Primary Capacitor sizing
uzzors2k, Sat Mar 03 2007, 09:53AMI'm planning on making a new SGTC, powered by an offline run flyback transformer, which can supply about 300W @ 20kV. The secondary resonant frequency should be around 571 kHz. All of the calculators (yes, I'm a noob.) I've found assume one is using a 50/60 Hz AC transformer. However, since this is not the case the capacitor size will only be limited by the flyback's power output. So how big can I go before the flyback can't charge the capacitor fast enough? I assume a larger primary capacitor means more power?
Re: Primary Capacitor sizing
..., Sun Mar 04 2007, 03:11AM
Well, you are designing a DC coil, not one that has to work at 50/60hz. What that means is that you can run at whatever break rate you want to
So, pick a break rate. In general, higher bps = fatter sparks, lower bps = longer sparks, however the DRSSTC people have shown there is a 'sweet spot' that gives longest sparks.
Then you take the # of bps, say 100, and find out how many joules you need in your tank cap. So, for 300w and 100bps, you need a 3j cap. (keep in mind that watts are joules/seconds). So then you decide what voltage you are going to set the spark gap at (10kv is probably about right) and then size the cap for that.
finally, you then use a calculator to find what size primary you need for the given cap size and secondary resonant frequency.
Have fun!
..., Sun Mar 04 2007, 03:11AM
Well, you are designing a DC coil, not one that has to work at 50/60hz. What that means is that you can run at whatever break rate you want to
So, pick a break rate. In general, higher bps = fatter sparks, lower bps = longer sparks, however the DRSSTC people have shown there is a 'sweet spot' that gives longest sparks.
Then you take the # of bps, say 100, and find out how many joules you need in your tank cap. So, for 300w and 100bps, you need a 3j cap. (keep in mind that watts are joules/seconds). So then you decide what voltage you are going to set the spark gap at (10kv is probably about right) and then size the cap for that.
finally, you then use a calculator to find what size primary you need for the given cap size and secondary resonant frequency.
Have fun!
Re: Primary Capacitor sizing
uzzors2k, Sun Mar 04 2007, 01:15PM
Just the advice I was looking for, thanks!
uzzors2k, Sun Mar 04 2007, 01:15PM
Just the advice I was looking for, thanks!
Re: Primary Capacitor sizing
uzzors2k, Fri Mar 09 2007, 06:02PM
I'm ready to build the tesla coil this weekend, but I just need somw things checked first. I decided to use 58 plastic plates to make a 62nf primary cap, which with a 1.2µH primary should resonate at 570kHz. What kind of primary shape should I use; pancake, helical or cone? If I use a pancake coil I'll only need 3 turns to hit 1.2µH, sounds a bit low to me, what about coupling and such?
uzzors2k, Fri Mar 09 2007, 06:02PM
I'm ready to build the tesla coil this weekend, but I just need somw things checked first. I decided to use 58 plastic plates to make a 62nf primary cap, which with a 1.2µH primary should resonate at 570kHz. What kind of primary shape should I use; pancake, helical or cone? If I use a pancake coil I'll only need 3 turns to hit 1.2µH, sounds a bit low to me, what about coupling and such?
Re: Primary Capacitor sizing
J. Aaron Holmes, Fri Mar 09 2007, 06:28PM
62nF! Wowzers. Maybe I'm wrong, but at 20kV, 60nF is like "pole pig territory". With a flyback, I'm thinking it's gonna take a long time to charge that 60nF, and you'll be left with a minescule breakrate unable to facilitate proper streamer growth (not to mention hideous peak currents associated with a 1.2uH primary--these can be hard on your caps!). I'd recommend trading a lot of that primary C for a lot more primary L.
EDIT: In fact, assuming 20kV is your RMS voltage and your peak is something like 28kV, and assuming your target power is 300W with a break rate of 100 (3 joules per "bang"), then I compute a required cap size of only about 7.5nF, which is 1/8 the size you're using! That would make 62nF good for over 2.4kW Maybe not "pole pig territory", but real close!
EDIT #2 (sorry): Actually, RMS is probably a hard thing with your flyback, since it's not driven with a sinusoidal source. Perhaps somebody will hop on here and finish what I've begun here. But if 20kV were your peak and not 28kV, then your cap size would probably be about 15nF, which is still only a quarter of what you're using.
Regards,
Aaron, N7OE
J. Aaron Holmes, Fri Mar 09 2007, 06:28PM
62nF! Wowzers. Maybe I'm wrong, but at 20kV, 60nF is like "pole pig territory". With a flyback, I'm thinking it's gonna take a long time to charge that 60nF, and you'll be left with a minescule breakrate unable to facilitate proper streamer growth (not to mention hideous peak currents associated with a 1.2uH primary--these can be hard on your caps!). I'd recommend trading a lot of that primary C for a lot more primary L.
EDIT: In fact, assuming 20kV is your RMS voltage and your peak is something like 28kV, and assuming your target power is 300W with a break rate of 100 (3 joules per "bang"), then I compute a required cap size of only about 7.5nF, which is 1/8 the size you're using! That would make 62nF good for over 2.4kW
Maybe not "pole pig territory", but real close!EDIT #2 (sorry): Actually, RMS is probably a hard thing with your flyback, since it's not driven with a sinusoidal source. Perhaps somebody will hop on here and finish what I've begun here. But if 20kV were your peak and not 28kV, then your cap size would probably be about 15nF, which is still only a quarter of what you're using.
Regards,
Aaron, N7OE
Re: Primary Capacitor sizing
uzzors2k, Fri Mar 09 2007, 08:20PM
I can't quite remember the maximum output voltage, so I'm just going to set the spark gap for 10kV to be nice on the components. 20kV is just a number I based on striking distance while arcing, it may be a lot lower however. 10kV @ 60nf is within the power capability of the flyback (or so I hope). However, you mentioned large peak currents, would these destroy my capacitor? I do feel that 60nf and 1.2µH is pretty unbalanced, but then again the secondary is pretty unbalanced too.
Any suggestions for primary coil type, this is my first proper Tesla coil so I'm still pretty green...
uzzors2k, Fri Mar 09 2007, 08:20PM
I can't quite remember the maximum output voltage, so I'm just going to set the spark gap for 10kV to be nice on the components. 20kV is just a number I based on striking distance while arcing, it may be a lot lower however. 10kV @ 60nf is within the power capability of the flyback (or so I hope). However, you mentioned large peak currents, would these destroy my capacitor? I do feel that 60nf and 1.2µH is pretty unbalanced, but then again the secondary is pretty unbalanced too.
Any suggestions for primary coil type, this is my first proper Tesla coil so I'm still pretty green...Re: Primary Capacitor sizing
Dr. Drone, Sat Mar 10 2007, 02:16AM
Dr. Drone, Sat Mar 10 2007, 02:16AM
Re: Primary Capacitor sizing
Tom540, Sun Mar 11 2007, 04:39AM
Looks like someone shops at Trader Joe's
Tom540, Sun Mar 11 2007, 04:39AM
Looks like someone shops at Trader Joe's
Re: Primary Capacitor sizing
uzzors2k, Mon Mar 12 2007, 08:47PM
So clean Christop_eR, those primaries look really professional. I sure wish I had some acrylic, just wait 'til you see my ghetto coil.
uzzors2k, Mon Mar 12 2007, 08:47PM
So clean Christop_eR, those primaries look really professional. I sure wish I had some acrylic, just wait 'til you see my ghetto coil.
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