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Registered Member #3963
Joined: Wed Jun 22 2011, 11:06AM
Location:
Posts: 3
Hello, everyone this will be my first of probably many questions on this site. I am an aspiring young engineering student looking to build my first tesla coil. I've chosen a design similar to Steve's DRSSTC-.5 It's challenging for an amatuer, but I'm confident. I have one question that's been bothering me for a while now. In the older driver there is a RC circuit connected to the PRE pin on the flip flop. I believe it said on the site it was to delay the input to the pre pin so the interrupt signal would be passed correctly. I've been looking at the truth table for this flip flop, and I've been wondering why this is necessary. On the newest driver design the output is on Q now instead of Q NOT. If D is always held high, is there any reason I can't just connect PRE together with D on high? I think the output from this circuit would be the same as the older one. I propose the feeback flip flop could be wired similar to the way the flip flop in the over current detector is. Am I at all right about this? Someone set me straight.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I can't think of a good reason why that wouldn't work. Seems like when CLR goes low, it'll activate the system, and when it goes high as the interrupter turns off, it'll wait for a clock to actually turn off, which is the goal of the system.
Not sure why the feedback is tapped to the interrupter through a 10k resistor, though.
The newer design is a different flip-flop (74hc74 vs 74hc109) which behaves differently, look at the logic table and it has to be on Q rather than Q inv
Registered Member #3963
Joined: Wed Jun 22 2011, 11:06AM
Location:
Posts: 3
I thought it might work. I guess I'll have to wait and see when I start building. I'd like to test it, but I have no oscilloscope and I wish I was better with PSpice. Maybe some smart person can do it in PSpice and tell me if it works.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
The 7474 implementation on the current sense system isn't quite operating in the same way the main enable flip-flop works, but I did come up with a way to use a 74109 in a way that doesn't require an RC timing network. By keeping J, K, and PRE high, and putting an inverted interrupt into CLR, the Q_inv output seems to make an appropriate enable-output. Maybe there's something I'm missing?
Attached ltspice simulation. Top line is the non-inverted interrupt, middle line the current feedback, and the bottom line the output that would be used as an enable on whatever gate driver is used. Just etched a new driver tonight so I won't have a chance to test this in reality for a while.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I was working on a coil tonight and for some reason it just hit my why this wouldn't work as well as the setup with the timing RC circuit, if the bus voltage is off, and then turned on, no oscillations could start without power cycling the controller since the flipflop would remain in a steady state, since nothing would turn it off a short while after each interrupter pulse ends, the controller would just be in a latched state hence the need for the turn-off time delay setup.
Registered Member #4054
Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26
Guys, be warned! On LTSPICE, the 74HC74 model is wrong and do not work properly... I spent a day fiddling with it without success and then switch to Electronic Workbench and voila, no problems. On steve rev 1.3b schematics, the purpose of the setup is to allow for "pre" and "set" to both be high a brief moment thanks to the RC constant. When that happens, on the next "clock front" from the CT Q is updated with D level which is connected to GND. Since the clock front occurs when primary current is zero, this in effect synchronize the interrupter signal with current zero crossing... It's not elegant, but that definetaly works. This is the reason my board has the option to substitute the logic circuitry with a motor control microcontroller, as it would make a very simple board. It also allow endless possibilities as far as switching delays, advance and so on are concerned....
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
serge petiot wrote ...
Guys, be warned! On LTSPICE, the 74HC74 model is wrong and do not work properly... I spent a day fiddling with it without success and then switch to Electronic Workbench and voila, no problems. On steve rev 1.3b schematics, the purpose of the setup is to allow for "pre" and "set" to both be high a brief moment thanks to the RC constant. When that happens, on the next "clock front" from the CT Q is updated with D level which is connected to GND. Since the clock front occurs when primary current is zero, this in effect synchronize the interrupter signal with current zero crossing... It's not elegant, but that definetaly works. This is the reason my board has the option to substitute the logic circuitry with a motor control microcontroller, as it would make a very simple board. It also allow endless possibilities as far as switching delays, advance and so on are concerned....
Serge
Keep in mind though that uCs don't like noise or the large amount of EMI that it would experience so close under a DRSSTC. It would likely reset it's self often. This brings up another problem, most uCs default to all pins on or all pins off on ether power up or a reset, this can spell disaster for your bridge because during a reset the uC would turn on both low and high side IGBTs at the same time.
With all that being said you would also need some very split second timing with no lose in data. Dedicated logic always keeps track of your data even up into the MHz range. But with a uC, depending on your your programming, your uC can lose data or skip cycles.
I personally would never use a uC to do direct control of power electronics, it's just to unpredictable to run reliably.
Registered Member #4054
Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26
Hi Axelro, well, the uC is a poxy little PIC18F2431 that we use for motor control in my company; its got highspeed ADC. It's to be programed with MPLAB C18, with interrupt driven current zero-crossing detection.Might have a few routines in assembly, for time critical fonctions, such as updating the outputs to the MOS driver. I was originally considering a DSP, but I thought it's a bit overkill.
Eric, let's agree to disagree, we use uCs in all our power electronics applications here at MDP Electronics. You just have to know what your doing regarding EMC and routing of the boards. The pins on uC default to inputs on powerup, so you have to make sure to use external pullups for the desired state before you configure your outputs. Also, internal F is 40 Mhz, so that's 10 MIPS for the PIC (1 instruction takes 4 clk cycles) which is confortable for a 200Khz coil. To be honnest, we could not do without them anymore. uC will give you a flexibility as far as switching your bridge that you won't get from hardwired circuit. As I said my board has both option, and I'll learn usind Steve's circuit for starters. It's about time DRSSTCs move into the 21rst century.
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DRSSTC Flip Flop Question
