv and also another point at the 11x2 point to give 18.2nf 16.5kv so i can then chose between a capacitor value suited to either of my NST's can this be done?

Sulaiman

Thu Apr 02 2009, 07:53PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Make your mmc as versatile as possible.
(Although it's rare that the new use actually coincides with 'planned-for' options;)
e.g. consider making 3 individual 'modules' of
10, 11 or 12 series (capacitor in parallel with balancing/discharge resistor)
that can be easily paralleled or seriesed (?).

Try to minimize resistance and inductance introduced by layout and connections,
allow good airflow, (forced-air / fan is better)
High voltage at high frequency has many unexpected ways of going where it wants !,
allow AT LEAST 1mm/kV in air and 3mm/kV over surfaces,
'smoothen' any sharp points/edges to reduce corona.

Don't be tempted to use any kind of spray/paint/encapsulant etc. for your early experiments. It hinders subsequent repair/re-design.
Don't make your first TC too nicely - there are sooo many 'subtleties' that I doubt you could design and build an 'optimized' version first time. (I can't)
Don't over-think ... 10% inspiration & 90% perspiration is about right.
(Eddison)(Nikola Tesla's arch-enemy !)

Ryan Parle

Thu Apr 02 2009, 11:41PM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

Thanks Sulaiman.

I've now ordered 30 capacitors and 50 bleed resistors. i couldnt really afford to get any more capacitors so i will leave room on my mounting board/s to add a few more in series and may make an aditional string at a later date for more adjustablity.
I was planning on putting each string on its own piece of acrylic with a stud at each end to allow the strings to be hooked up in parallel. somthing like what has been done here:

although i was thinking that the components could do with being spaced out a little more, what do you think?

Steve Conner

Fri Apr 03 2009, 07:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you're going to be using some strings, and leaving others disconnected, then yes, they will need spaced out a lot more. I'll leave you to figure out why.

Ryan Parle

Fri Apr 03 2009, 09:19AM

Registered Member #2038 Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60

Ok well by now you've probably figured that i'm no HV boffin.

my only guess is the high frequency & electrostatic charging the capacitors even when they are disconnected? but then the bleed resistors should prevent that, so i guess i'm wrong on that!

For now i've only got the correct amount of caps to work well with my 10kv/50ma NST so i probbably wont find out until i buy some more caps.

1 2 next

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