I was looking for an aluminium welding project that wouldn’t kill anyone if it failed. And also some speaker stands that wouldn’t take up any desk space.
I started with a 1m length of 3″ x 3″ x 1/8″ box section.
After a whole afternoon of hacksawing, jigsawing and filing it was reduced to 2 columns and some wedge shaped pieces.
The pieces were then welded together into a giant C clamp shape and a 3mm plate was welded to the top to support the speaker. I decided to only tack weld the plate because I was worried the heat of a full weld would warp it.
I find the fillet weld the most difficult. This doesn’t exactly look great but it’s my best yet.
The stand base screws to the underside of the desk with some hefty wood screws.
The desk is completely clear and there is plenty of room underneath for oscilloscopes, soldering irons and so on.
I got fed up with the thumb controls on the torch that came with my welder and decided to try a foot pedal. They are extremely expensive to buy so I decided to make one out of an unwanted wah pedal.
The main difference between a wah pedal and a TIG foot pedal is that the switch engages as you first apply pressure to the pedal, telling the welder to open the gas valve and go through its preflow and ignition cycle. Depressing the pedal further then controls the welding current.
To allow this operating mode I threw away the existing stomp switch and replaced it with a microswitch operated by a cam.
The other difference is that it needs a rather strong return spring to avoid igniting the welder by accident. I found this out by trial and error. D: Keeping with the musical instrument theme, I used some Strat tremolo springs from eBay.
I connected it to the socket for the hand controller as a spare plug for this was supplied with the welder. It thinks it is a torch with 1 button and thumbwheel. This seems to work fine, but I will try the foot pedal socket if I ever get round to figuring out the mating plug for it.
It was time to put it all together! (This actually happened in March- these are post hoc posts 🙂 )
First Odin’s control electronics had to be converted to run off 24V DC instead of the original 240V AC. (And mounted in a Eurorack while I was at it…) This wasn’t too difficult as they already used 20-something volts DC internally, derived from the mains with a traditional iron cored transformer and rectifier, and regulated to 15V.
I added DC input sockets to the driver and gate drive amplifier modules, and changed the fan for a 24V one too. The original 240V AC inputs are retained in case the PFC breaks down and I need to change back to the old power supply.
The PFC will be situated at the operator’s position with long cables for 750V and 24V running to the coil. This made everything simpler, as there was no need for remote control and the circuit breaker on the PFC could be the emergency shutoff for the whole system. But it did leave the 24V cable vulnerable to strikes and general pickup of the extreme levels of EMI around a Tesla coil. My solution was to make a DC input module using a surplus Traco 40W DC-DC converter to give galvanic isolation, and lots of EMI filtering on both input and output.
The red module is the receiver for my Teslink system that sends multiplexed control signals over a Toslink optical fibre. I finally got round to completing it (and making a Eurorack mounting transmitter too)
The idea is that the PFC accepts single or 3 phase power at anywhere between 208 and 415V, and supplies 24V DC to the Tesla coil electronics from its own control power supply. I didn’t want the hassle of having to change taps on control power transformers, or rather the carnage of connecting it to 415V with the taps set to 240. (I have done this before- it was messy)
The Tesla coil primary was set up using a water-filled steel pan as a dummy load.
It didn’t explode! 😀
The next step would have been to take the PFC and immersion heater bucket to a lab with 3 phase 415V power. Unfortunately this was made impossible by the COVID-19 lockdown. The debut was to have been the Nottingham Gaussfest, but this was also cancelled. Insert corona joke here 🙁
Before I could get on with building the PFC into an enclosure, I had one last design decision to make: What sort of EMI filtering to use. The size and shape of the EMI filters would affect the rest of the mechanical design. Ok, that’s management speak for “How am I going to get all this cr@p into the 3U rack enclosure I’ve already purchased?”
Now, I deal with EMC in my day job and am vaguely familiar with the standards and test procedures, but this is a one-off handmade power supply for a Tesla coil. It’s never going to get tested for emissions, and the emissions from the Tesla coil will dwarf the contribution from the power supply anyway.
So the main purpose of the EMI filters is to protect the PFC from malfunction or damage caused by the Tesla coil emissions. These tend to be common mode transients caused by ground strikes, containing frequencies up to the 10s of MHz. There isn’t a great deal of VHF or UHF energy due to the length of the spark channel. So they really aren’t super hard to filter out.
I prefer to connect the filters so the Y capacitors (jargon term for the capacitance between lines and ground) are at the end connected to the outside world. My reasoning is that I’d rather any incoming transients were dumped to chassis ground through the capacitors, than potentially flashing over a choke.
I started by trawling the RS, Farnell and Mouser catalogues for ready-made EMI filters. I ended up with a Delta 30TDVST2 for the input and a Schaffner FN2200-25-33 for the output. These both had the Y capacitors at the load end, so would have to be used backwards from the maker’s recommendation.
I soon discovered a serious problem with both filters: a very high Y capacitance. This isn’t a problem in the intended industrial application, but a bit of a show-stopper for mine. When the PFC is used on a single phase supply, the high capacitance causes enough earth leakage current to trip any RCD. Note that the Y capacitance of the DC output filter also contributes to the leakage, because the DC output is not isolated from the mains and has AC superimposed on it.
I couldn’t find any better filters, so I broke them open and set about reducing the Y capacitance.
I lifted the connection between Y capacitors and earth, and added a 68nF capacitor in series, with a 2.2M discharge resistor. This should give a total leakage current budget of around 10mA at 240V AC. (Odin has 2 68nF capacitors from DC bus to ground already, which contribute too)
The modified filters no longer tripped my house RCDs, so the job was done. In hindsight, I wouldn’t buy ready-made filters again. It would have been cheaper to buy the parts and make them.
The PFC engine is working, but there are a few other things needed to make it usable. (Operationalise it? Or Heaven forbid, weaponise it? 🙂 )
Auxiliary power supply: The PFC needs a small amount of power to run its own control electronics. I decided to use a Meanwell WDR-120-24 switching power supply to provide 24V DC. This is an industrial grade unit that will accept any input voltage from 200 to 500V AC.
The WDR-120-24 is a bit more expensive than the usual 85-265V input range units, but vital for my goal of being able to run the PFC off either 230V single phase or 400V 3 phase power, without any kind of voltage selector switch that could cause carnage if set wrongly.
Precharge: The bus capacitance of the DRSSTC is very substantial. Odin has 4700uF after a recent upgrade. The PFC itself will also need another 1000uF to allow it to work without the DRSSTC connected. All of this has to be charged to the peak value of the mains voltage before the PFC can even start, in an orderly manner without tripping any breakers.
I chose a capacitive ballast for this job, consisting of 22uF motor run capacitors with 10 ohm resistors in series. The capacitors do most of the current limiting while the resistors protect the capacitors and main contactor from the surge when the capacitors are shorted out. The resistors are attached to the main heatsink and protected by the overtemperature cutout.
The precharge controller is based around a time delay and voltage sensing relay. (Schematic in a future post) The voltage between D1 and D2 must get over 200V, and the voltage between D2 and U2 below about 20V, before the sensing relay will pull in. This energises the main contactor, connecting the PFC input rectifier directly to the mains, and powering up the PFC controller through its auxiliary contact. The PFC then goes through its own soft start procedure, charging the DC bus capacitance to full voltage.
Dump load: The large DC bus capacitance also needs discharged when the system is powered down. My previous coils all relied on bleed resistors and took over a minute to discharge. For this build I decided to try some PTC thermistors from Epcos. (Details in a future post.)
The main advantage of PTCs is that, unlike normal resistors, they limit their own temperature and won’t catch fire or explode if the switch controlling them accidentally turns on while the DC bus is powered. This allows me to switch them with a SCR which was already present in the bypass diode module.
EMI filtering: This is as much to protect the PFC from damage by the huge transients generated by the Tesla coil, as to protect the mains from the hash thrown out by the PFC. My search for suitable off-the-shelf EMI filters is documented in another post.
The low power test was a success, so a larger dummy load was obtained- 3x 240V, 3kW water heaters connected in series.
The PFC was placed on a special tea tray test fixture and temporarily wired into the electric cooker supply in the kitchen.
According to my smart meter the result was 8.1kW input, though the other house appliances were using a few hundred watts at this point. The line current waveform looked nice. I used an EMI filter this time so the garbage at the switching frequency is reduced.
In the previous post I mentioned that I couldn’t drive the MOSFETs directly from the controller chip because they need a negative voltage to turn off. The easiest way to provide this was with an isolated gate driver using the excellent Silicon Labs Si8271 IC.
Due to the tiny size of this IC and the requirement for a very tight layout to minimise stray inductance, I designed a PCB for it. This was also an excuse to finally start learning Kicad after years of using Eagle.
Power is supplied by a Murata MGJ6 DC-DC converter connected to J2. +15V to pin 1 and -5V to pin 2. The same DC-DC converter powers the opamp on the voltage feedback isolator board.
R7 is a small value like 22 ohms. I installed a 10k resistor in place of C6 to pull down the input if it became disconnected. R2 was not fitted and R1 was a 0 ohm link, the opposite of the schematic (I got the enable pin logic the wrong way round)
I uploaded the Gerber files to JLC– a Chinese manufacturer based in Shenzhen who ship worldwide- and had the boards the following week for a cost of about £5. JLC are 1/10 the cost of any UK PCB house and have really lowered the barrier to entry for hobbyists.