TC Driver / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

TC Driver

Move Thread

LAN_403

Frosty90

Sun Dec 07 2008, 07:02AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Hi all,
Thanks for being patient and answering all my questions! so here is another:
The driver i have built uses a 74hc14 to detect the signal from either a CT or an antenna. This is then 'ANDed' with the interuptor signal and fed to 4 tc4420 mosfet drivers. It works properly, and the logic functions perfectly well, but when a 50:50 square wave is put in to the driver (i.e as a test signal), the output from the driver is at about 60% duty cycle (obviously due to the hysterysis of the 74hc14). Is this a problem? Am i correct in thinking that this means the mosfets wont be 'zero switching' because the drive signal is now not symmetrical? I must admit that a similar setup worked for me in the past, and it obviously works for hundreds of other people, but sometimes i see scope screen shots on the forums, with nicely symmetrical waveforms, none of the 60% stuff. So is this right? If not how can i fix it? If not is it bad enough to worry about?

cheers,
Jesse

MRacerxdl

Sun Dec 07 2008, 12:48PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

I think is not a problem, because if you think, when you put a sine wave on input, you wont get 50% duty cycle.
But I dont know, I dont have scope, so I didnt checked on my coil too see what I get. Maybe I am wrong.

LithiumLord

Sun Dec 07 2008, 02:13PM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

Frosty90 wrote ...
It works properly, and the logic functions perfectly well, but when a 50:50 square wave is put in to the driver (i.e as a test signal), the output from the driver is at about 60% duty cycle (obviously due to the hysterysis of the 74hc14).

This is not a hysteresis issue. The hysteresis is only visible in the circuits that have a non-square waveform input as it's nothing but a difference in positive- and negative-going thresholds. Also, unlike a single-threshold logic, the schmitt triggers are designed for improved processing of the waveforms with prolonged slopes - the hysteresis is used to exclude any noise in case of minor ringing over the slope, also it nearly doesn't affect the signal symmetry for the exp-shaped strobes as the hysteresis gets the turn-on and turn-off delays for such signals nearly equal (so those things are perfect for designing RC delay circuits).
Thus, the most likely possible reason is a too high output resistance of your generator, so the square wave gets messed up by both the wires' and diodes' capacity, as well as any other stray or non-stray capacity induced there, getting the signal way different from a square wave. What frequency are you running it? On higher frequencies, those effects are stronger and are also accompanied with the effect of different turn-on/off times of the ICs used there which get more pronounced in that case.

Mestre Racerxdl wrote ...

I think is not a problem, because if you think, when you put a sine wave on input, you wont get 50% duty cycle.

All wrong. Not only the sine wave will be transformed into a square one only if it's midpoint is straight between the threshold voltages of the input gate (some phase shift will be present in that case however), but also how are you supposed to get a sine wave in any of the mentioned sstc feedback concepts? Because of the CT has to be loaded, after the clamping zeners it gives a nearly square wave. For an antenna, you'll get f=arccos((Vant-Vttl)/Vant), where Vant is the voltage to be induced on the antenna by the capacitive coupling (sine waveform) and Vttl is the TTL-level clamp voltage. Antenna, being unloaded, produces around 1kV input voltage so f, the angle covered by a slope, will be very small.

ps Frosty, AND-gate is not the best way of a modulation cicuit as it may cut off the power whenever it wants. Myself in my circuit featuring deadtime delays I added a 74hc112 on the output getting the advantage of it's third state in asynchronous RS-mode. That's pretty easy, also it doesn't take too much chips as 112 is a dual flip-flop (who cares, I got 7 chips on that board, excluding separate driver and coupler ICs :) ). Also you can just use an OR gate before the AND one to mix the interruptor signal with the driver chip input so it will never go offline if that driver is high.

Frosty90

Mon Dec 08 2008, 06:05AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Ahh ok thanks. yes it makes sense now, of course the hysterisis wont do that, i dont know what i was on about there! i think you are right about the signal being messed up ue to capacitence etc, i was actually running it from a fairly long wire connection, at about 350Khz. anyway i tested a quick lashup of the whole thing with 24v on the DC bus and it works moderately well, it will run off CT feedback with a startup signal from a 555.

In terms of modulation, im not actually just anding the interupt and the drive signal together, i did actually endup adding synchronization with a D flipflop so it can only go low when the drive signal transitions. Anyway it seems to work ok now except the gate driver chips are getting quite hot, so im going to rebuild with some 9A ones (tc4421 i think) in a to-220 package and put them on a heatsink.

Cheers
Jesse

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob