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What are the Determinates for a SGTC Power Supply?

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quicksilver

Tue Aug 10 2010, 12:09AM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I was wondering about this question for along time. Generally when a modern SGTC is made, a NST is used that produces voltage and current in a manner that can be used via a spark gap and capacitor to feed into the SGTC design.
What are the criteria for a power supply for a Tesla Coil? The design elements alter the 60Hz of a NST to an end result of a higher frequency but yet a flyback may also be used and it's output ARE high frequency to begin with. But yet a NST or a Pole Pig produces it's power at 60Hz.
There are many high voltage supplies. A plasma power supply produces high frequency power (DC). Can it be used in a SGTC? I understand that the total design elements are different for a SSTC or a VTTC but the common SGTC appears to have the ability to use both AC & DC as well as high frequency. What is to prevent any source of high voltage that will manage a spark gap to be used as a power supply?
What ARE the criteria for determining what may be used in a SGTC? I appreciate someone making this clear to me: I have wondered about this for awhile.

HV Enthusiast

Tue Aug 10 2010, 12:41AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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You can use whatever power supply you want for a Tesla coil. Its not so much the power supply by itself, but the power supply, switch, and capacitor together.

Generally speaking, a 60Hz power supply (transformer) is sized, along with a spark gap, so that the primary capacitor is just about charged to its peak voltage (of 60Hz transformer output) when the spark gap switches.

Technically, the power supply is completely independent of the Tesla coil and has practically zero effect when the spark gap is conducting. The power supply's only purpose is to charge up the primary capacitor. Its just a basic cap charging circuit.

The spark gap is just a switch that dumps the energy from the capacitor into the primary coil. It can be controlled either statically (via distance of electrodes), dynamically (based on position of electrodes with spark gap motor), or controlled through some other external circuitry such as a zero cross circuit with delay (to trigger spark gap tube), etc...

AC transformers are most commonly used because they are cheap, readily available, require no other circuitry for their implementation (i.e. diodes), and are very robust.

quicksilver

Tue Aug 10 2010, 04:06PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Thank you. This tracks quite well with what I understand.

Therefore there should be no reason why a given voltage DC plasma display power supply would be inappropriate? Given the other parameters and an appropriate MMC, is there any significant designs that are a substantially INAPPROPRIATE supply? I appreciate your time in this. I have been wanting to experiment but yet not destroy some of the more unique PSU I have.

(posted within this thread - as is the schematic)

If not; obviously - please don't waste your time. My only worry is that I had wanted to experiment with AC vs DC supply (& some of the DC supply's are costly, etc). I certainly realize the need to formulate a MMC (or super large cap) that is appropriately higher voltage & capable of high frequency. as well as protection mechanism such as safety gaps.

And once again I do appreciate input on this subject because I really haven't seen to many really unique power supplies in SCTC & searching came back with little to no real answers as to "why" a substitute would be inappropriate (other than economy of construction).

quicksilver

Sat Oct 09 2010, 12:47AM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Capacitors (for instance the CDE492 series) list a DC and an AC voltage design spec in their data-sheet.
Is there any reason for using a rectifier, etc for switching the input voltage to CD prior to use in a SGTC?
The data frequently plugged into a MMC design program or formula often blurs the distinction from AC / DC when determining the appropriate cap.
Would there be any advantage to changing the power supply to DC prior to use?

monokel

Sat Oct 09 2010, 06:45AM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

quicksilver wrote ...

Would there be any advantage to changing the power supply to DC prior to use?

If you use AC you must dimension the MMC capacitance and carefully to not obtain a bad
power factor. I assume you use a synchronous RSG (at high BPS rates you may also use an asynchronous RGS).
If you have chosen one sort of caps for a MMC the cost for the MMC is proportional to the energy that can be stored in the MMC at the maximal DC voltage. In order to get a good ratio between the real output power and the
cost of the MMC a high firing rate (BPS) of the rotary is desirable. On the other hand,
e.g. if you habe 8 beats per mains period, the MMC isn't charged up to the same voltage
at alle these 8 beats. In fact, at high BPS rates the MMC has to hold a maximal voltage
that is 1.41 times the MMC voltage of a DC powered SGTC with the same output power.
A 1.41 times higher voltage of the MMC means a 2 times higher cost.
Maybe in small coils the cost of the MMC in not so crucial but in big coils it can
be hundreds of dollars resp. Euros.

If you habe an AC powered SGTC and the BPS in not 2 times the mains frequency
the charging current will not be zero at all firing events. Because the current runs
eigther through the leakage inductance of a neon sign transformer or through an external
inductor, this may be a problem: The quenching of the spark at the rotary might be difficult
and arc trailing may occure. If the BPS is 2 times the mains frequency the phase angle of
the firing events must be set correctly in order to obtain both a good power factor and
a zero charging current at the firing event. Changing this phase angle can be critical;
resonance effects with high over-voltages and currents can occure.

If you rectify a one-phase HV voltage with a bridge rectifier you also have a bad power factor. But a three-phase
rectifier can have very good power factors. Thus I recommend a DC supply for SGTCs especially
for high power coils.
In DC powered SGTCs with RSG the MMC is always charged up to the same voltage; thus it is not
necessary to over-dimension the allowed MMC voltage as in AC powered SGTCs with a ratio BPS / mains frequency
other than 2 or 4. Further, it is possible that the charging current is zero at the firing time; thus
arc trailing can be made less probable.
In DC powered SGTCs you cannot use simply circuits with static spark gaps except you make sure that
the charging current is zero at the firing time and remains zero while firing.

Maybe the pages of Richie Burnett are useful:
http://www.richieburnett.co.uk/tesla.shtml

I have calculated currents, voltages and powers of both AC and DC powered SGTCs with RSGs
analytically. This JavaScript calculator may help designing an AC powered SGTC:
http://www.geniculatum.ch.vu/rotary/rotary-calculator-ac-en.html

I have also made several diagrams showing currents and powers in an AC powered SGTC with synchronous RSG.
Unfortunately the text is in German but the curves may be useful with the following legend:
http://www.geniculatum.ch.vu/rotary/InduktiveStrombegrenzungNKontakte.pdf
N is the ratio of the BPS rate and the mains frequency ( >= 3 and integer in the diagram)
P_wirk is the real output power of the coil.
P_schein is the apparent power the power supply must provide.
u0_eff is the RMS supply voltage.
w_netz is 2*Pi*mains frequency = 377 Hz at 60 Hz mains frequency resp. 314 Hz at 50 Hz mains frequency.
L1 is eigther the leakage inductance of a neon sign transformer or an external inductance.
C1 is the MMC capacitance.
Take the diagrams with caption "Ohne Blindstromkompensation" rather than those with the caption "Mit Blindstromkompensation".

Regards, monokel

quicksilver

Sat Oct 09 2010, 09:29PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Thank you very much for a in-depth response: you've given me quite a bit to weigh out in determining the benefits vs. the idiosyncrasies & cost factors.

I can understand the concept of using DC where a large coil is concerned however I have a rather well made power supply for a plasma sign (see above) that delivers 11k @ 5 ma and thus falls at the lower end of the spectrum.
I have debated whether it's worth the time and cost to experiment with it as it delivers a very brilliant 4cm arc for it's low current & appears to be made with a great deal of care in that it is composed of both a "flyback" and "multiplier design" (see above link to schematic).

I had thought that DC MAY be an advantage - in that many cap designs are obviously directed toward DC usage and the low current may give me some "flexibility" in MMC design. However as you pointed out that may not be the case with a smaller coil & static gap. (A rotary gap presents problems enough to push a cost factor in it's own right.)

I have been amazed how some very low power "flyback-model" powered small coils gave great results for the (proportionately) very small current involved. But perhaps the fore-thought and components available to the designer were quite vast.

It seems a shame not to give it a go; provided I could work out the details without spending too much.

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