CONNERLABS

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  • PFC Part 1: Background

    I decided it was time to build a new power supply for Odin with power factor correction. (See all posts on the PFC)

    Power factor what? To explain, we have to go back to the turn of the last century and the War of the Currents. Alternating current won so all of our mains supplies are AC. However, solid-state Tesla coils, like most power electronics, run off DC. Odin needs several kilowatts of power at about 750V DC.

    The simplest way to achieve this is to use a voltage doubling rectifier on the 240V AC mains. I used this in my previous large solid-state Tesla coil, the OLTC2, with one small refinement. I used SCRs instead of diodes to allow soft starting and varying the output voltage.

    The major drawback of this circuit is a very poor power factor, especially at reduced output voltage. It is particularly bad in a DRSSTC, as it has a huge capacitor on the DC bus for energy storage, and the rectifier charges this directly. I improved the power factor somewhat by connecting a large iron cored inductor in series with the incoming mains. This worked well enough, and you can see details of the old power supply here here and here.

    Gate pulses, line voltage and line current waveforms from the old PSU running at low output voltage.

    I used this same power supply topology in Mjollnir, Little Cook and Odin. There was no room inside the smaller coils for the passive PFC inductor, so I used a much smaller one, making the power factor even worse. The shortcomings really became apparent in a few shows I did with Odin. It performed really badly when running off a generator, and there were two venues with 3 phase supply that I couldn’t use.

    After much thought I decided to go for the simplest possible solution: a boost PFC. These are very common in larger consumer electronics, and controller ICs and design information are readily available. It is also easy to modify for 3 phase input. Essentially you just replace the rectifier with a 3 phase one. The line currents aren’t nice sine waves any more, but the power factor is still greatly improved over a plain rectifier with the same filter capacitance.

    Probably the biggest drawback for Tesla coil use is that the output voltage can only be greater than the peak value of the rectified input voltage. You are limited to a minimum of about 400V when running off 240V single phase, and 600V from a 415V 3 phase supply. This is a problem for me at least because I like to check the tuning at reduced voltage after setting the coil up at a new venue.

    Eventually I persuaded myself that I could live with this and set to work designing a boost PFC.

  • Arc Attack x Tesla Fuzz

    I built the Tesla Fuzz especially for a collaboration between Arc Attack and some talented metal guitarists. And here they are!

    Joe DiPrima and Gabriel Guardian explain how it works

    Alex Campbell of Seek Irony performs some neoclassical shredding that makes the Tesla coil sound almost good.
    Thunderstruck is on every Tesla guitarist’s set list. 😉

  • And we’re back

    Finally got round to fixing the server yet again

  • Funky light fitting 1

    First in a series?

    This was made from oak battens, a suspension kit for a 600x600mm LED panel, and lamp holders, cable and ceiling rose from Creative Cables.

    I wanted the Edison bulb look, but needed it to run off low voltage due to my sketchy wiring. I found some 12V E27 LED bulbs on EBay, but the choice was much more limited than mains voltage ones, and they had to ship from China. The ones I bought are completely unregulated so the brightness varies drastically with voltage. See Big Clive’s teardown of the same bulb.

    I ended up using them in 3 series pairs with a regulated 24V supply from a boost converter hidden in the ceiling rose.

    DIY boost converter made with a LM2576HV

  • Finally painted the transformers

    The Crown SXA has had black power transformers and silver/rust coloured output transformers for like a decade now 😉 The black Hammerite spray paint is great.

  • 12V lighting

    One of the joys of living in an old house is dodgy wiring. Another joy is ornate lath and plaster ceilings that might collapse at any minute. 🙂 My problem is a combination of the two. The overhead lights in most rooms are powered by 1930s vintage rubber and cloth covered wiring that can’t be replaced without trashing the ceilings.

    As a token gesture to electrical safety, I added a RCD to the lighting circuit, and believe it or not it started tripping at random. This left me with a few options:

    Stop using the ceiling lights altogether

    Have them rewired and the ceilings replastered

    Run the lighting circuit off a lower voltage that wouldn’t tax the ancient rubber and cloth insulation.

    A lower voltage would of course limit the power that could be delivered before overloading the wiring. So the second part of the plan would be to replace the light fittings for LEDs to achieve a reasonable light output with less power.

    I decided to go for 12V (in hindsight 24 would have been better- I ended up replacing the power supply with a 15V one as a compromise…)

    First step was to source a reputable brand of power supply festooned with safety approvals. It will be left on 24/7 unattended, and it would be ironic to go to these lengths to avoid the wiring catching fire, only for the PSU to catch fire. 🙂

    I bought a few 600x600mm LED panel lights from TLC Electrical, and chucked the 240V drivers that came with them for some 12V drivers from Ebay.

    They are still plenty bright enough.

    The power supply was mounted next to the consumer unit and connected to the existing 240V lighting circuit with a FCU.

    Oh dear… At this point I realised the thickness of the old wires was mostly insulation and the actual copper cross section was puny. To the point where a 11 amp power supply could potentially overheat them if one of the light fittings shorted out, leaving us no better off in terms of safety than before.

    I connected it up anyway 🙂

    The first of the new light fittings in place

    The other light fittings were modified with bodges like this

    Later I added some fuses to address the issue of the PSU being able to overheat the wiring.

    (footnote: this is still working in 2021 and I made some funky low voltage light fittings 🙂 )

  • The Tesla Fuzz

    An interface for connecting an electric guitar to a Tesla coil.

    Schematic (PDF)

     

  • Extra Coneage

    8″ speakers never really did anything for me, so I decided to wedge a 10″ into the Ninja Corvette.

    It was just possible by carefully chiselling away parts of the cabinet.

    It sounds a lot better than the 8″. Unfortunately I have no idea how to make a good grill for it now!

  • Odin

    My new Tesla coil “Odin” is up and running.



  • Wolfson Pi Audio Card – first impressions

    Ever since the Raspberry Pi came out, I’ve been experimenting with its audio capabilities. The latest audio gizmo available for it is the Wolfson Pi Audio Card, which promises 24 bit, 192kHz recording and playback, with analog and digital I/O, for a very reasonable price. So of course I ordered one straight away. 🙂

    After waiting a month I finally got my hands on it. The software installation is somewhat unclear so I will document what I did here. I didn’t want to use the Wolfson official image as it was a massive 8GB download. I started with a copy of the image that I developed for PiTunes, and applied this patch to it, which adds the Wolfson kernel and the support files for the audio card. I then changed mpd.conf to use audio output device hw0,0 (it was previously 1,0 for the USB audio device) and added a call to SPDIF_playback.sh in my .bash_login file, to set the card up for digital output.

    I also removed the invocation of pikeyd from /etc/rc.local, as the keypad and encoder were not present. They can’t be used anyway, since the Wolfson audio card hogs all of the GPIO pins. It doesn’t really matter, as MPD can always be controlled remotely.

    On firing this up, I was surprised to find that it worked first time! 🙂 I verified the output to be bit perfect at 24 bit, 96kHz. This is possibly the best value for money HD audio source you can get anywhere: you should be able to pick up a Raspberry Pi, a Wolfson Audio Card, a wifi dongle and a hard disk for under £100.