Building a Solid State Tesla Coil, some questions about the reference desing - Page 1
Forum section: electronics / projects

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 24, 2013, 01:41:44 pm
content: A friend and me want to build a table size Solid State Tesla coil that is still able to generate >15" sparks (see picture) but i have some questions regarding the design we want to use as a reference:http://www.richieburnett.co.uk/sstate.htmSince the author's email does not work I thought why not ask here.-First when I look to the H-bridge driver (sstc01.gif) I see that he switches the ZTX totem pole from the emitter outputs of the TL494, why is that and why not use pin 8 and 11 as output?-Second thing I see he is using the TL494 to generate 350kHz but it is specced as 300kHz max. Since he is a power electronics engineer I assume he has a good reason to do so but what?-Also he uses a 220ohm as a pull-up/down on the emitter outputs, why such a low R value (12/220=54mA)? As a failsafe?-Why is there a 100n//10uF in the AB lines but only 10R//10RR in the CD output lines?-What kind of isolation transformator can I use for isolation beween the driver and h-bridge, something like (http://nl.farnell.com/murata-power-solutions/77205c/transformer-pulse-1-1-1-8dip/dp/1454878). Or is it also ok to remove the 100n//10uF and 10R//10R and use forward diodes and/or TVS diodes to protect the driver circuit? I dont have a lot of transformer experience-In the power stage (sstc02.gif) he rectifies the 240VAC with 2x1N5408 in series, why is that and won’t they burn up since they are only rated 3A? And why no diodes in the neutral line? Now the negative sine part is dissipated by the 1N5408 right?Thanks a lot and I'll keep U updated ofcourse!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 24, 2013, 02:28:22 pm
content: 4HVis the best place for tesla coil advice - I think Richie hangs around there as well.

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 25, 2013, 09:37:01 pm
content: Quote from: mikeselectricstuff on September 24, 2013, 02:28:22 pmThx, I will have a look there as well. My questions are not that much about the tesla coil itself but more about the tl494 configuration and transformers in general

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 27, 2013, 08:23:30 am
content: No-one?

author: Paul Price
author_rank: Super Contributor
timestamp: September 28, 2013, 02:37:47 am
content: -First when I look to the H-bridge driver (sstc01.gif) I see that he switches the ZTX totem pole from the emitter outputs of the TL494, why is that and why not use pin 8 and 11 as output?The idea here is to switch between 0 and 12V to drive the pre-driver MOSFETs gate capacitances to get the fastest turn-on using the current gain of an emitter follower output. It also switches off all the pre-driver MOSFETs if the TL494 is removed from the socket for troubleshooting.

author: Paul Price
author_rank: Super Contributor
timestamp: September 28, 2013, 02:40:46 am
content: -Second thing I see he is using the TL494 to generate 350kHz but it is specced as 300kHz max. Since he is a power electronics engineer I assume he has a good reason to do so but what?The designer discovered that the 494 can be pushed to operate at a frequency for the Tesla Coil to operate at it's most-efficient or resonant freq.

author: Paul Price
author_rank: Super Contributor
timestamp: September 28, 2013, 02:43:53 am
content: -Also he uses a 220ohm as a pull-up/down on the emitter outputs, why such a low R value (12/220=54mA)? As a failsafe?No, not a failsafe, just the best R value to discharge the pre-driver MOSFET gates capacitance within the current drive limits of the 494.

author: Paul Price
author_rank: Super Contributor
timestamp: September 28, 2013, 02:44:53 am
content: -Why is there a 100n//10uF in the AB lines but only 10R//10RR in the CD output lines?Your posted schematic is truncated and fails to display the parts in your question.

author: Paul Price
author_rank: Super Contributor
timestamp: September 28, 2013, 02:52:23 am
content: -What kind of isolation transformator can I use for isolation beween the driver and h-bridge, something like (http://nl.farnell.com/murata-power-solutions/77205c/transformer-pulse-1-1-1-8dip/dp/1454878).Or is it also ok to remove the 100n//10uF and 10R//10R and use forward diodes and/or TVS diodes to protect the driver circuit? I dont have a lot of transformer experienceAnswer: No, this will cause smoke and sorrow.You can probably find the needed transformers in many discarded models of desktop PC power supplies and get them for free. They are very commonly used to drive the bipolar or MOSFET half-bridge switches. Try them in your circuit to see if they can develop 12-18V of drive voltage to the power stage MOSFET gates before applying the 240VAC. You will need a scope for that. You need to be sure the gate-source voltage delivered by the transformer is >10V and <20V peak.

author: Paul Price
author_rank: Super Contributor
timestamp: September 28, 2013, 03:00:56 am
content: In the power stage (sstc02.gif) he rectifies the 240VAC with 2x1N5408 in series, why is that and won’t they burn up since they are only rated 3A? Answer: That's enough, the MOSFETs are are only turned on for a short time and let resonance do the rest, the total 240VAC average current is low.And why no diodes in the neutral line? It is a half-wave rectifier, so no diodes are needed on the neutral side because of this.Now the negative sine part is dissipated by the 1N5408 right? Answer: No. No negative sine part.

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 30, 2013, 06:43:44 am
content: Quote from: Paul Price on September 28, 2013, 02:44:53 amThey are on the most right side of the controller schematic.

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 30, 2013, 06:45:31 am
content: Quote from: Paul Price on September 28, 2013, 03:00:56 amThx a lot for your answers!But why would u use 2 diodes in series to rectify it while it effectively acts as one? Then why not put them in parallel to do some (crude) load sharing?Onlly thing I can think of is the blocking voltage but they are rated 1000V each.

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2013, 08:27:21 am
content: Be prepared to blow up lots of mosfets.I really want to make a SS coil as well, (I currently have a 360W NST spark gap coil)After looking at SS coils i pretty much decided to use vacuum tubes for the switching.Valves are much harder to damage than mosfets and can switch really high voltages which helps when it comes to building the primary coil. Low voltage primary coils have to be quite tightly wound around the secondary which makes things tricky.

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 30, 2013, 08:43:36 am
content: I am. Biggest problem in my particular solid state tesla design is the VSWR right?But in my schematic are the MOSFET's not reasenably well protected against it by the diodes?

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2013, 10:13:28 am
content: Protection looks ok to me, but i've not built one yet so i'm no expert.

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 11:08:18 am
content: But why would u use 2 diodes in series to rectify it while it effectively acts as one? Then why not put them in parallel to do some (crude) load sharing?My best guess with so much super HV arcing going on, coupled electromagnetic transients could cause a failure of just one diode.It might work just as well without the 2nd diode...for a while.

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 11:12:59 am
content: -Why is there a 100n//10uF in the AB lines but only 10R//10RR in the CD output lines?The transformer primary connections are the same for both AB and CD lines, a push-pull connection to both transformers.

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 30, 2013, 11:44:39 am
content: Quote from: Paul Price on September 30, 2013, 11:12:59 amSorry, I didnt make myself clear, what I meant was: why are there a 100n//10uF in the push-pull output lines to drive transformer/MOSFET?The 10R I get but why a 100n//10uF if u drive the isolation transformer with a square ~350kHz signal?

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 12:35:06 pm
content: -Sorry, I didnt make myself clear, what I meant was: why are there a 100n//10uF in the push-pull output lines to drive transformer/MOSFET?The 10R I get but why a 100n//10uF if u drive the isolation transformer with a square ~350kHz signal?If the unit is powered down or if transients caused waveform distortion or even transients because of freq. adj pot noise, then the 10uF cap would protect a MOSFET pair from remaining turned and conducting an ever increasing current, saturating the transformer and then destroying the MOSFETs. The 10uF cap charges up and stops current flow under abnormal waveform conditions.---------------Using tubes instead of MOSFETs is not an advantage. If you enclose the MOSFET circuitry within an aluminum or steel case, it could be both robust and reliable. The tube's high anode voltages on the primary of the Tesla Coil want very much to arc-over or overheat, they might require a silicone coating which keeps the wires warming, not to mention the very deadly high currents and primary voltages you could be working with. The ozone ions created in the Tesla Coil experience serve well to exacerbate the situation.OP:I am. Biggest problem in my particular solid state tesla design is the VSWR right? Ans: No, many other problem areas.-----------------------------------------------But in my schematic are the MOSFET's not reasenably well protected against it by the diodes?-------------------------------------------------Ans: Yes.

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 12:41:07 pm
content: In Los Angeles they had a 6-ft cone shaped monster Tesla Coil at the Griffith Park Observatory. They stopped demonstrating its operation because they said the HV discharges caused dangerous X-Rays to be produced. I always thought that the very strong EMI caused by the 4-ft arcs would just cause a Gieger tube circuit to malfunction.What do you think?Does operating a Tesla Coil challenge your reproductive capability?

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2013, 01:03:21 pm
content: You can definitely get xrays from a tesla coil under the right circumstances.Im not sure exactly what those are though, i think it has to do with arcing some types of metal.Anything below about 3000V on the primary coil and you really start to find it hard to get good coupling to the secondary coil. Above 6000V+ is ideal.Using mosfets/igbt to switch these sort of voltages gets expensive fastThis is why all the SSTC that run on 230v direct are quite small and inefficient.The bigger ones often use a 2-4 stage voltage multiplier or MOTs to run at around 1-2kv. But that's getting pretty close to the upper limit for a single mosfet/igbtIf you want to switch 6kV onto the primary without selling your car for a large bank of series mosfets a vacuum tube SSTC is a good alternative.At least, this is what i found when i looked into the issue 6 years ago. Maybe there are better mosfets now?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 30, 2013, 01:09:58 pm
content: Quote from: Psi on September 30, 2013, 01:03:21 pmI don't think there is any x-ray risk without a vacuum - TCs can certainly produce voltages that will generate x-rays from vacuum tubes etc.

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 30, 2013, 01:13:59 pm
content: Quote from: Psi on September 30, 2013, 01:03:21 pmThat means I should place my primary coil very tight and with minimal isolation around the secondary coil.

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2013, 01:22:52 pm
content: Yep, close to the secondary but you do want lots of electrical insulation between the two. It does depend a bit on your coil though, you want to find the sweet spot.Not enough primary-secondary coupling and you lose power but too much and it will overcouple which limits performance also.If you plan to feed 230v to the primary coil it will need to be closely wound around the secondary in a helical design instead of a pancake.Below is a calculator for spark gap coils that maybe of use. It lets you play around with coil specs and takes you through each calculation so you can build the tesla coil close to optimal theory wise.http://webspace.webring.com/people/gl/lemagicien/tcpage/tcc/teslacalc.htmlEdit: sorry, i originally posted the wrong link here, its corrected now.

author: DutchGert
author_rank: Frequent Contributor
timestamp: September 30, 2013, 01:29:47 pm
content: Quote from: Psi on September 30, 2013, 01:22:52 pmAnd use the whole height of the secondary coil instead of only the bottom?

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2013, 01:33:47 pm
content: na, maybe 30% of the height.That online tool i posted should let you calculate the specs of different types of coil dimensions, turns, etc.Try to keep in mind a tesla coil is more like a radio transmitter and radio receiver than a transformer.

author: ConKbot
author_rank: Super Contributor
timestamp: September 30, 2013, 01:45:13 pm
content: Quote from: Paul Price on September 30, 2013, 12:35:06 pmAlso, a cap in series with a GDT can help prevent any issue due to the input to the transformer not being an exact 50% duty cycle (or more precisely, the volt-second product not being being zero since the SSTC probably wont have a simple 50% duty cycle square wave output. )

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 02:13:28 pm
content: And use the whole height of the secondary coil instead of only the bottom?------------------------The overriding principle is that the primary windings should not reach into any height of the secondary winding that would permit an arc-over from the very HV winding to the top of any other winding.I would imagine it would be possible for the 230V output to drive an additional transformer winding that creates a 3-6KV secondary winding over the tertiary HV Tesla Coil winding. You would create this transformer asconcentric primary-secondary-tertiary winding over the bottom of the super HV winding.Would take some tweaking to get the coupling right, but the primary to secondary calculation would be just a turns ratio calculation. Could be constructed on a thin insulating tube that could be raised or lowered under or along the bottom of the tertiary super HV Tesla winding, while the primary-secondary windings would be tightly magnetically coupled.

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2013, 10:46:35 pm
content: Quote from: Paul Price on September 30, 2013, 02:13:28 pmhm.. do you mean (mosfet switched 230v) -> (230:6000 transformer) -> (primary coil) ?that should work but would need a 230V -> 6kV transformer that can handle the primary switching frequency (somewhere between 100khz -> 1mhz)ie, it's not a simple 50Hz transformerFinding something like that for sale will be almost impossible, so it will need to be built around the appropriate core material.That's not really a problem, just takes a while and requires good insulation between the HV winding layers and put under oil.