Step 15: Full-system troubleshooting

Do a Thorough Check

First do a visual inspection of your board. The vast majority of problems are caused by faulty component installation, which can be caught before you apply power to the coil. Thoroughly check the following:

  • Signal diodes (1N4148s) and bridge rectifiers are oriented correctly.
  • Electrolytic capacitors are oriented correctly.
  • 5V and 15V regulators are in their correct locations and have not been swapped.
  • Correct chips are in each socket, and chips are installed in the proper direction.
  • Soldering looks neat and clean.


Next, let’s do a multimeter check of the IGBTs. If something doesn’t check out here, DO NOT power up the board. Something is wrong with your IGBTs or their installation.

  • Meter between pins 1 and 3 of each IGBT and verify that you read a short.
  • Confirm that with the + probe of your multimeter on the middle pin of Q1 and the - probe of multimeter on the middle pin of Q2, you read open.
  • Meter between pins 1 and 3 of X1, the large 3-pin connector. You should not read a dead short.


Finally, check a few important mechanical things:

  • There are about 450 ohms between the toroid and the ground prong of the power plug.
  • The breakout point is mounted properly and is pointing in a direction away from the antenna. You should never arc to the antenna.
  • The wire at the top of the secondary is taut and not sagging into the secondary tube. Likewise, there are no drips of glue or stray bits inside the secondary tube.
  • The antenna and primary connector are plugged in. You’re using the full length of the optical fiber to distance yourself and the interrupter from your Tesla coil.
  • Your USB cable is short and/or coiled tightly to avoid picking up noise. The control laptop is unplugged from its charger.


You can now attempt to turn on the coil, as most failure modes result in the coil doing nothing or underperforming, rather than damaging the board. If you want to be more conservative, follow the complete troubleshooting procedure in Step 16.

Before you follow the startup procedure, read through all of the procedures and failure modes described below to know what to expect!


Startup and Shutdown Procedure

  • A. Start with no cables connected to your tinyTesla coil, and AC power cable UNPLUGGED.
  • B. First, confirm that your interrupter is working by connecting it to a laptop and verifying that it responds normally by observing the output of the fiber optic transmitter. As you raise and lower the power, the output should become brighter and dimmer accordingly. Set whatever computer program you’re using to only slightly above its lowest power setting and pause output to the interrupter.
  • C. Connect the fiber to the coil and the interrupter, and space the interrupter the full length of the fiber from the coil.
  • D. FIRST plug the power cable into the coil, THEN plug it into AC power. Caution! The board is now energized. Proceed with extreme caution. Note that the coil can be loud when it turns on. This procedure ensures that you’re not handling the coil at the moment you power it up, in case it behaves unexpectedly. If the fuse blows or you notice something else unusual happen, UNPLUG THE COIL IMMEDIATELY and proceed to the troubleshooting steps ahead.
  • E. Use your laptop to send a MIDI signal to the interrupter at low power. You should see and hear a small, clean-sounding spark coming from the breakout point. If it looks OK, slowly increase the power while watching for any problems such as flashover on the secondary, arcing to the antenna, arcing inside the coil, or any other strange behavior.
  • F. To stop operation, use your control software to stop sending MIDI signals to the interrupter. Always stop your coil using the interrupter rather than cutting AC power!


Every time you turn on the coil

Reference this startup procedure every time you turn on the coil!

Every time you turn off the coil

To turn off the coil, FIRST cut the interrupter signal first, THEN cut AC power. Cutting off the coil from AC power while the interrupter is still sending a signal can cause indeterminate states in the logic circuitry as the voltage rails sag, and can blow your IGBT bridge!


Common Problems and Solutions


If it just works…


Hooray! Good job following the directions. Share your success on our forum, and if you take great pictures or video we’ll showcase your build on our website and social media!


If nothing happens…


If you paid no attention to the direction of your primary winding, 50% of the time, your coil will do nothing! The phasing on the board needs to match the direction in which your primary is wound. Simply rotate the phasing jumpers and try again.


Rotate the jumpers 90 degrees


If switching the phasing doesn’t help, don’t panic! The majority of issues are caused by poor soldering or an obvious mistake during assembly. Check the following:

  • Is your primary plugged in?
  • Is your interrupter actually outputting a signal?
  • Is the antenna positioned too far away?
  • Have you tried both phasing directions?
  • Is your coil plugged in and AC power working?
  • Is the fuse installed?


If you don’t find any obvious errors, continue to the troubleshooting guide in step 16.

Coils do nothing half the time!

50% of the time, your board’s phasing will not match the direction of your primary. Simply rotate your phasing jumpers and try again.


If you see an arc down the coil...


Stop! Your driver is working (hooray!), but it’s outperforming your secondary. If you run the coil for any longer when you see arcs down the secondary, you’re going to degrade the wire’s enamel, and in the worst case, burn through the fine wire of the secondary. Fortunately there’s an easy solution, and that’s to add more varnish. Add varnish until you think there’s enough, and then add one more coat. There’s no such thing as too much varnish. If you need to patch just one spot, you can use some hot glue, electrical tape, or kapton tape. Put packing tape on top of the varnish to add extra insulation.


Taking good care of your secondary is essential to a long-lasting Tesla coil. As soon as there is an arc on the secondary, the insulation and wire enamel are compromised. The problem will only get worse with time unless you treat it right away. Stop operating immediately and patch the burned spot with a dab of hot glue, epoxy, varnish, or even tape. It’s possible to get a completely arc-free secondary that lasts a long time if you take care of it properly.


If you hear an arc but don’t see it...


Stop! Do not turn up the power! Increasing power on a coil that’s misbehaving never ends well. Always stop and fix the problem before turning it up.

Several things could be happening when there’s a weak output. The coil could be arcing on the inside rather than from the breakout point, which is caused by stray drips of glue or sagging wire on the inside of the secondary tube.

Problems with the logic or gate drive signal could be causing the coil to perform very weakly. Check your 15V rail carefully using the procedure in Step 16. Check that nothing is shorting on the underside of the board.


If the spark sounds hissy or crackly…


  • Try moving the antenna around. Bringing it closer to the coil often improves performance, but remember not to bring it too close, or you will get arcing to it.
  • Check your grounding. The top of the secondary to the ground pin of the power cord should read ~600Ω. If it reads significantly higher you have a break somewhere which you need to hunt down and fix.
  • Consistent poor performance despite hearing gate drive buzz (see Step 15) can indicate an issue with gate drive signals. Check your 15V rail carefully.
  • You may have a bad solder joint which is picking up noise and affecting your driver’s logic signals. Inspect your soldering on both the interrupter and the driver board. It never hurts to re-flow your joints with a good iron to fix potential cold joints that are near-impossible to see.
  • Your USB cable may be poorly shielded and may be picking up noise.


Keep in mind that a little bit of hissing or crackling is inherent to the operation of the coil, and that audio quality will not be perfect.


If it makes a loud bang upon plugging into AC power...


See, this is why we tell you to plug your AC cord into your coil first, then into an outlet! Fortunately this failure mode really isn’t likely to occur if you inspected your board properly and did a multimeter check of your IGBTs.

Probable causes of this failure mode include the following:

  • IGBTs not properly isolated from the heat sink with sil-pad. Or, you used a metal screw instead of a nylon screw to fasten your IGBTs.
  • You installed an electrolytic capacitor backwards, resulting in the capacitor blowing.
  • A catastrophic solder bridge caused a short that made your fuse blow instantly.
  • You plugged in your coil while the interrupter was latched up at 100% duty cycle.
  • You plugged in your coil without the interrupter’s cable installed, resulting in continuous light entering the optical receiver, latching up the coil.


Unplug your coil from AC power immediately, wait 5 minutes for the capacitors to discharge, and then examine your board carefully for errors. If you’ve made a catastrophic assembly mistake you should probably get help from somebody with more experience.


The fuse blows during operation...


There are a few possible causes of spontaneous failure during operation.

Interrupter latch-up happens when the microcontroller crashes, leaving the transmitter outputting high. A momentary large, loud spark from the coil and green flash from the fuse blowing is likely to be the interrupter latching up to 100% duty cycle rather than it’s typical <10% duty cycle. You can confirm the interrupter latched up by observing a continuous light from the optical transmitter. Next time you power up your coil, take care to coil your USB cable and unplug your control laptop from its AC charger.

IGBT overload can occur if you draw too much current from the coil. If you run the coil at full power and draw an arc from it using a grounded metal object for example, you may stress the IGBTs too much.

Other causes of failure during operation include overloading the coil by drawing too much current from it (particularly if you’re drawing an arc), or running it for too long, causing overheating. You may also cause a failure if you operate the coil at full power while there’s a serious issue with your coil, such as if the spark is very crackly or weak.

Unplug your coil from AC power and wait 5 minutes for the capacitors to discharge. Check your IGBTs to check whether they have blown. Unplug and re-plug-in your interrupter to verify it functions normally when you restart your control software.

If your fuse blows, DO NOT simply replace the fuse and try again. Meter your IGBTs.

How to Check your IGBTs

Use a multimeter to check that your IGBTs behave like a diode between pins 2 and 3. On a resistance measurement setting, the pins should read open in one direction and a near short in the other direction. Some multimeters have a diode test feature which shows you the diode voltage drop, which is a better test. A complete open or short in both directions indicates dead IGBTs.



Help, I don’t see my problem here!

We can’t predict every single failure mode that may happen. If you encounter a problem that isn’t covered here, take a look at our online help forum. Someone else may have had the same problem and figured out a solution.

Please post on the forum rather than emailing us for support, so that everyone can learn from how you resolved your issue!



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