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Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
You need to know what peak/RMS current to expect. I get a good approximation by determining the reactance of the primary at the resonant frequency of the secondary. Plug this in to get an approximation of the peak/RMS current. Use the rectified and smoothed DC Bus voltage if you're going to run CW on smoothed DC, but you can use the RMS voltage if you're not going to run CW or aren't using a smoothing cap (or are using small dc blocking caps on a half bridge). Remember that it is just an estimation, but it's always served me well.
I'm a bit confused. Isn't reactance at its peak at the resonant frequency? So wouldn't that give me the lowest value of the current, not the highest?
EDIT: Oops, I'm thinking the reactance of the primary itself, not the primary coil/capacitor together
SECOND EDIT: I've done the math but I'm not sure I did it correctly. It should be simple, but my current seems much higher than I expected -
X_l = 2*pi*f*L. My primary L = 27.805uH and resonant frequency of the secondary is 217.37kHz. Thus the reactance (X_l) = 37.975 ohms
X_c = 1/(2*pi*f*C). My capacitor C = 20nF. Thus the reactance is 36.609 ohms
I_max = V_rms/(sqrt(R^2+(X_l - X_c)^2))
*NOTE* I will not be running in CW mode
R of primary is as follows: Gauge: 12 (AWG) Length: 15.55 feet Thus total resistance of primary R = 0.025 ohms
I thought you said you'd be making a SSTC not a DR?
There's no primary cap in a SSTC so your primary impedance is nonzero and is thus the main determining current limiter for a loaded SSTC. (Coupling factor affects loading).
For a DR the calculations are much more complex.
Edit: the resonant circulating currents see only resistive losses and switching losses for an unloaded coil, so as your peak primary voltage rises so does the peak primary current. This is why ZCS and ZVS switching are so important. This current figure is only for your switches though, your rectifier doesn't see circulating currents.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
I thought you said you'd be making a SSTC not a DR?
There's no primary cap in a SSTC so your primary impedance is nonzero and is thus the main determining current limiter for a loaded SSTC. (Coupling factor affects loading).
For a DR the calculations are much more complex.
Edit: the resonant circulating currents see only resistive losses and switching losses for an unloaded coil, so as your peak primary voltage rises so does the peak primary current. This is why ZCS and ZVS switching are so important. This current figure is only for your switches though, your rectifier doesn't see circulating currents.
I think someone asked me that a couple of times already. I was originally planning on building a simple ISSTC, but since I have most of the parts already, I decided to just go ahead and try for a DRSSTC. That is why I'm using a primary feedback transformer, which I mentioned in a previous post. I also talked about tank capacitors for a while, which I wouldn't need if I was building a regular SSTC
By the way, the new ferrite cores arrived today so I was able to wind a new GDT. I am quite happy with it too--I was able to borrow a better scope for testing. Here are a few pictures:
The GDT:
GDT primary and two secondary waveforms taken at 273kHz, the resonant frequency of my coil. (Sorry for the lack of color--apparently the scope I used wasn't capable of saving to a color image):
And while I was at it, I decided to test the old primary feedback transformer I had wound (on what we decided were probably bad cores). Guess what? You guys were right--output was absolute garbage:
So I decided to re-wind it using the new cores I received:
And tested at 273kHz:
Thinking I should have tested it using a sine wave though, not a square wave. Am I correct? What is the most common method of testing primary feedback current transformers?
Square wave voltage through an inductor causes a sine wave current to flow. Sorry I missed the decision to go through with a DR coil, there was a bunch of back and forth on it earlier, lol.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
Square wave voltage through an inductor causes a sine wave current to flow. Sorry I missed the decision to go through with a DR coil, there was a bunch of back and forth on it earlier, lol.
Yes, that was my fault--I couldn't make up my mind at first, so I'm sure I confused a few people!
So the waveform looks good then? That would be great news!
I am very pleased with how the GDT turned out, except I goofed a bit with one of the secondaries, and there are 1-2 fewer turns. If it causes too much of a problem I can re-wind just that secondary.
So determining the necessary current ratings for the bridge rectifier--Are you saying the one I mentioned earlier would be sufficient, provided it is not run continuously (which I'm not planning to do anyway)?
Now, I think I read somewhere that it is not recommended to exceed 90% duty cycle unless you design it to run in CW mode. Can anyone verify this? It makes complete sense to me, and I plan to implement a limit on the software-side (Arduino).
I'm also finishing up the MIDI processing, so I now have manual (freq/duty cycle adjust) and MIDI interrupter modes. I'll post a simulation or a video at some point in the near future.
SECOND EDIT: I've done the math but I'm not sure I did it correctly. It should be simple, but my current seems much higher than I expected -
Your math is mostly correct within the model you assume. A series tank, like your primary, has a reactance of zero at its resonance, so the current will have a sharp peak around there. The values you calculate depend very much on the precision of the values you put in. Depending on your feedback scheme you might not be running at exactly the secondaries resonant frequency, so the kind of math you used will likely result in values way off.
*NOTE* I will not be running in CW mode
If you disregard the effect of the secondary, your primary tank will ramp up its current in a linear fashion. The primary voltage will increase by 4*Vbus each cycle, Vbus being the amplitude of the voltage you feed into the tank. A short burst will not lead to infinite currents. The current will also be limited by the primary copper resistance, possibly at some very high values.
Primary current is also limited by secondary loading, particularly when arcs break out. This has been discussed here: The effect is, that secondary loading will induce a resistance in the primary tank, reducing its Q significantly. That also causes a reduction of primary current.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Uspring wrote ...
DerStrom8 wrote:
SECOND EDIT: I've done the math but I'm not sure I did it correctly. It should be simple, but my current seems much higher than I expected -
Your math is mostly correct within the model you assume. A series tank, like your primary, has a reactance of zero at its resonance, so the current will have a sharp peak around there. The values you calculate depend very much on the precision of the values you put in. Depending on your feedback scheme you might not be running at exactly the secondaries resonant frequency, so the kind of math you used will likely result in values way off.
*NOTE* I will not be running in CW mode
If you disregard the effect of the secondary, your primary tank will ramp up its current in a linear fashion. The primary voltage will increase by 4*Vbus each cycle, Vbus being the amplitude of the voltage you feed into the tank. A short burst will not lead to infinite currents. The current will also be limited by the primary copper resistance, possibly at some very high values.
Primary current is also limited by secondary loading, particularly when arcs break out. This has been discussed here: The effect is, that secondary loading will induce a resistance in the primary tank, reducing its Q significantly. That also causes a reduction of primary current.
This is very good to know. I was trying to figure out how my primary would handle 87 amps, but it sounds like it's not likely going to see that, at least not for long periods of time.
I'm not sure I ever posted an updated JavaTC output, so here it is. The primary is still a bit off, and I'll have to fiddle with it a bit to match the resonant frequency, but it's somewhat close so far.
Don't waste too much time on getting the primary tuned exactly to the secondary. The arc has a considerable capacitance and will lower secondary resonance. Generally the primary should be tuned maybe 10% lower for best results. Initially start off with approximately equal frequencies and then experiment with lower primary fs.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Uspring wrote ...
Don't waste too much time on getting the primary tuned exactly to the secondary. The arc has a considerable capacitance and will lower secondary resonance. Generally the primary should be tuned maybe 10% lower for best results. Initially start off with approximately equal frequencies and then experiment with lower primary fs.
Good to know, thanks!
I'll post back here when I make more progress. I've made some modifications to my Arduino interrupter and will soon post the project. I just have to tweak a few more things first.
Regards, Matt
UPDATE:
My capacitors and IGBTs arrived today, so I'm just about ready to start construction. Unfortunately the perfboard and some of the other materials are still at my home in Vermont, and I'm currently living in Boston for work. I won't be able to retrieve the until next month.
In the mean time, I have made progress on my interrupter. It's based around an Arduino, though the Arduino is really only used for the MIDI options and to select modes. There are 4 modes:
Mode 0 - Output off Mode 1 - Manual. This mode uses a 555 timer and a 393 comparator to generate a square wave with completely independent frequency and duty cycle adjustment. Mode 2 - USB MIDI. This mode takes MIDI that is played over a MIDI connection from a computer, sent over USB to the Arduino, and converts it to a square wave that is usable as an interrupter output. This mode requires extra software, which I may cover in a future post. Mode 3 - External MIDI. This mode takes MIDI signals directly from an instrument and converts it (using some external circuitry) to a serial signal that is read by the Arduino. The Arduino then converts it to a square wave for the interrupter. This mode is not yet functional.
Here's a photo of the prototyped setup:
Eventually this will all be going onto a proto-shield that can mount directly to the Arduino, making it much neater and more portable.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Hi guys,
I finally got around to breadboarding the entire setup (minus the actual Tesla coil) and have noticed the UCC27425 gets quite hot. I am using a 1uF DC blocking cap in series with the output, but I'm wondering if 1uF isn't enough?
Once again, here's my circuit:
Currently I have a DC power supply set to 12v connected to the UCC27425 power rails, as well as between the + and - rails of the bridge. I am simulating feedback with a function generator set to produce a 237kHz square wave. I replaced the primary and tank capacitor with a variable resistor set to 470k, though I'm wondering if I should use an inductor instead.
I get a square wave (though it has a bit of ringing) on the output of the H-bridge.
Any thoughts as to what may be happening to the 27425 to cause it to heat up so much?
Also, when connected as shown in the schematic, the interrupter has no effect on the output of the 27425.
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First SSTC design - Need some critique
