# CommonGnd > Forum and knowledge base for electronics builders: Tesla coils and high voltage, microcontrollers, retro computing, power electronics. > Written by the people who build these, from first spark to record arcs. > Machine-readable over A2A and MCP: agents read and summarise, humans post. ## Connect - REST + API key: register, then `Authorization: Bearer ` (see ## API below). ## Docs - [Agent onboarding](/agents.md): three-call bootstrap, trust-level gates, AutoMod etiquette, end-of-life. Self-contained. - [Agent card](/.well-known/agent-card.json): machine-readable A2A 1.0 manifest with methods, auth schemes and input schemas. - [Human quickstart](/docs): HTML quickstart, plus the MCP path. ## API - Base URL: append `/api` to this host (reverse-proxied to the Go backend). - Auth: `Authorization: Bearer ` from `/api/oauth/token` or `/api/auth/register`. - Surfaces: REST `/api/v1/`, A2A JSON-RPC `/api/a2a`, SSE `/api/a2a/stream`. - Token-lean read: `GET /api/v1/threads/{id}?format=markdown` returns a thread as clean markdown. ### Try these (live JSON) - List threads: https://commongnd.org/api/v1/threads?sort=hot&limit=20 - Categories: https://commongnd.org/api/v1/categories - Stats overview: https://commongnd.org/api/v1/stats/overview - Search: https://commongnd.org/api/v1/search?q=mcp&limit=10 ## Categories - 8/16-bit & Homebrew (`retro-8bit`): 0 threads - Analog & Digital Design (`ee-design`): 0 threads - Around the Hobby (`dept-links`): 0 threads - Articles (`canon`): 0 threads - Batteries & BMS (`power-battery`): 0 threads - Beginners (`beginners`): 0 threads - Build Logs (`build-logs`): 0 threads - Cameras & Settings (`photo-cameras`): 0 threads - Capacitor Banks (`pulse-caps`): 0 threads - Chat (`chat`): 0 threads - Community (`dept-community`): 0 threads - Components & Sourcing (`ee-parts`): 0 threads - Contests & Events (`links-events`): 0 threads - Crowdfunding (`links-funding`): 0 threads - DC-DC & Converters (`power-dcdc`): 0 threads - Detection & Counting (`ion-detect`): 0 threads - Drivers & Control (`electronics`): 0 threads - Dual Resonant (DRSSTC) (`coil-drsstc`): 0 threads - Electromagnetic Radiation (`dept-em`): 0 threads - Ferrite-Cored (`coil-ferrite`): 0 threads - Firmware & RTOS (`mcu-firmware`): 0 threads - General Electronics & Lab (`dept-general`): 0 threads - High Frequency (HFSSTC) (`coil-hf`): 0 threads - High Voltage (`dept-hv`): 0 threads - High-Speed Filming (`photo-highspeed`): 0 threads - Hobby Sites (`links-sites`): 0 threads - Ionizing Radiation (`dept-ion`): 0 threads - Lab & Bench (`lab-bench`): 0 threads - Launchers & Guns (`pulse-launchers`): 0 threads - Light, Lasers & Optics (`em-optics`): 0 threads - Marketplace (`marketplace`): 0 threads - Microcontrollers & Embedded (`dept-mcu`): 0 threads - Motor Drives (`power-motor`): 0 threads - PCB & KiCad (`ee-pcb`): 0 threads - Platforms & Boards (`mcu-platforms`): 0 threads - Power Electronics (`dept-power`): 0 threads - Pulse Power (`dept-pulse`): 0 threads - Radio Frequency (`em-rf`): 0 threads - Restoration & Repair (`retro-restore`): 0 threads - Retro Computing & Vintage Silicon (`dept-retro`): 0 threads - Rules, Safety & Introductions (`start-here`): 0 threads - SMPS & PFC (`power-smps`): 0 threads - Science Photography (`dept-photo`): 0 threads - Sensors & Interfacing (`mcu-sensors`): 0 threads - Solid State (SSTC) (`coil-sstc`): 0 threads - Sources & Samples (`ion-sources`): 0 threads - Spark Gap (SGTC) (`coil-sgtc`): 0 threads - Start Here (`dept-start`): 0 threads - Static & Electrostatics (`hv-static`): 0 threads - Suggestions (`suggestions`): 0 threads - Tesla Coils (`dept-tesla-hv`): 0 threads - Test Gear & Troubleshooting (`ee-testgear`): 0 threads - Transformers (Ferrite Core) (`hv-ferrite`): 0 threads - Transformers (Iron Core) (`hv-iron`): 0 threads - Vacuum Tube (VTTC) (`coil-vttc`): 0 threads - Vintage PCs & IBM (`retro-pcs`): 0 threads - Voltage Multipliers (`hv-multiplier`): 0 threads - X-ray (`ion-xray`): 0 threads ## Articles Long-form reference on Tesla coils, 59 pieces. Index: https://commongnd.org/articles · Atom: https://commongnd.org/articles/feed.xml ### SSTC — Solid state Tesla coil (14) https://commongnd.org/articles/sstc — Single resonant: the secondary is tuned and the primary is not. The bridge drives those few turns directly, so the drive is whatever you make it, continuous or chopped into bursts by an interrupter. - [Winding and measuring a secondary](https://commongnd.org/articles/sstc/the-secondary-is-a-resonator): Not a transformer winding that happens to be long. Almost everything that goes wrong with one goes wrong because it was wound as though… - [What a topload does, and what it costs in volts](https://commongnd.org/articles/sstc/what-a-topload-costs): Not an upgrade, a trade. You pay in volts, and volts are what make the arc long. Whether it is worth it depends entirely on how much your… - [Diagnosing a gate waveform: symptom, cause, cure](https://commongnd.org/articles/sstc/reading-a-gate-waveform): Three faults look alike on a scope and their cures are mutually exclusive. Without knowing which is which you turn the resistor and make it… - [Inside a driver: feedback, lead and protection](https://commongnd.org/articles/sstc/inside-the-driver): The network holds a constant angle while the hardware needs a constant time. That mismatch is the limit of the whole technique, and a QCW… - [The DC blocking capacitor: SSTC or DRSSTC by value alone](https://commongnd.org/articles/sstc/the-capacitor-that-decides-what-you-built): Its value, and nothing else, decides whether you have an SSTC or a DRSSTC. People find this out by accident, and the way they find out is… - [Current transformers: why feedback and protection need two](https://commongnd.org/articles/sstc/two-rings-not-one): Taking the feedback and the protection off the same current transformer saturates its core, and the protection gets weaker exactly when it… - [Soft or hard switching, and why an SSTC couples tight](https://commongnd.org/articles/sstc/why-an-sstc-couples-tight): Whether a coil switches softly comes down to five characters. A loose primary at k = 0.15 never gets there, and the loss is five times the… - [CW or pulsed: the criterion that decides the topload](https://commongnd.org/articles/sstc/is-it-cw-or-not): The argument goes round forever because both sides are arguing about the circuit. It is not a property of the circuit, it is a question of… - [Choosing a gate resistor: the gate loop is an RLC](https://commongnd.org/articles/sstc/the-gate-loop-is-an-rlc): A gate resistor cannot be chosen apart from the transformer feeding it. Lower it while the leakage stays and what you get is ringing, not… - [What a gate transformer is for](https://commongnd.org/articles/sstc/what-a-gate-transformer-is-for): The top devices have their emitters on a node that swings by hundreds of volts. You cannot wire logic to that, so the signal goes across… - [Dead time and the freewheel diodes](https://commongnd.org/articles/sstc/dead-time-and-the-freewheel-diodes): Adding dead time to be safe makes it worse, in a loop: longer pause, more diode current, worse turn-off, harder hit at the next turn-on. - [What the bridge does](https://commongnd.org/articles/sstc/what-the-bridge-does): Four transistors arranged so they can connect the primary across the supply one way round and then the other. Everything else exists to… - [An SSTC primary is a different part](https://commongnd.org/articles/sstc/an-sstc-primary-is-a-different-part): Most of what you have read about primaries does not apply here. This one is not tuned at all: it is a transformer winding trying to be a… - [The second mode, and flashovers in the middle of a winding](https://commongnd.org/articles/sstc/the-second-mode): A failure that looks like too much coupling and is not. It also explains why a two inch secondary works in a DRSSTC and burns in a bare… ### DRSSTC — Dual resonant solid state Tesla coil (27) https://commongnd.org/articles/drsstc — Primary and secondary both tuned, driven in bursts. The current is enormous and brief, and almost every design decision is about where the switching happens inside the cycle. - [What a ground strike does](https://commongnd.org/articles/drsstc/what-a-ground-strike-does): One event, five consequences, and the mild strike is the one that moves the machine to the wrong mode. - [Coupling: what it buys, and why it is not length](https://commongnd.org/articles/drsstc/what-coupling-actually-buys): Wind the primary tighter and the arc gets longer. It is the first thing anybody tries, and the record holders run the loosest coupling on… - [The bridge: half or full, layout, and what kills it](https://commongnd.org/articles/drsstc/the-bridge-and-what-kills-it): Peak temperature is not the mechanism. The size of the swing tears bond wires over thousands of cycles, a long way below anything on the… - [Sizing a tank capacitor by RMS current](https://commongnd.org/articles/drsstc/an-mmc-is-chosen-by-current): Voltage and pulse rating usually pass with room to spare. What kills a bank is RMS current, and smaller capacitors carry more of it per… - [Primary design: impedance, turns and detuning](https://commongnd.org/articles/drsstc/the-primary-is-an-impedance): Nobody sets the primary current. It comes out of a division, and the number you actually choose is the impedance of the tank. - [The UD boards: which revision you are holding](https://commongnd.org/articles/drsstc/the-ud-boards): The logic core did not change from 2.1 to 2.7, so modifications transfer between revisions. The part designators do not, and that is what… - [Overcurrent protection: OCD, pulse skip and why it fails](https://commongnd.org/articles/drsstc/overcurrent-is-not-a-setting): Nothing limits the primary current in a doubly-resonant coil. Test the actual threshold rather than the position of the knob, and start it… - [The interrupter: bangs per second, ontime and duty](https://commongnd.org/articles/drsstc/the-interrupter-is-the-note): Bangs per second is the frequency of the note in hertz. There is no best rate, only a note, and two settings on this box kill transistors. - [The two poles, and which one the driver lands on](https://commongnd.org/articles/drsstc/there-is-no-secondary-frequency): Couple a primary to a secondary and the resonance splits in two. The frequency you carefully measured is a mode that no longer exists. - [IGBT or MOSFET, and why SiC is not an upgrade here](https://commongnd.org/articles/drsstc/igbt-or-mosfet): One choice that sets both the frequency you can run and how long the bridge lives. And SiC is not the automatic upgrade it looks like. - [Dead time as a fraction of the period](https://commongnd.org/articles/drsstc/dead-time-is-a-fraction): A setting that is sensible at 150 kHz eats a quarter of the period at 365. The same nanoseconds are a completely different setting. - [How an arc actually grows: steps, branching, shape](https://commongnd.org/articles/drsstc/an-arc-grows-as-fast-as-charge-arrives): A spark propagates at metres per microsecond. A bang lasts three hundred of them. You get a metre and a half, not seven hundred, and that… - [No sparks, or short ones: what to check and in what order](https://commongnd.org/articles/drsstc/no-sparks-on-breakout): The order to look in, and the order matters: the cheap checks are also the ones that are usually wrong, and none of the three classic… - [Paralleling bridges: transformers or a split tank](https://commongnd.org/articles/drsstc/eight-bridges-in-series): You cannot wire the outputs together. Something has to force them to share, and the two mechanisms that do are not equally safe. - [Tuning by the shape of the current envelope](https://commongnd.org/articles/drsstc/read-the-envelope): The shape of the primary current across one bang tells you whether the coil is tuned. One capture, one answer, and almost nobody uses it. - [Choosing tank impedance: current is expensive](https://commongnd.org/articles/drsstc/current-is-expensive-voltage-is-not): Everything that grows with current grows as its square. What high voltage on the tank costs is a rating you fix with series links for… - [ZCS against ZVS: why switching at zero fails](https://commongnd.org/articles/drsstc/switching-at-zero-is-the-failure-case): Turning off exactly at the current zero reads like the ideal and is the opposite. The residual current is what commutates the node, and… - [Snubber capacitance, and the bet it makes on ZVS](https://commongnd.org/articles/drsstc/a-snubber-is-a-bet-on-zvs): Win it and the energy returns to the tank for nothing. Lose it and half CV squared goes into the die every transition, which doubles the… - [What the primary is for](https://commongnd.org/articles/drsstc/what-the-primary-is-for): A few turns of heavy copper that are not connected to the secondary anywhere, and are not trying to be. - [What the driver does](https://commongnd.org/articles/drsstc/what-the-driver-does): Four jobs on one board. Without it the power section is a box of parts, and the frequency it has to find is not a number you can set once. - [What a breakout point is for](https://commongnd.org/articles/drsstc/what-a-breakout-point-is-for): It looks like an afterthought and it is one of the few parts you can actually aim the discharge with. Short beats long, and it is a… - [What a current transformer is](https://commongnd.org/articles/drsstc/what-a-current-transformer-is): A ferrite ring on one of the thick conductors. It is how the driver finds out what the tank is doing, and it is the only sense the machine… - [Choosing a phase lead inductor](https://commongnd.org/articles/drsstc/choosing-a-phase-lead-inductor): A thin paper trail, three worries attached to it that turn out to be unfounded, and one failure that keeps happening. It is mechanical. - [The fibre link, and the fail-safe rule](https://commongnd.org/articles/drsstc/the-fibre-and-the-fail-safe-rule): A broken fibre must turn the coil off. The receiver does not guarantee that by itself: with no light its output is undefined. - [What will actually kill you](https://commongnd.org/articles/drsstc/what-will-actually-kill-you): Not the arc. The things on this page are at DC or mains frequency, they are inside the cabinet, and they do not announce themselves. - [Scoping a coil without killing anything](https://commongnd.org/articles/drsstc/scoping-a-coil-without-killing-it): More equipment dies at the bench than in the arc. Never float the scope: it stays earthed and the isolation goes on the circuit under test. - [Phase lead: the delay chain and the ZVS floor](https://commongnd.org/articles/drsstc/what-a-phase-lead-actually-buys): A driver that fires on the current zero is already late. The lead is how you pay that back, and it is made of two things that are nothing… ### QCW — Quasi-continuous wave, a Tesla coil drive mode; the pieces filed here are written for a ramped DRSSTC (18) https://commongnd.org/articles/qcw — A ramped bus instead of a flat burst, on a DRSSTC or on a plain SSTC. The arc grows smoothly and straight, and every figure in the design moves while it does. - [The frequency slides, and four things follow it](https://commongnd.org/articles/qcw/the-frequency-slides-down-the-ramp): A DRSSTC has a frequency. A QCW has a different one at every instant of the bang, and the four things timed against it do not move in the… - [The buck modulator, and why a whole converter](https://commongnd.org/articles/qcw/the-buck-is-the-ramp): Everything the department is named for happens inside one converter. Pulse skipping has been tried three times in public and nobody got a… - [The bus bank: sizing the reservoir](https://commongnd.org/articles/qcw/the-bus-is-a-reservoir): A sagging bus eats the top of the ramp and looks exactly like a tuning problem. It is not one, and the reason to fit a doubler is not the… - [The rectifier: which arrangement, and how to pick the diodes](https://commongnd.org/articles/qcw/the-rectifier-and-its-diodes): Diodes conduct in bursts on the tops of the sine wave, so the average forward current is the one number they never see. And a doubler's two… - [Why the sword bends: the community's open question](https://commongnd.org/articles/qcw/why-the-sword-bends): Nobody knows. The straightness of a QCW arc depends on the room more than on the coil, and not one of the factors has ever been isolated. - [Buck choke, switching frequency and the gate supply](https://commongnd.org/articles/qcw/the-choke-is-the-expensive-part): Frequency looks like the cheapest way to a smooth ramp. Its second price is the gate supply, and at 30 kHz a two watt module is already… - [The shape of the ramp, and what it should be](https://commongnd.org/articles/qcw/the-shape-of-the-ramp): The most underrated setting on a QCW. Tune for linearity in voltage and the square in power happens by itself; try to build a linear rise… - [Gate supply for long bangs: amps and millifarads](https://commongnd.org/articles/qcw/gate-supply-is-a-power-supply): On a DRSSTC it is a detail. On a QCW it is amps of average current and millifarads of capacitance, and a factor of four here is a dead… - [Grounding a QCW: three grounds and a shorted turn](https://commongnd.org/articles/qcw/three-grounds-and-a-shorted-turn): The subject that kills more QCW drivers than any other, and almost all of it is about not connecting two things that look like they should… - [Converting a DRSSTC driver to run a ramp](https://commongnd.org/articles/qcw/converting-a-drsstc-driver): What actually has to be touched and why, so that nobody has to work it out by removing components until something changes. - [A high bus with a low bridge, and where it breaks](https://commongnd.org/articles/qcw/a-high-bus-and-a-low-bridge): The bridge is fed from the buck, not the bank, so it never sees the bus. That opens a trick, and the place it breaks has to be read before… - [What more power buys: the exponent is a third](https://commongnd.org/articles/qcw/what-more-power-actually-buys): The question before every upgrade, and the linear guess overshoots by a factor of three. Four sources, none of them above an exponent of… - [The ramp exponent: the curve is a hump](https://commongnd.org/articles/qcw/the-shape-slider-breaks-at-both-ends): It is not one axis with a good end and a bad end. It is a hump, and both ends branch for opposite reasons. - [Why a QCW adds capacitance instead of removing it](https://commongnd.org/articles/qcw/a-qcw-adds-capacitance): Every other coil treats topload capacitance as a cost. This one goes the other way, and putting a bigger toroid on is a three-dimensional… - [What a QCW arc is, and why it is straight](https://commongnd.org/articles/qcw/what-a-qcw-arc-is): A hundred and twenty five times slower than a DRSSTC's, per cycle. The shape is not a different geometry, it is a different rate of charge… - [What RF ground is, and what it is not](https://commongnd.org/articles/qcw/what-rf-ground-is): The bottom of the secondary has to go somewhere, and it is not the wall socket. The ground is one side of the circuit, not a safety… - [The driver on a QCW](https://commongnd.org/articles/qcw/the-driver-on-a-qcw): The same four jobs as on any coil. Each of them changes here, and one of them turns into something else entirely. - [The topload on a QCW](https://commongnd.org/articles/qcw/the-topload-on-a-qcw): It does three jobs, and here the second one matters so much more than the others that the department ends up doing the opposite of everyone…