[TCML] New Coil Questions
bartb at classictesla.com
Wed May 27 20:47:42 MDT 2009
Welcome back to coiling! Comments interspersed.
Tyler Pauly wrote:
> Hello all,
> I've been out of the tesla coil "circuit" for a while now but I'm looking to get back in. I've had a 10kVA pig sitting in my garage for a while and I want to put it to use. Some questions....
> 1) Would it be okay to go over rating on the pig? I assumed as much, but figured it wouldn't hurt to ask. I'm thinking somewhere in the range of 12-13kVA if I do.
Yes, certainly. Just don't go overboard on run times. Although it sounds
good to do this, you have to be ballasted for the current, so that is
another matter altogether.
> 2) Is the most straightforward route for the tank capacitor an MMC, or are there other (easy-to-find) capacitors that would work as well?
MMC's can be done, but it gets expensive due to the number of caps to
achieve a desired C for a decently large pig coil if the 942C common
values are used. There are some larger single cap MMC values that might
be worth taking a quick look at, but usually their voltage rating is a
bit lower, so you end up needing more caps in series anyway. Just run
the numbers and probably best to go to Cornell Dubiliers website and
select the 942C pdf they have. This will show all cap values and related
data. I personally don't use MMC's for pig coils. I use professional
pulse caps (I obtained them prior to the MMC thrill, but they have
proven their worth over the years). It mainly comes down to a cost issue.
> 3) I have an equation that tells me the capacitive reactance of my xmfr. Do I want my tank capacitor to have that value, or would the resonant effects be a bad thing? What value should I shoot for?
Likely, the capacitive reactance value you speak of is the capacitance
at which the transformer becomes resonant with. This is actually not a
smart route. If your tank resonates with the driving transformer, high
voltages will be seen at the cap and the transformer (putting both into
harms way). For a pig, you'll likely use a smaller than resonant cap
(STR). A 10KVA pig with a 14.4kV output will have a resonant cap at
about 0.128uF. This is the value "not" to use. Going larger makes for an
expensive and large cap. So consider .08uF to .1uF to keep the cap
somewhat out of resonance and still have a large bang size for big sparks.
> 4) Where have most of you guys gotten your variacs? Talking about the 0-280V 28A ones. I can't seem to find any, and I would like 2. I assume they'll be expensive once I find them...
I found my variacs at an old electronics surplus seller in Minnesota and
paid only $50ea for a 240V 28A Powerstats (bought 2 and a couple 12A
variacs). However, I'm in California these days. Not sure where to get
them out here.
> 5) Are the CDE 942 caps still the best choice in MMC building? Who are some suppliers? I can only find/think of Digikey.
Check with DC Cox on this list. He sells them periodically and will
probably get you the best price.
> 6) I know the coil's secondary will be into the 3:1 realm, but how big should it be? I'm mainly asking for 12kVA usage with the pig, as that's what I hope to run it with.
I would recommend forgetting the 3:1 ratio and head to 5:1. The lower
ratio will simply put the top load that much closer to the primary. Try
to attain as much distance as possible away from the primary yet keeping
within a decent performance geometry. Going to long gets lossy. 5:1 is a
good design. 6:1 can be good also, but you really don't want to go past
that. Use at least 18G wire for a big coil and make the diameter large
(12 to 15 inch).
> 7) Can I wire an arc welder in series with my outlet leads to current limit? I would think so, but again, it doesn't hurt to ask.
Yes, but I don't use an arc welder myself. So I can't comment much here.
> 8) My pig only has 1 HV bushing, so should I just split the wiring I attach to it to get 2 leads off of it to make the primary circuit? Sorry for the bad wording, I hope I get my question across.
Wire the case to RF ground and run the HV lead (hv bushing end) to the
tank cap. I have a dual bushing pig, and yet I still tie one of the hv
bushings to the case which I keep at RF ground (allowing only 1 hv lead
run out to the coil). The inside of the primary and the bottom of the
secondary are also forced low to RF ground which keeps a known potential
between the two coils. Don't run green mains ground to the pig. That
would actually tie mains ground to RF ground and you will end up with
nasty voltage spikes on your house. Stop mains ground back at the
> Thank you for bearing with me, if you got through all that. And thanks again if you answer any of them! :P
Again, welcome back to coiling Tyler.
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