OLTC project / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

OLTC project

first previous 4 5 6 7 next last

Move Thread

LAN_403

Marko

Sat Jun 24 2006, 11:45PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

With 2500V IGBT one needs to lower the input voltage of a mot just a little.

To get 2 kilovolts it would take a pretty big voltage multiplier to get enough power ($$$ caps)

Steve Conner

Sun Jun 25 2006, 01:04AM

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

You only need 1kV since DC resonant charging automatically doubles it for you. My OLTC 2 used a 600V DC bus made by voltage doubling the 240V AC supply.

Jrz126

Sun Jun 25 2006, 11:37PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

I calcuated the resonant freq of my testing secondary to be 189.95 khz. Now how do I determine what frequency to drive my IGBT at?

Steve Conner

Sun Jun 25 2006, 11:43PM

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

The OLTC isn't like other SSTCs where you drive the switches around the resonant frequency. If you want 120 sparks per second, you feed 120 pulses per second to the IGBT, so you could say you're driving it at 120Hz.

However, the length of the pulse for best performance is related to the resonant frequency of the coils and the coupling between them. You want the pulse to end when the primary current reaches its first notch, usually after 3-4 cycles of the resonant frequency.

Since OLTCs lacked bang energy, we ended up operating them at insane breakrates to try and make the sparks bigger. I never ran my OLTC 2 at breakrates higher than 600Hz because the power supply couldn't take it, but I've seen other people use well over 1000Hz.

GimpyJoe

Mon Jun 26 2006, 12:40AM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Firkragg wrote ...

With 2500V IGBT one needs to lower the input voltage of a mot just a little.

To get 2 kilovolts it would take a pretty big voltage multiplier to get enough power ($$$ caps)

You can eliminate the need for a variac by simply making a new tap point on your MOT secondary. Just do a cut off turns and re-solder when you have the output voltage you want.

Marko

Mon Jun 26 2006, 11:57AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Maybe, if MOT likes unwinding.
Problem is that this involves sawing the core in half, and resonant charging with doubler or etc. is a less pain than that.

Steve Ward

Mon Jun 26 2006, 04:28PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Re: Cardboard forms: I use a cardboard form on my largest coil. The cardboard performs just fine at standing off the voltages, but you must properly prep the form. Make sure you have a dry tube, not something thats been sitting outside. First thing you have to do is take a knife and "score" (shallow cut) along the seam of where the paper wrapping overlaps (its a spiral around the tube). This should allow you to remove the outer paper easily with careful peeling. Once you get rid of that outer layer of paper (you want it gone cause it has black, conductive, ink), you are ready to begin the process of coating and sanding. Coat the tube with your typical polyurethane varnish, then give it a light sanding. Repeat this process about 3 times or more, until you have a suitably smooth surface to wind your coil on. After winding the coil, seal it again with 3 or more coats of the varnish. You may also wish to coat the ends (the edges) as well as the inside of the tube with varnish to keep it from absorbing any more water. So far my coil has withstood 144" sparks from an 8" diameter 45" long winding. I used the concrete form tube from menards, but the stuff from home-depot has worked fine too. I think its all pretty much the same, as ive heard other successes with cardboard too.

Dr. Drone

Mon Jun 26 2006, 07:21PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Jrz126

Mon Jun 26 2006, 11:38PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

Thank you all for the info...

I found a 555 circuit that allows me to vary the duty cycle with a pot. I had the on time set to about 50usec. with 144 bps. It ran pretty good off of a car battery charger, pulling about 4-8 amps. When I use my variac, it hums like mad...Am I drawing too much current through it? how do I make sure I dont pull too much current from the wall outlet?

HV Enthusiast

Tue Jun 27 2006, 01:08AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

i use cardboard sonotubes for a number of coils also. i don't bother removing the outer layer though. i think the entire tube, being considerably hygroscopic, will have a higher conductivity than a little black ink. if it makes you feel better, then by all means remove it, but i don't think its necessary.

first previous 4 5 6 7 next last

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