A coat of paint for the “Power Tank”

Fittingly the paint colour is Valspar “Thunderbolt” left over from painting the front door at home. Still waiting on some more industrial looking socket outlets to replace the chrome ones.

I’ve been warned that after going to Powerpole connectors for 12V DC distribution, I’ll never want to touch a cigarette lighter plug again. That sounds about right.

Off-grid power bank

I’ve been renting a 20ft container to use as storage and workshop space, and wanted some electrical power.

The prototype was not very elegant. πŸ˜€ The basic components are a 135W solar panel on the roof, a 12V 100Ah AGM leisure battery, a Victron MPPT charge controller, and a 1kW pure sine wave inverter.

The inverter is a reasonably priced “Mercury” branded unit from TLC Electrical. I believe the Chinese OEM is Ningbo Kosun. I may do a more detailed review in another post.

Lead-acid batteries aren’t great for geek points, but I thought it would be the best technology for the job. I only visit about once or twice per week and rarely drain the battery completely. It can also get very hot in there in summer, so the battery figure of merit I’m interested in is basically calendar life when fully charged at high temperature. Lead-acid offers a lot of that for the money.

The first problem I found is that the Victron’s load output is wimpy. It won’t even run a 12V compressor. However the Victron has a good algorithm for turning off the load when the battery gets low to avoid damaging depth of discharge, and I wanted to make use of that.

So my first addition was a large MOSFET solid-state relay that would allow the Victron load output to control a much heavier load at 12V. I made this myself using 3x IRFP7430 MOSFETs in parallel, driven by a Si8712 isolated driver chip. The MOSFETs are rated at 40V, 195A, 1mOhm Rds(on). Turn-on and off seems to be rather slow. The small package on the right is a flyback diode to avoid a large transient when turning off an inductive load such as a motor. The MOSFETs would probably be ok with this, but it could damage other loads on the 12V bus.

The solid-state relay does not switch the supply to the inverter. This is connected directly to the battery to minimise voltage drop.

I modified the inverter to have a remote on/off switch, which was very easy as the internal on/off switch just connects 12V from the positive input terminal to the control circuitry. I simply hacked it so the control circuitry got its 12V feed from the Victron’s load output via a front panel toggle switch. This allows the Victron’s low battery algorithm to turn off the inverter alongside the other loads, and allows me to bury the inverter inside the enclosure with no access worries…

Note that I couldn’t find the official remote control panel for the “Mercury” inverter. I bought what I thought was the correct one on eBay, and on plugging it in, there was smoke… Luckily it all came from the remote and the inverter still appeared to be in good shape.

I also got this 500 amp battery monitor in Renogy’s Black Friday sale. 500A is overkill but it was cheaper than the lower current versions. The accuracy at low currents turned out to be fine.

I wanted a visual indication that the inverter was on, and also some geeky statistics relating to the AC output. This power meter module from Amazon did the trick with its cheery blue backlight. It is powered from the AC supply, and this increases the 12V power draw of the inverter by about 1 watt.

A British Army F632 ammo box was procured to fit all the parts in and… they didn’t fit. To gain some extra space, I added a wooden frame to the front made from CLS studs. I am quite proud of the Stratocaster jack cups as PV input connectors.

I provided 3 DC outputs: two Powerpole connectors protected by 32A circuit breakers and a cigarette lighter with a 10A breaker. Household AC circuit breakers do seem to work on 12V DC. I’ve seen them used up to 48V DC.

The inverter feeds two British standard socket outlets via RCBOs (earth leakage trips). In order for these to function the inverter’s neutral is connected to earth. Battery negative is also connected to earth and in use the earth stud is connected to the container. I believe this is the best overall solution from an electrical safety point of view.

The RCBOs do consume some AC power to perform their earth leakage monitoring function, but the amount is tiny, and doesn’t increase the inverter standby consumption noticeably. I tried Axiom and British General brands, and the BG ones had the lowest consumption.

These also have a 6A overcurrent trip function (the O in RCBO) but the inverter’s own current limiting would probably always operate before they tripped.

The inverter was mounted on a plywood divider and some holes punched in the sides of the box to promote airflow. I also removed the AC socket outlet from the inverter, leaving a large square hole for extra airflow, cut out the fan grill from the inverter casing, and reversed the fan. Hopefully these changes will make up for jamming it inside a relatively small compartment.

To be honest I have no idea if reversing the fan helped the inverter’s own cooling. Stock behaviour is to suck hot air out of the enclosure, and the old rule of thumb is that it’s always better to blow in cooling systems, but the inverter seemed like a quite well balanced and optimised design otherwise, so I can’t believe the designers left much on the table with sub-optimal cooling.

Really it was for my own convenience at a system level, the fan is at the 12V input end of the inverter and I wanted the hot air exhaust at the opposite end to the battery terminals, so I could have shortest possible battery wiring and a neat “signal path” from DC to AC without the inverter’s hot air blowing onto the Victron charge controller.

You might notice the lack of fuses on the battery positive terminal. Every connection to the battery is ultimately protected by a fuse or circuit breaker, but not directly at the terminal.

In use, you can see the inputs from the PV panel and the earth wire leading to an earth clamp on the container. (A steel framed building as far as BS7671 is concerned I guess.) The 12V and 240V wiring are not done yet.

The power bank itself could also do with a coat of paint and some more industrial looking unswitched AC outlets. The chrome ones are a bit flashy. Weighing in at 45kg (perhaps Power Tank would be a more appropriate name?) it also needs some serious handles to assist in moving it around…

I love the way the meter backlights come on when the inverter switch is flipped.

Appliances the inverter has run successfully:

  • Toaster, obviously
  • 1kW electric heater
  • 400W “Eco Henry” vacuum cleaner
  • Clarke CDP102 pillar drill
  • Clarke 6″ bench grinder
  • 1200W travel hair dryer (output sags to 220V, hair dryer only produces 1kW)
  • Ikea TILLREDA induction hotplate (only up to 40% power…)
  • A selection of guitar amps (works surprisingly well with no noise issues)

Tesla Space Gun 2000

For this year’s Gaussfest I decided to make a dub siren and connect it to Odin.

A dub siren is basically a very simple analog synth used to make sound effects for dub reggae. The original ones were a simple circuit with two 555 timers, but there are all sorts of variations on the theme. I was especially impressed by the Rigsmith GS1, which seems to contain some sort of toy sound effect IC.

I’m sure I had something similar mounted on the handlebars of my Raleigh Chopper in the 80s. How hard could it be to build one?

After some Googling and searching eBay, I found a surplus dealer selling some promising looking chips: the HK620 and HK623.

To make my dub siren I copied the data sheet application circuits almost exactly. The only change I made was to replace the timing resistor (“Rosc” in the datasheet) with a 1M pot in series with a 47k fixed resistor. I also added a 3.3 volt regulator so it could run off the standard 9V guitar pedal supply.

Buttons… So many buttons…
And a Hammond diecast box and some other bits and pieces

It sounds identical to the Rigsmith! Have they been shopping at Budgetronics too? πŸ˜€

Testing with a delay pedal and guitar amp

The Futurama FU-3 (part 4)

In this part the FU-3 gets its final faceplate…

I started with a 3mm piece of aluminium pre-cut to size by Metal Supermarkets. The existing screw holes were easily replicated with PEM nuts, but the old faceplate vibrated horribly, so I wanted to add 2 more mounting bolts, and oh dear, the drilling for the top one just missed the panel.

This was a perfect excuse to zap something with the TIG welder.

Zap and tap

With this done, a coat of gold paint and a pasting with letter punches…

Gaussfest 2021

This bonkers event was organised by Extreme Electronics at Papplewick Pumping Station in Nottinghamshire. One of the few venues where Odin‘s full power can safely be deployed…

Check #Gaussfest on Twitter for more πŸ™‚

Odin set up in the boiler house. All steam powered, no EMC worries πŸ™‚
The control position next to the boilers
Tapping into 3 phase 415V supply
Can you identify the tune being covered?
More importantly, can YouTube’s content ID algorithm?

Look out for Mike “Electricstuff” with fingers in his ears

Boiler house switchboard… Yes my setup moved the meters πŸ™‚
Mmmm, asbestos
This generator was a bit too small to power Odin πŸ™
This one was too big
My performance did actually use about 10kW
An event like this would not be possible without defibrillators and a tea room.

PFC part 11: 3 phase test

Finally the long awaited test happened πŸ™‚ (you can read all posts about the PFC here)

We plugged it into the 3 phase outlet and it started up normally! πŸ˜€

When smashed with 750V the Tesla cabin heater would draw an impressive amount of power while warming up. Unfortunately the steady state power draw was only about 2kW, probably something to do with the rather weak fan cooling it.

The PFC line current waveforms at (roughly) this 11kW output power level. No surprises here, they look exactly like the theoretical ones for this circuit. (Except strictly speaking the red one is upside down πŸ™‚ ) The theoretical power factor for this waveform is 0.95.

We don’t have a 3 phase power analyser in the lab, so I used 2 single phase ones on the input, according to the old “2 wattmeter method“. To be honest this didn’t work very well, as the power drawn by the PTC heater was always changing, and it was impossible to make sure the 2 meter readings corresponded to exactly the same time. Also, the PF reading is rubbish due to the inherent 30 degree phase shift: to get the actual PF you have to plug the wattage readings into a complicated formula.

When the heater reached steady state, I measured an input power of 2000W, an output of 1900W, and a power factor of 0.96. From an academic point of view it would have been nice to measure the efficiency at higher powers, I expect that 100W is mostly switching losses and the efficiency will increase with heavier loads.

The main goal was to get confidence that the PFC would work at its first gig, and this has been achieved πŸ™‚

PFC part 10.5: Don’t contact me, I’ll contact you

Another problem I have with the cabin heater testing (And the PFC testing πŸ™‚ ) is that the PFC is not designed to start up into a load. This isn’t a problem for the intended application, as the Tesla coil won’t draw any current until it’s commanded to. But to power a resistive load, I have to use a switch to connect the load after the PFC has started up.

For the previous immersion heater tests I used an ordinary 240V AC rated switch. This will make a DC circuit, but won’t break it: the arc simply won’t go out until the entire switch is incinerated. Of course if you’re reading this there’s a fair chance that you actually enjoy burning things to a crisp with electric arcs, but I have to be on my best behaviour to get the keys to the 3 phase outlet at work, and proper DC rated contactors are getting cheaper anyway thanks to the proliferation of electric vehicles.

The Kilovac LEV200 has hydrogen filling and a permanent magnet to blow out the arc. It’s good for several hundred amps at 900V DC.

As one might expect, it makes a satisfying clunk.

Tesla Model 3 heater testing

I thought I’d better test the characteristics of the PTC elements in a scientific manner as opposed to just applying 750V DC from the PFC and seeing what happens.

When measured with a multimeter at room temperature, each element read about 600 ohms. This would imply a rather low power output if that was the minimum resistance.

For my next experiment I connected one of the elements to my old Xantrex 600V bench power supply. The 600 ohm cold resistance would imply a draw of no more than 1A at 600V if the element was purely PTC. But to my surprise, the current draw actually began to increase as the element got hotter, eventually hitting the PSU’s 1.7A current limit at only 200V.

This means that the elements must actually start off as NTC, and transition to PTC at a higher temperature. That kind of makes sense, as a car heater matrix has to be able to start up from very low temperatures. Purely PTC elements would presumably exhibit an immense current surge when the heater is turned on after leaving the car parked overnight in Canada. πŸ™‚

I was wondering why Tesla bothered to implement individual control of the 6 elements, and I guess the turn-on surge is the answer to this too: by turning them on sequentially the surge can be made 6x smaller. It’s not like a Tesla traction battery (or even an IGBT) would care if the turn-on surge was 10A or 60, but maybe it allows them to use a lower rated fuse to connect the heater to the battery. Fuses rated for high voltage DC are expensive so the savings made here might outweigh the cost of 5 IGBTs and drivers.

Anyway, I couldn’t test above 200V due to the limited output current. The next larger PSU I have is the PFC, and it has a minimum output voltage of 400V (600 on 3 phase) so I will just have to “send it” as the kids say nowadays.

Tesla Model 3 cabin heater teardown

I needed a high power dummy load that was a bit more health and safety friendly than my bucket of immersion heaters. I investigated lots of possibilities until I eventually found a Tesla Model 3 heater matrix on eBay. The Internet said it could draw up to 6kW, and being PTC, it shouldn’t catch fire if I forget to turn the fan on, which would look great on a risk assessment. So I went for it. πŸ™‚

The lid is held on by penta-lobe screws with a tamper-proof peg in the middle. There was nothing in my collection of tamper-proof bits that would fit, but a sturdy flat-bladed screwdriver worked quite well after Dremeling a new slot or just jamming it in hard enough to break off the peg. πŸ˜€

Ooh, fancy! What does all this stuff even do?

There appear to be 6 separate PTC elements, each with low side switching by a 600V IGBT and non-isolated gate driver.

To the left of the IGBTs is a voltage divider and low side current shunt, and on the right an isolated CAN interface and DC-DC converter. Handling CAN communications and A/D conversion of the voltage and current signals, we have an 8 bit ST microcontroller (probably sharing its 0V rail with DC bus negative)

Of course I immediately set to work reverse engineering the CAN protocol so I could command it to connect its elements in 2 groups of 3 for 800V input. Oh wait that’s not gonna work πŸ™ we need a hardware solution…

The heating elements were connected to the PCB by spring clips that released quite easily, but the PCB was stuck firmly to the aluminium enclosure with thermally conductive glue. I freed it with the old embedded programming trick of heating the enclosure with a heat gun and prying with a paint scraper. (sadly this doesn’t work any more in Python 3 πŸ˜‰ )

With the PCB removed we can see the terminals for the heating elements, looks as if some sort of spade terminal should fit nicely. (Also note the little nodule at bottom right which appears to be for cooling the CAN transceiver chip.)

And after a pulsating second half the score is Wago 221, CANbus 0. πŸ˜€

Compensating the PG508

I got it to work and amplify, but the loop gain left a lot to be desired, so I decided to start over on the compensation. I also hooked up an unregulated power supply and a different output stage, partly because I wanted to see how the PSRR was doing, and partly because I wanted to reassemble the Ice Block with its original driver board.

It survived πŸ™‚

Now, you should never anthropomorphise amplifiers, they hate it! I swear that this one “wants” to blast electronic music from the 90s at high volume though. πŸ™‚

Found this in my junk pile, retro or what πŸ™‚

Having given the Ice Block its output stage back, I had to find another one for my experiments. A search of the junk pile yielded the remains of a Maplin 100W MOSFET amp kit. I’d have preferred to try BJTs, but the PG508 prototype was already set up to work with lateral MOSFETs.

Put it together and what have you got?

I tried the time-honoured method of soldering RC networks in random places, or maybe places that seemed to make a difference when touched with a damp finger. πŸ™‚ This improved it somewhat, but it still wasn’t doing a great job of correcting the output stage’s copious (and vintage correct!) crossover distortion.

Cordell to the rescue!

I eventually cracked Bob Cordell’s “Designing Audio Power Amplifiers” and spent an afternoon pondering Chapter 9, “Advanced Forms of Feedback Compensation”. It struck me that the PG508 topology is very similar to figure 9.7, except that the input stage doesn’t exist as such: the feedback node is the VAS input.

It also struck me that I’d already ended up with RC networks in the places shown in fig. 9.7 by trial and error, just with completely different values. R4 and C2 were in the original Tektronix PG508, and R5 C3, R3 C1 were my additions. So the obvious course of action was to change them to the values suggested by Cordell and see what happened.

Initial results weren’t great: it oscillated at 20MHz, but this was squelched by reducing R4 to 51 ohms. Having done this, performance was excellent: the 16pF C1 gave the extra loop gain I was looking for. I’d started out with 100pF here as that’s the value used in a Douglas Self Blameless amp. The Blameless input stage typically has 5-10x the gm of the PG508’s non-existent IPS, though. So funnily enough C1 needs to be 5-10x smaller to get comparable loop gain.

With these modifications the measured performance was 0.03% THD at full power at 10kHz, and 0.00something at 1kHz. The 10kHz figure seems high, but it’s now in the ballpark for a well functioning driver doing its best with a vintage MOSFET output stage. (Cordell’s AES paper quoted 0.02% at 10kHz with the Hawksford error correction turned off.)

Note that this THD figure is no better than I got with the old compensation and the Ice Block output stage. This just means that the Ice Block output stage must have about 3-5x less distortion than the single 2SK135/2SJ50 pair used here.

Slew rate was also improved, and stability with a capacitive load was just about acceptable: with 0.1uF slapped on the output it showed a few cycles of damped ringing but didn’t oscillate.

The circuit at the end of a hard day of soldering capacitors at random (and trying to find a LM317 or 7912 : ) )

I also took the opportunity to test out the opamp front end inspired by the Quad 405 and Cordell Super Gain Clone. I used an OPA2604 as it was the best opamp I had around. This works very nicely: it reduces the DC offset to 2mV, undoes the phase inversion inherent in the PG508 circuit, and increases the overall gain from 10 to 50.

Note that the opamp must be a FET input type because of the high impedance of the DC feedback path. Also, as the circuit has 2 LF time constants (the 1M/1u and the 47k/2u) with feedback around them, it functions as a 2nd order active high pass filter. It rolls off at 12dB/octave and can resonate if the time constants are too close together.

I basically copied this part from the Quad 405, so it must have done the same thing. I guess it was desirable to have a good rumble filter here in the days of vinyl. Arguably it still is in the era of small vented speaker cabinets and dubstep. πŸ™‚