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Building a DRSSTC Pt. 7 - Feedback

Blog entry posted in 'Building a Dual-Resonant Solid State Tesla Coil', Aug 2, 2014.

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Hello everyone, and welcome back to the blog!

Today's entry will be about the feedback circuitry. In solid state Tesla coils, some sort of feedback loop is required in order to have the driver trigger at the same frequency of the secondary coil. Some designs use "secondary feedback", whether it be an antenna to pick up the RF emitted by the secondary, or a current transformer at the bottom of the secondary, around the lead going to RF ground. While this works, I will be opting for Primary Feedback, which involves a current transformer around one of the leads going to the primary coil. This is not possible for ordinary SSTCs since the primary circuit does not resonate, but with a DRSSTC the primary resonates at the same frequency as the secondary. Therefore, we can get the same feedback signal with either the primary or secondary feedback.

The primary feedback transformer generally has a 1:1000 turns ratio. The signal from the transformer is then passed into some cleanup circuitry to convert the signal into a square wave, which is then (indirectly) fed back into the "INA/B" pins of the driver circuitry that we looked at in the last entry. It is important to note that there is more to the driver than I showed in the graphics, and we'll discuss this a bit later when we look at the interrupter.

Most feedback designs use a voltage regulator (zener diodes, for example) and some sort of buffer such as the 74HC14 hex Schmitt Trigger inverter. Well-known Tesla coil and SSTC pioneer, Steve Ward, uses the following feedback circuit utilizing two of the Schmitt Trigger inverters and some diodes to clip the voltage to the 5V required by the buffer.

Feedback.png

D1-D4 regulate the voltage, and D5 & D6 clip it to the 5V rail. This ensures that we don't damage the chip by driving it at too high of a voltage.

It is not shown in the schematic, but it is also necessary to have a decoupling capacitor directly across the 74HC14 power pins. We'll take a closer look at how this works once we get to the final schematic.

Besides the feedback for the signal, we also need a way to monitor the current on the primary and make sure we don't exceed the limits for our IGBTs. Therefore, most DRSSTC builders also implement a second current transformer which feeds into a comparator. If the current is measured to be above a certain threshhold, it triggers a 555 timer in one-shot mode, which pulls the input signal LOW. This shuts off our driver and prevents the Tesla Coil from running until the system is reset. This is probably one of the simplest ways to prevent damage to our transistors and other parts of the setup. Once again, we'll be able to see this better in the final schematic.

That's about it for this entry. We'll keep it short and to-the-point. The final design entry will be about the interrupter, and then we can move on to the actual build, which I'm sure is what you all have been waiting for! :D

As always, please feel free to leave comments, questions, and/or feedback!