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SSTC Troubles

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freeskier89

Wed Aug 22 2007, 01:32AM

Registered Member #958 Joined: Thu Aug 16 2007, 11:15PM
Location:
Posts: 3

Hello,

I am pretty new to tesla coils and I thought I would make a relatively small SSTC. I made a schematic for it that is very similar to others you will find on the net, and I have not yet got it to operate. I am using a 3.5" x 12" secondary and 6 or so turns of 12awg wire as the primary. Currently the primary is wound directly over the bottom of the secondary. (I will increase the distance when I know it is semi-functional).

For preliminary tests I powered it using a 24V 6A current limited supply. I tried adjusting the frequency control potentiometer (multi-turn precision) from one side to the other, and there were never any signs of streamers or corona. It appears to be slightly working though. My Voltmeter is reading over 1kv on the toroid, and if I ground one end of the voltmeter, and I hold the other in the air about 2 ft from the toroid, it is reading up to 400V. I have been a little reluctant to give it full power (full-bride rectified mains) until I see something at the secondary.

Unfortunately I do not have an oscilloscope to debug (student-budget doesn't support one at the moment) and that is why I tried to stick pretty close to some schematics shown online. I am wondering how you more experienced guys think I should proceed. Should I give it more power, or do you notice something very wrong with the circuit?

Some probably relevant details:
-Secondary is grounded with a 8ft grounding rod
-Gate Drive Transformers are these:

-The circuit was constructed on a custom dual layer pcb. (I'll post the image for that if you think it would be helpful)

Thanks!

Sulaiman

Wed Aug 22 2007, 11:29AM

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

Welcome,

That's a fairly complex system that you've built.
I've been in electronics for decades and if I built something that complex that worked first time
I'd be both pleased and surprised !

I understand the problem of the expense of an oscilloscope
but for such a project I can't imagine de-bugging it without an oscilloscope.

Most of us here to give any real help would ask you to 'scope various signals

So the obvious next step IS to get an oscilloscope
Even the oldest valve-based oscilloscopes would be capable of 'seeing' what's going on
so I reccomend you look for ANY working oscilloscope that you cam afford.
Try to find a 'ham fest' or similar in your area
Ebay is a good start but postage costs are a problem.

If you fill in some of your personal details maybe there is someone who can help / tell of a cheap source.
Often members here get new 'scopes and offer their old one cheap /free (you still have to pay for postage)

Having said that, there are a few things that you could try.

Measure the ac voltage across the primary.
With a 24V supply the primary should have a +/- 20V (approximately) squarewave across it
a typical ac voltmeter should read about 22Vrms.
Even doing this will probably give a 'wrong' answer as the circuit should be operating above 100 kHz
and most ac voltmeters do not work properly at such a high frequency.
The measurements that you made (1kV, 400V etc.) are probably incorrect due to the voltmeter!

Make a bridge rectifier using 1N4148 or similar
(schottky diodes or known 'fast' diodes are ok, typical 1N400x diodes are a little too slow)
connect the ac side to the primary
put a capacitor of 1nF to 100nF across the dc side of the bridge rectifier and a resistor of 10k to 100K
now measure the voltage on the dc side - it should be about 20V dc.
If not then that's a problem

If you have a radio that receives 'Long wave' you may be able to 'tune-in' to the oscillations.
If you give details of the secondary (diameter and height of winding, number of turns, size of topload)
then we can estimate the frequency required to resonate it.
If you're lucky that falls within the range of a radio receiver and you can then set the correct operating frequency.

So give the details of your secondary, build the bridge rectifier and measure the primary voltage
and maybe more help is possible.

freeskier89

Wed Aug 22 2007, 10:23PM

Registered Member #958 Joined: Thu Aug 16 2007, 11:15PM
Location:
Posts: 3

Thank you so much for the lengthy reply! :) I guess I am just going to have to get an o-scope. It will definitely be crucial in the long run, and there will be another RF project that I will be working on where it could be extremely useful. I will have to get one online, as opposed to locally because I live in a small town in Wyoming. I might try the bridge-capacitor test in a couple of days though. I would today, but school started today and won't have time for it until the weekend.

Thanks again for the assistance. I appreciate it!

Shaun

Fri Aug 24 2007, 03:26PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

An 8ft ground rod would work well but it seems to defeat the purpose of making an SSTC small...unless you are just talking about your mains ground.

freeskier89

Sat Aug 25 2007, 04:18AM

Registered Member #958 Joined: Thu Aug 16 2007, 11:15PM
Location:
Posts: 3

I know... but, for preliminary testing, before I am more familiar with TCs, I wanted to make sure it was completely isolated from mains.

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