AD-OLTC The Rebuild...

So after blowing up the AD-OLTC I though it was a good time, not just to repair it, but rebuild the HV side. So..

Things that needed inproving..

The Coil

The size of the old coil was 11" and I was getting 21" sparks.. And them I'm supprised when it arced over ? 😉 ..

Time for a bigger coil..
The next size up in easily available wire was the deciding factor for size, 3.15mm. I needed about 2000T so pluging in all the figures produced 25" long coil...

Here is the coil 3/4 wound, I would it by hand, the tape is to prevent the coils from loosening if I let go.. To keep things simple I decided to wind the coil on 4.25" PVC waste pipe.

The Primary

I would need to change the coupling of the coil to obtain maximum spark length, So this is the base of the coil, the fatter peice will hold the primary coil and the top peice will hold the secondary, The slit is to alow the earth from the bottom turn of the secondary to the earth post in the centre of the coil.

The primary is held high so that all of the other components can be underneath it. This allows for a strike rail, and an insulated disk around the base of the coil.

Im going to try a single primary, as this simplyfies the wiring, and I beleive that the current sharing in the IGBT's will be OK for one of them not to take all of the load. This also gives me the option of firing them all in parallel if required..

Sanity Check

I put all of the semi finished bits together and did a quick shot to see what it'l look like...

Ouch!!. It used to be a desktop coil.

The scary thing about this is that there is almost nothing at the base of the coil, I may have to add outriggers..

The finished Secondary coil has 1970 turns, and I have applied 5 coats of varnish. In this picture I had just rubbed down the coil between coats, hence the appearance. 

Replacing Blown Silicon.

A shot of all the components that have laid down their lives at the Uk Teslathon...

I've repaired the damage to the controller, another 4046 and a 4011 to add to the silicon cemetry. Also the main power slider had developed a strange fault where by it read about 25K (it was a 10K slider) and at its lowest point it would only read 10K to 0v, this ment that the scale started at 1/2 power.
It does make me wonder if this was the cause of the inital primary arcs is the power suddenly jumped up due to the faulty slider?

When I took it apart it looked like there had been some power flowing through it. The only wat this could have happened is if it had touched the earthed case. (The electronics are referenced to bridge rectified mains 0V not earth).

So I need to check the insulation from the slider to the case..

Re Tuning

I started with the set of capacitances I had with the old OLTC around 2.5uF, As I tuned It was obvious that the capacitance would need to be smaller to bring the coil into resonance. I ended up with 1.5uF a whole 1uF lost.
After asking around various suggestions were put forward as to why, stray inductance in the primary was the one I thought was the issue, So I re designed the primary layout a few times to reduce the stray inductances.

It made no differance at all...

But it did reduce the losses in the primary, so now the coil will ring for longer, so not too bad an excercise after all.

I beleive that the problems I am having is due to some mistakes in some figures I put into Win Tesla to calculate the inductance of the secondary.

I have now measured the secondary at 51mH, where Win Tesla makes it 69mH, I think that this could be the cause of the lower primary capacitance.

I'm going to have to run with it for now, and get a bigger topload to bring down the Frez at a later date..

I took this picture whilst tuning to show that at high BPS

I can almost get cw operation.

The bangs are 20 cycles apart at 7.5Khz (max BPS)

AD-OLTC Continued

So Amberley Teslathon happened. The OLTC ran quite well, untill I had another problems with the controiller taking out the 4046 again. I beleive this is caused by a spark jumping from the operator to the slider of the power control. This is connected directly to the input of the 4046 so blows the chip. Im going to try two diodes to the supply rails to hopefully shunt this away to somewhere safe.

So The OLTC spent the afternoon in TT mode. I made a star from piano wire for demonstration purposes.(picture)

I did set it up to do power arcs in TT, the video is here..(460K)

The IGBT's are still running hot and im not getting the spark length I should be. I need to increase the coupling, probably by allowing the secondary to drop down into the centre of the primary.

I replaced the 4046 and 4011 again, and I've put in two diodes to the supply rails and a 0.1uF capacitor on the VCO input to the 4046 to prevent a strike from killing the IC again. (I'tl probably kill something else now.)

I've found a circular sandwich box, that has a diameter of 140mm for an external primary to give me enough room to sink the secondary below the primary. 140mm is a slightly larger primary that I would have liked , but it was the best I could find, So I have wound the primary around it to try.

To allow the coupling to be changed, I have drilled a series of holes in the inner former this will allow me to put in a set of screws to change the height of the secondary and give me different coupling arrangements.

I've tuned up to the new primary dimensions and roughly set the coupling.
Unfortunatly the primary being a larger diameter then before has increased inductance and has dropped my primary cap size to 1.3uF 🙁 .

Increasing the coupling worked, I can get ring up in 2.5 cycles which means that I can run at the higher BPS's without excessive heating again.

Increasing the BPS is strange. I get a linear response (spark length to BPS) up to around 5Khz. At this point I'm starting to get the next bang during the ringdown of the secondary from the previous one. From this point on I get alternate points of better / worse spark length as the position of the next bang is either in or out of phase with the secondary, but the peaks are BIG!! . I'm back up to a similar performance as I was before (around 20"), even with a smaller Primary Cap.

 I do have to run with my interrupter to keep the input current draw down as 7K BPS gives me 2KW average and I don't think the IGBT's would stand that sort of abuse.

Click the picture for a movie of the AD-OLTC running at high BPS.

I was playing with my OLTC calc and I found something very obvious.
 If you have thicker wire the inductance decreases. 

The first OLTC I built, following Steve Connors circuit, had a double wound primary to share the current between the two IGBT's, but it also had a lower inductance. When I started the upgrade I went to a single coil as I knew (now) that current sharing in IGBT's was not an issue. I noticed instantly that the inductance had increased, but as two coils in parallel have the same inductance as a single coil, I dismissed the fact that the conductor was twice the size.

So I remade the Primary (AGAIN) with a double winding of break pipe (I must get shares). It has reduced my primary inductance by 0.43uH and increased my Primary Cap to 1.71uF still not up to 2.4uF but better.. but this is now limited by the larger diameter of the primary.

The successive mods to the primary have done it :-D , I'm back to where I was before the Secondary blew..

This picture is of  arcs to a grounded ariel 19" away from the topload breakout.

(click for a better image).

I took my AD-OLTC to the Gaussfest (a teslathon).

The coil worked like a dream, I could run it at full power (7K BPS) without any interruption. and I got a confirmed strike of 23" Not bad for a small non brick OLTC.

(click for a better image).

A mother board coming under some HV at about 17" away

(click for a better image).

The AD-OLTC running at 7KBPS (flat out) this is at a power of 2.7KW , the intense yellow light on the RHS was a drawing pin I used as a breakout.