Clarification on the Primary Circuit

Non-kit projects based on the public Gen1 designs

Clarification on the Primary Circuit

Postby sredmond » Tue Mar 25, 2014 11:43 pm

Hi oneTesla! I posted this in the comments, and Kayla suggested I repost it to the forums so that everyone can share in the knowledge.

I'm really interested in the idea of a musical Tesla coil, so I'm building one in my lab! I've been using the oneTesla resources for inspiration and guidance, and I must say the documentation is extremely impressive. After about a month of fiddling with optocouplers, I finally got the interrupter board to work perfectly (well... not in running mode, but that's fine). I'm using an Arduino Mega rather than an Uno in order to support up to six notes polyphonically :) However, I've run into a few challenges while building the coil's primary circuit. Specifically about the images here: http://onetesla.com/driver-guide

What source is connected to the current transformer T? I get that it is some sort of antenna that is responding to the oscillating E&M waves from the oscillations in the Tesla coil (in order to trigger IGBT switching at zero current crossings), but what is it *actually*. Is it a specific antenna, or just any wire in the air, or something more complicated?

Second, are the "voltage regulators" you refer to in the User Manual the same as the protection diodes, D1 and D2? Or are the "voltage regulators" something else I'm missing completely.

Third, how do G1 and G2 really work? I get that they are inverters, but I looked through the datasheet for the 74HCT14 (that's the part... right?), and I didn't see anything about squaring up the signal. I do understand, though, why it's better for the signal to be square-ish when it clocks the flip-flop and the gate drivers.

Fourth, I still am not entirely clear of the purpose of R1, R2, and R3. In the guide it says that it allows the flip-flop to be "tickled" into activation just from the interrupter signal at the start, and then still works when the top of the signal pathway is dominated by the feedback current, but why are they placed the way they are? I think I could understand this; it's just not clicking yet.

Lastly, I would like to buy a gate drive transformer, rather than build one, because without a metal toroid, I'm not confident that I can phase the transformer correctly, and that part seems pretty vital to have built correctly. That said, what specifications should I look for in a gate drive transformer (besides that the turn ratio should be 1:1:1)?

From one Tesla coil enthusiast to another, I really appreciate all your help in this endeavor.

After seeing Kayla's response, I also had another wave of questions.
1) What is the part number of the voltage regulator? Does it matter, or are we just using the diodes to clip the range of voltages that will pass to the rest of the primary circuit?

2) Can this circuit be run off of AC power (from the wall - 120V, 60Hz in America)? Do I need to run the circuit through a transformer first? (I have a 15,000V Neon Sign Transformer). Also, where does the power supply actually *go* on the circuit? It has to be somewhere on the right side of the circuit, but is it directly between the inductor and tank capacitors? Do, then, the bus capacitors act as the doubling/rectifying capacitors?

3) Kayla suggests that the primary current is routed through the 300:1 current transformer, but how do I redirect current from the primary (wherever its source is), to the left side of the circuit?

4) Finally (sorry for all the questions), is it okay if I use a breadboard for all of this? I made sure to order DIPs (well... after some struggling with hand-soldering SMT chips), but, from experience, will a breadboard be able to handle the extreme voltages and currents? I can use protoboards, but I would like to avoid soldering as much as possible.

Thank you!
-Sam

P.S. Any advice on primary coil dimensions/inductance or secondary coil dimensions, or even topload dimensions? I'm modifying a spark gap Tesla coil (which works fine - sparks ~ 6in), but I'd totally be willing to make a new Tesla coil, if it will lead to bigger sparks.
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Re: Clarification on the Primary Circuit

Postby martijn89 » Wed Mar 26, 2014 2:57 am

The voltage regulators are not the same as the protection diodes, look for 7815 and 7805 regulators.

You can feed it with mains power, the circuit rectifies it to dc to the bus capactors. The IGBTs feed that (depending on logic) in to the osscilating circuit (tank capacitor <--> primary coil) in such away it adds to the power.

Lastly the only power from the right side (high voltage/power side) to the left (logic) side is the sensing of current flow via the current transformer. And your question of breadboard usage, for the power side most definitely not (too much power), even on the logic side it would be ill advised, an bad contact could drive the circuit wrong.

If i did not answer a question it is because i am not sure enough my info is correct. I am assembling mine next week.
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Re: Clarification on the Primary Circuit

Postby Heidi » Wed Mar 26, 2014 1:29 pm

To answer some additional questions:

The primary current is fed through the 300:1 current transformer (listed here http://onetesla.com/products/components/current-transformer.html).

An inverter has two states: high and low, or 5V and 0V. It goes low when the input signal is high (>1.4V), and high when the signal goes low (<0.85V) --take a look at table 10 in the datasheet. So, if you feed the inverter something like a sine wave, it will turn it into a square wave. This is what we mean by "squaring up the signal." There are two inverters to get back the original signal, but with cleaned-up edges.

Basically, the resistors form a voltage divider that combine the feedback signal from the transformer and the interrupter signal. They're sized in such a way that allows the feedback signal from the transformer to dominate if both signals are present, but the interrupter signal is enough to trigger the inverter if there is no feedback signal.

You may want to take a look at the Eagle files at http://onetesla.com/downloads.
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Re: Clarification on the Primary Circuit

Postby martijn89 » Thu Mar 27, 2014 5:26 am

And the question about the current transformer, in my understanding it is an coil with the primary feed running trough the center. This gives an voltage dependent on the current flowing in the primary configuration.

So there is no antenna, it is current sensing, i do believe it would be possible to use an antenna but have no idea how. Don't forget that if you are using an antenna the secondary voltage becomes dominant so you have no idea how much current flows in the primary making the switching guesswork.

In an sstsc it might be useful i think, mainly because the primary and secondary have the same frequency? (i think)
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Re: Clarification on the Primary Circuit

Postby Frost273 » Fri Mar 28, 2014 7:54 am

sredmond, whare are you from?
Building Gate Drive Transformer on your own is not that hard.
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Re: Clarification on the Primary Circuit

Postby sredmond » Mon Mar 31, 2014 2:11 am

Frost, I am from the Bay Area, just a little south of San Francisco. I would like to build my own gate drive transformer, but I've been advised against it. I may just do it anyway - what's the harm? Additionally, why do I even need a gate drive transformer? Couldn't I just split the differential output across the gate drivers to the base and emitters of the IGBTs? Or am I really just trying to separate the logic and power sides?

Martijn, I think I understand what's going on with the current transformer now. So basically, the power line (from the power side) runs through a coil of wire. Since the power line is AC, there is a changing magnetic flux through the coil of wire, which then induces a current in the coil of wire. The amount of current induced in the coil of wire can be changed by varying the geometry of the configuration. I'm glad that I don't need an antenna, since that would be hard to get right.

Additionally, Martijn, if I can't use a breadboard, what *can* I use? I don't have printed circuit boards - only proto circuit boards and breadboard. I also have alligator clips galore, but they probably can't withstand it. Do I just connect everything with cables? I also looked through the schematic at http://onetesla.com/media/wysiwyg/onete ... ver220.png but I can't seem to find where the voltage regulators go! They show them in the bottom right, but I can't find those parts anywhere on the circuit! Any help would be greatly appreciated.

Lastly, are all the electrical connections on pin 8 of the gate drivers necessary? Seems to me like they are all connected to ground anyway. Also, will I need to buy heat sinks, or will a separate fan be enough?
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Re: Clarification on the Primary Circuit

Postby Alex » Mon Mar 31, 2014 5:58 pm

The gate drive transformer is needed to bring the gate voltage of the igbt 15V higher than the emitter voltage (which is around 370V), to turn it on.

you can solder your driver onto a protoboard. (breadboard can result in dodgy connections)
you can just connect your bridge with wire and solder, or use some copper sheet.

The gate driver connections are necessary

you will need heatsinks and at least one fan. you will also need sol-pad to isolate the igbt from the heatsink (if all of your igbt's are sharing a heatsink)

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