Designing an Auto Match Unit for RF supply / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Designing an Auto Match Unit for RF supply

first previous 5 6 7 8 next last

Move Thread

LAN_403

Ash Small

Sat Nov 27 2010, 08:21PM

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

I understand that variable caps of 600 nano Henries are available (this would probably do as I could increase the inductance of the coil a bit)

I could do as Proud Mary suggested, and switch caps in and out (I've made 50 nano HV caps before and I know I could make these a lot bigger especially if they were lower voltage)

Again, it comes down to doing some experimentation to see how many plates and what size the plates would need to be.

The other option is to increase the inductance of the coil, but I'd prefer to explore the other avenues first.

( variable caps don't have to be air vane, they can have a dielectric.)

Thanks for your comments though, Radiotech. (do you have any links etc regarding your previous post?)

Proud Mary

Sat Nov 27 2010, 09:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

I understand that variable caps of 600 nano Henries are available (this would probably do as I could increase the inductance of the coil a bit)

You've got yourself into a bit of a muddle here, lad.

Capitance is measured in Farads, and inductance in Henries.

As for variable capacitors, standard air spaced types offer a maximum of 3 x 500pF i.e. a three ganged capacitor totalling 1500pF when using a 'receiving' type. More expensive and hard-to-get transmitting types with wider spacing between the vanes to stop flash-over often have to be looked for for a long time, because they are no longer manufactured to any great degree. There are vacuum capacitors used in modern transmitters going up to perhaps 2000pF - 2nF - and costing upwards of several hundred pounds apiece, which occasionally appear on the surplus market.

I feel that completely unnecessary layers of complication have been added to what should be a very simple design that could easily be worked out on the back of a beer mat.

To get you pointed back in the right direction, you should re-think your design on the basis that a 150 -1500pF variable capacitor is what you have to work with. You can connect fixed value capacitors in series or in parallel with it if you need less or more variation, but you have to accomodate the fact that variable capacitors are only available in a limited number of values.

radiotech

Sat Nov 27 2010, 09:23PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Jara and Mics`` Mic= uHy, Jar = 1/900 F.

Ash Small

Sat Nov 27 2010, 09:47PM

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

My mistake, Proud Mary, a typo, I meant farads.

I can put a cap in parallel because I doubt I'll need adjustment from zero to 690 nano farads.

The other option is, as I mentioned previously, to increase the inductance of the coil.

There must be a solution here somewhere because I'm basically copying conventional commercial designs. If the maths says I need a 0.7 micro Farad variable capacitor then that is what I need.

Steve Conner

Sun Nov 28 2010, 03:10PM

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

Radiotech: What is a "bogey" value? Here in Scotland a bogey is something you "howk" out of your nose.

13.56MHz is too low a frequency to cook your eyeballs. You'll just give yourself some deep and painful RF burns, worst case.

You do not need a 0.7uF variable capacitor, even if you could find one. With such a high capacitance and low inductance, your tuned circuit will be extremely lossy. If I was building this thing, well, I have a 4-400A tube and a couple of Jennings vacuum variable caps in my junk box- a 500pF and a 1500pF- so I'd start there.

Edit: Some confusion about the operating frequency? 0.7uF is way too big a tank capacitor for 13.56MHz, but it is in the ballpark for an induction heater at a few hundred kHz. 4hv members have built such things using a Celem water cooled capacitor or two, and a few turns of water-cooled copper pipe. My own one only had 0.2uF and could have done with more.

Ash Small

Sun Nov 28 2010, 04:08PM

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

Ok, back to the drawing board, I suppose.

I'll wind some coils and measure the inductance using this circuit:

I don't have any water pipe here at the moment, so I'll just use wire.

Ash Small

Sun Nov 28 2010, 07:57PM

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

My DMM is playing up, it still measures ohms but not farads. I don't think it's the battery, but I'll try a new one tomorrow anyway.

It could be a day or two until I get it sorted or get a replacement.

radiotech

Sun Nov 28 2010, 11:47PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Well, guess that shows that we have been out
os Scotland too long since great grandad was sent packing
for enjoying the grape too much....

However down Glasgow way, bogey -- an estimated standard score--
didn't golf come from there?

Ash Small

Mon Nov 29 2010, 04:58AM

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

I've made a bit of progress now, my DMM appears to measure from around 0.5 nanofarads to 30 microfarads.

I've re-calculated, based on a coil inductance of 0.2 microhenries, which gives a capacitance of 0.688 nanofarads for resonance.

I need to wind a few coils and measure the inductance to check that I'm in the right ballpark, and then sort out a couple of suitable variable capacitors to play with.

Ash Small

Mon Nov 29 2010, 10:00PM

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

I thought I've managed to make a bit of progress here, but.....

I wound a coil, didn't have any waterpipe to hand so used enamelled copper wire wound around a wine bottle and held in place with insulating tape, five turns at 3/4 inch (20mm) centre to centre. I then connected it to a 0.7 microfarad cap (says 0.684 on it and my meter said 0.72) in parallel, and connected a series resistor of 1 K Ohm, connected it to a 0 to 1 Meg signal genarator and a 10 Meg 'scope.

Generator was set to 500KHz and I had a line on the 'scope (no amplitude, just a line), so I adjusted the signal generator upwards and got a peak at 630KHz, which equates to 0.1 microhenries.

The trace looked a bit dirty, so I adjusted it back down again, and got another peak with three times the amplitude at 107.5 KHz (or thereabouts). This looked a lot cleaner and equates to 3.13 microhenries.

Steve McConner says his coil is 0.2 microhenries, other websites say similar coils are 2 microhenries.

I've put some photos below (I hope). The first is the 630 KHz trace, the second is the 107.5 KHz trace, with identical amplitude settings on the 'scope.

Which figure should I go with?

first previous 5 6 7 8 next last

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