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How close must primary & secondary resonances be?

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Doug86

Thu Oct 08 2009, 08:41PM

Registered Member #2424 Joined: Tue Oct 06 2009, 08:02AM
Location:
Posts: 17

Hi all,

I'm currently helping james (audio:deviant)build a DRSSTC using the driver and half bridge from a minibrute. Finally got the driver board sorted and all tests checked out ok.

We found the resonance of the secondary assembly to be 109Khz however the lowest we can tune the primary to (with the tap right at the end) is 118Khz.

as I see it (please tell me if im wrong) our options are:

1: build a new primary with an extra turn.

2: remove some turns from the secondary. (would rather not)

3: possibly change the primary capacitor.

What would you guys do?

Also, I'd like to test if everything else is working properly, can we expect any sparks at all with the current setup?

Many thanks :)

Z28Fistergod

Thu Oct 08 2009, 09:14PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Using a smaller topload seems like it would be the most readily accessible option. That, or add a few caps to the MMC; you've got plenty laying around, right? I would try to avoid messing with the primary or secondary myself.

I don't think you would get much of anything out of it that far out of tune.

StevenCaton

Thu Oct 08 2009, 09:43PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

Don't forget that if you plan on having the coil make large sparks, then the resonant frequency of the primary circuit is going to have to be lower than the resonant frequency of the secondary resonator. Depending on spark size, your primary circuit Fo might have to be 20% lower.

Doug86

Fri Oct 09 2009, 01:35AM

Registered Member #2424 Joined: Tue Oct 06 2009, 08:02AM
Location:
Posts: 17

Many thanks :)

We formed a makeshift topload out of two old frying pans (given a good once over with the grinder) and bolted it on... 146Khz, primary tapped acordingly.

First time ive seen a teslacoil for real and I am amazed (and totaly hooked)

Coil ran beautifully up to around 170V on the variac (around 14" sparks) where the primary started to arc to the secondary so we shut it down.

There are around two turns of the primary left above the tap point and the arcs seemed to be coming from the unused turns. Do you think these turns could be causing the problem?

also is there any disadvantage in running a small topload?

Thanks for all your help so far :)

teravolt

Fri Oct 09 2009, 01:50AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

add primary capacitance and add another turn or two if you can. As you may know adding caps will increase primary current and adding turns will decrease current. Do you have pictures?

StevenCaton

Fri Oct 09 2009, 02:12AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

also is there any disadvantage in running a small topload?

Definitely It could significantly limit your spark length if you have way too small of a toroid.

Z28Fistergod

Fri Oct 09 2009, 03:22AM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Doug86 wrote ...

Many thanks :)

We formed a makeshift topload out of two old frying pans (given a good once over with the grinder) and bolted it on... 146Khz, primary tapped acordingly.

First time ive seen a teslacoil for real and I am amazed (and totaly hooked)

Coil ran beautifully up to around 170V on the variac (around 14" sparks) where the primary started to arc to the secondary so we shut it down.

There are around two turns of the primary left above the tap point and the arcs seemed to be coming from the unused turns. Do you think these turns could be causing the problem?

also is there any disadvantage in running a small topload?

Thanks for all your help so far :)

Like SteveC said, bigger top-loads can lead to longer streamers. So, now that you have gotten your plasma fix, you can work on getting the primary circuit within tuning range of your secondary with an appropriate sized top-load. Either with more capacitance or more inductance.

At this point you have a lot of fiddling to do to get the most out of the coil. You need to adjust the coupling to reduce flash overs, and primary to secondary arcing, and then adjust the primary tap to compensate. It's a long, fun process, so try to have a good time. If you have extra turns that are causing primary to secondary arcing, and you're not sure if you will need the tuning range, you can just bend them out of the way. When your sure you wont need the extra turns, cut them off.

What I found, and maybe you will too, is that my coil runs best when the coupling is reduced to where primary to secondary arcs almost completely disappear. You might be surprised at just how low the coupling needs to be for peak performance.

Doug86

Fri Oct 09 2009, 10:51AM

Registered Member #2424 Joined: Tue Oct 06 2009, 08:02AM
Location:
Posts: 17

Thanks to all the replies so far, We couldn't have got this far without this forum :)

teravolt wrote ...

add primary capacitance and add another turn or two if you can. As you may know adding caps will increase primary current and adding turns will decrease current.

The primary capacitor is made of 12 0.33uF 2kv caps wired like this:

Mmc

To give 0.11uF at 12kv.

By removing one of the paralell pairs the capacitance would increase to 0.132uF (lowering the resonant frequency) at the expence of only being good for 10kv correct?

what kind of voltage can you expect accross a primary cap in a coil like this?

Z28Fistergod wrote ...

What I found, and maybe you will too, is that my coil runs best when the coupling is reduced to where primary to secondary arcs almost completely disappear. You might be surprised at just how low the coupling needs to be for peak performance.

Thanks Z28Fistergod, could you clarify what you mean by coupling? do you mean the physical position of the primary and secondary? or the electrical resonances?

teravolt wrote ...

Do you have pictures?

Yes!

The origional topload and coil before the elecronics went in.

Makeshift topload.

Many thanks again.

Z28Fistergod

Fri Oct 09 2009, 12:02PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Coupling refers to how much energy is transferred from one circuit to another. In this case the coupling is magnetic, between your primary and secondary. You adjust it by raising or lowering the secondary coil.

teravolt

Fri Oct 09 2009, 05:58PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I noticed that you are followwing the mini brute gide lines if I were you continue using the large topload and add another string to your MMC. I am not shure why you have such a differance in frequency. If you want to you can reduce the amount of caps just to test it but I would not go full power. This may help with your analisis. what type of equipment do you have

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