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IamSmooth

Thu Nov 16 2006, 01:37AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I am collecting and putting together a TC and I have some general questions I hope some of the more experienced people wouldn't mind addessing for me.

1. I see that many use varnish on the secondary and some use A LOT of varnish (>20 coats). Does it matter for performance? I believe that varnish adds to the stray capacitance of the coil. Is this a good thing?

2. I have seen various primary designs: helix, spiral, cone. I can think for some reasons for this: available materials for the builder, coupling, flash-over, aesthetics. Is there a theoretical *best* design or is it just apples vs oranges?

3. My design (15kv/120ma primary with a synchronous 120bps spark gap) could use a resonant cap value of about 0.021uf and a LTR value of about 0.055uf. As long as Ls*Cs = Lp*Cp is it acceptable to have a tank capacitor *close* to one of these values or should it be as exact as possible. I have two 0.025uf Maxwell capacitors and if necessary I have some smaller ones I can chain in parallel/series to make up the difference; otherwise, I was going to use 0.05uf as my tank.

4. I have seen a Freau formula for spark length as L (inches) = 1.7*sqrt(WallPlugPower)
Is the Wall Plug Power in watts the value without using PFC capacitors? Clearly, if I use PFC then the wall plug power significantly drops. Which value is used?

Steve Ward

Thu Nov 16 2006, 04:00AM

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

1. I see that many use varnish on the secondary and some use A LOT of varnish (>20 coats). Does it matter for performance? I believe that varnish adds to the stray capacitance of the coil. Is this a good thing?

It has no real effect on the self capacitance of the coil since the majority of the dielectric from coil to ground is air anyway. I usually use about 5 coats of varnish, its mostly for protection against scratches. More recently i have found epoxy to work out nicely. One brand in particular, Envirotex (i think) works very nicely. You want the stuff that is made for covering counter tops or furnature. Epoxy needs only 1 coat, and doesnt stink up the place if you have to do it indoors (good in winter time).

2. I have seen various primary designs: helix, spiral, cone. I can think for some reasons for this: available materials for the builder, coupling, flash-over, aesthetics. Is there a theoretical *best* design or is it just apples vs oranges?

The geometry of a helical primary allows for the highest coupling (which is far greater than we desire for TCs). The cone provides higher coupling, but also provides added voltage standoff between windings compared to the helical. Flat gives a lower coupling, but the best voltage standoff of all. Basically, you want the coupling high enough for good energy transfer (you want it to be rather quick), but you dont want racing sparks or sparking between the 2 coils. For SGTCs, flat primaries usually offer enough coupling, and its not worth the hassle of building a cone (though i think they look cool). You can always raise up the secondary to lower coupling if its too high.

3. My design (15kv/120ma primary with a synchronous 120bps spark gap) could use a resonant cap value of about 0.021uf and a LTR value of about 0.055uf. As long as Ls*Cs = Lp*Cp is it acceptable to have a tank capacitor *close* to one of these values or should it be as exact as possible. I have two 0.025uf Maxwell capacitors and if necessary I have some smaller ones I can chain in parallel/series to make up the difference; otherwise, I was going to use 0.05uf as my tank.

Provided you stay clear of the resonant value at 60hz (to avoid resonant rise between NST and tank cap) then you should be OK. The .055uF value was likely determined from simulation to provide the best power use of the transformer (maximum bang energy possible given the supply constraints). Its not critical to get exactly this value, .05uF will work just fine. Going larger will give you a lower charging voltage. Going with a cap thats too small will probably not raise the charging voltage, but the lower capacitance will mean smaller energy storage per bang. If using maxwell capacitors, be sure they are rated for 40kV, otherwise im not sure if i would risk using them. The peak charging voltage will be around 20kV, but the high primary Q causes a nearly 100% voltage reversal, so the net voltage rating should be 40kV for this type of use. Note that the MMCs we all use are pushed way beyond their ratings, and this seems to work ok. I have heard of people killing the 35kV variety of maxwells with 15kvAC power sources. Just a warning, as i know how much it would suck to blow up your nice caps.

4. I have seen a Freau formula for spark length as L (inches) = 1.7*sqrt(WallPlugPower)
Is the Wall Plug Power in watts the value without using PFC capacitors? Clearly, if I use PFC then the wall plug power significantly drops. Which value is used?

Note, power (watts) is not necessarily the the VA. Your power (watts) should be about the same with and without PFC, but your current draw will be reduced with PFC. I believe that John assumes one will use real power, and not the VA. I know that my 15kV 120mA powered coil drew about 14A in operation and produced about 66" sparks using a 120BPS SRSG and i think a .045uF MMC (ive long since sold it). With some extra tweaking it should have been capable of more i think, but it was good enough at the time.

IamSmooth

Thu Nov 16 2006, 04:13AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Steve, your reply clears a lot up. Thanks

Steve Ward wrote ...

Note, power (watts) is not necessarily the the VA. Your power (watts) should be about the same with and without PFC, but your current draw will be reduced with PFC. I believe that John assumes one will use real power, and not the VA. I know that my 15kV 120mA powered coil drew about 14A in operation and produced about 66" sparks using a 120BPS SRSG and i think a .045uF MMC (ive long since sold it).

When I connected the PFC caps to one 15kv/60ma NST I reduced the amps drawn from about 8.9 to 2.1 when I was drawing an arc. This is only about 240 watts, yet I keep hearing everyone talk about the NST being 900watts (when I really think they mean VA). So, for each 15kv/60ma what value should I use for the "power" in the formula?

Steve Ward wrote ...

If using maxwell capacitors, be sure they are rated for 40kV, otherwise im not sure if i would risk using them. The peak charging voltage will be around 20kV, but the high primary Q causes a nearly 100% voltage reversal, so the net voltage rating should be 40kV for this type of use.

As far as my maxwells they are each rated for 50kv so I am guessing I will be fine.

Based on your comments I might go for a small cone with a 5-10% angle.

Steve Ward

Thu Nov 16 2006, 06:30AM

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

When I connected the PFC caps to one 15kv/60ma NST I reduced the amps drawn from about 8.9 to 2.1 when I was drawing an arc. This is only about 240 watts, yet I keep hearing everyone talk about the NST being 900watts (when I really think they mean VA). So, for each 15kv/60ma what value should I use for the "power" in the formula?

This is tricky. Capacitor charging is a bit different than drawing sparks. When drawing an arc with non PFC'd NST, the load looks mostly inductive. It doesnt help any that there are magnetic shunts, effectively there to limit the secondary current, but do not necessarily decrease the primary current draw, so you end up with a lot of reactive power, and not much real power, which you discovered. When people quote NST power, they probably should say VA.

Anyway, when a NST is charging a capacitor, you can get more real power out of it than when drawing an arc, this is due to the better impedance match. The capacitance cancels out some of the reactive component on the NST. I havent worked out the details for myself, but i believe you can actually extract about as much power (W) as they are rated for in VA. A lot of it has to do with the size of the tank capacitor and the BPS of the coil. When people say to use 2.5X the LTR value, i think this is to maximize the amount of power (W) used from the NST

I wouldnt really worry much about the power vs VA stuff right now unless you really must know how "efficient" your TC is. If you are looking for a value to plug into the Freau formula, id say just say that the VA rating is your input in watts, and that will probably be about the biggest spark you would expect. Please note that this formula is just an emperically derived guess at what the maximum spark length should be. Most coils dont meet the mark, and then again, some exceed it. Of course, not everyone measures power properly, if at all (they usually go by the VA rating of the NST). I should mention its possible to exceed VA rating of a NST given the proper setup, so some people might "cheat".

Hazmatt_(The Underdog)

Thu Nov 16 2006, 06:22PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

*** with regard to Cprimary ***

I am researching the sizing of the Cprimary capacitor, and if you are careful with your calculations you CAN find the value which gives you the nameplate VA. But you must be careful not to exceed this because the transformer will die.

IamSmooth

Thu Nov 16 2006, 07:13PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I have two more "general" questions if none of you mind:

1. I have a synchronous spark gap using an Oriental Motor. Will I have any trouble with magnetic or RF interferance if I place this underneath the primary? I plan to have the metal housing of the motor grounded to the earth.

2. I am going to use a 6 inch diameter secondary. The aspect ratio can be 1:3 to 1:5. I've seen some emperically derived formulas that suggest that certain ratios are better for a given input power level. I am aware of tuning Ls*(Cs+Ct) once I have picked the values to equal Lp*Cp. Is there any good reason to go 1:3, 1:4 or 1:5?

Steve Ward

Thu Nov 16 2006, 10:55PM

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

I am researching the sizing of the Cprimary capacitor, and if you are careful with your calculations you CAN find the value which gives you the nameplate VA. But you must be careful not to exceed this because the transformer will die.

Great, id like to hear about your results. It should be possible to simulate this, provided you can model every aspect of the NST (i still dont know how to model core materials yet, so im not sure).

1. I have a synchronous spark gap using an Oriental Motor. Will I have any trouble with magnetic or RF interferance if I place this underneath the primary? I plan to have the metal housing of the motor grounded to the earth.

Shouldnt be a problem since the primary B-field is oscillating much faster than the 60hz field that the motor runs on. Also, the steel case of the motor should block most of those effects anyway. I never payed any attention to this sort of issue.

2. I am going to use a 6 inch diameter secondary. The aspect ratio can be 1:3 to 1:5. I've seen some emperically derived formulas that suggest that certain ratios are better for a given input power level. I am aware of tuning Ls*(Cs+Ct) once I have picked the values to equal Lp*Cp. Is there any good reason to go 1:3, 1:4 or 1:5?

Id say it depends on what spark length you expect. You might get up near 6 foot sparks with that supply. In my experience, you can push a secondary to produce sparks about 3X its winding length, but in general it seems more efficient if you aim for only 2X or so. So you may want to go with a 30-36" winding... 30" is probably just fine. 6" diameter sounds good too... again, just going from experience. Remember, you can basically make Ls and Cs whatever you want. Cp is pretty well defined for the power supply, so you just change Lp. Typically, its better to go with lower operating frequencies so that Lp is large, making Zsurge (sqrt(L/C)) large, keeping the peak primary current lower. This lowers the spark gap losses to some degree, and many find this to be noticable with spark length per input power.

IamSmooth

Sun Nov 19 2006, 02:49AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I have seen the spark gap parallel with the HV terminals of the transformer and in series, interchanged with the capacitor position. I have read that the former way is better as there is less "stress" on the transformer. Can anyone explain why this is the case?

Colin 99

Sun Nov 19 2006, 09:40PM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

You want to have the spark gap across the NSTs to prevent the voltage across them from getting too high, otherwise you could damage your NSTs. The primary capacitor forms a resonant circuit with the NST, If the capacitor is across the NST, the voltage can exceed the rating of the NST and damage them. This happens with Cltr also - producing a resonant rise across the capacitor and NST. With the spark gap across the NST (set right and safety gaps), it prevents the voltage from getting too high on each half cycle.

Also a Terry filter is suggested between your NSTs and spark gap to prevent high frequencies form getting back into the NSTs. The Terry filter is an RC filter with MOVs and safety gaps across them.

Shaun

IamSmooth

Mon Nov 20 2006, 12:05AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Thanks Colin, but maybe I wasn't clear in my question. My coil is going to have a synch spark gap with a safety static gap in case the synch motor fails. I am going to have a Terry Filter with a static gap for each end of the HV terminals of the NST with a ground in between.

My question is simply is there any difference in switching the position of the capacitor and the spark gap (my sych spark gap) if I have the other items in place? I guess the question can also be does the capacitor stress the NST more if it is parallel or series with the NST if I already have a safety spark gap across the HV ends of the NST to catch HV spikes?

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