Sep 302018

So this is what I’ve come up with for the core controller.

There’s provisions for two versions on this board, one with an ATTiny861 which does high-speed (250kHz) PWM and can drive a buck, boost or buck-boost DC-DC converter.  It features differential I²C interfaces for the input and output INA219 boards, and LVDS for controlling the MOSFET boards.

The other version is built around the ATTiny24A, and just features the ability to turn on and off MOSFETs.  It can drive two statically, or PWM one (at a much lower speed), with the user supplying the driver logic.  Due to the the fact that this device does not do high-speed switching, I’ve forgone the LVDS control over a simple current loop.  The I²C is still differential though as that could be some distance away and is still somewhat high frequency.

The layout of the board is a small 5×5cm 4-layer PCB.

I had to go 4-layer as I needed to route signals both sides and didn’t want to interrupt the power planes.  The two inner layers are VCC and GND.  There’s de-coupling capacitors galore, although the two power planes will probably function as a decent capacitor in their own right.  ICSP is via the interface header at the bottom.

Sep 302018

I was originally thinking of one monolithic board which would have everything it needed.

There was provision for the lot, including a separate ATTiny24A so that you could omit all but one of the MOSFETs, swap the remaining MOSFET for a P-channel, drop the MOSFET drivers, one of the INA219s, and the ATTiny861, and you’d have just a monitoring board with a (low-speed) PWMable switch.  It’d plug into the same place and use the same host interface.  The one board could be made into just a boost, or just a buck.  Flexibility.

There was just one snag.  That’ll work for small power supplies with maybe up to 5A capability (~100W) but not for the 50A version.  The MOSFETs will fit, but the tracks will need to be huge, the board will be hideously expensive to make, and they don’t make inductors big enough.

Looking around for inductors, I did see these.  They’re not massive like the 10uH one I saw, and they’re not expensive.  The downside is they’re about 10% of what I really need.  I guess I’ll just make do.  They’re also not PCB-mount (mind you, a 40kg inductor doesn’t PCB-mount either).

Thus, it may be more sensible to separate the MOSFETs and high-power stuff from the controller.  Now here’s the rub.  We’re dealing with sub-15ns pulses.  PWM.

Years ago for my industrial experience, I did work on an electric harvester platform.  The system ran 48V.  The motors were rated at 20kW, and were made in house using windings wound from 5mm diameter enamelled copper wire and neodymium magnets.

We had loads of issues with MOSFETs blowing.  The MOSFET driver was mounted close to the MOSFETs, as I’m proposing to do here, but between DSP and driver, was a long-ish run of ribbon cable. @Bil Herd posted this article covering the challenges involving inductance on PCB layout.  That same problem applies to “long” cable runs too.

10 years ago when we were working on this project, I remember asking about if we had considered maybe using coax cable instead of a ribbon cable.  The idea was rubbished at the time.  Given we were PWMing 400A, I think there might’ve been something in that suggestion.

That ribbon (10~20cm of it) would have had a lovely inductance all of its own, and while I have no idea what frequency the PWM was running at (I might have the code somewhere but I can’t be stuffed digging it up), and we were fundamentally driving a single-ended signal over a fairly long distance.  Yes, ground was close, but probably not enough, a twisted pair would have been better, but even then not perfect.  We blew many MOSFETs on that project.  Big TO-263s!

An earlier article on differential signalling got me thinking: why not use LVDS for the PWM?  A quick search has revealed this receiver and transmitter (Mouser says two receivers on it, but I think that’s a typo).  The idea being that I send the PWM down a differential pair using LVDS.  155Mbps should be plenty fast enough (the ATTiny861 can only do 64MHz) and these parts will run at the 5V needed for fast switching.  In fact they require it.

Using twisted pairs, the inductance should cancel.  I’ll make a MOSFET board that just has these signal pairs:

  • +12V (for the MOSFET driver) and 0V
  • +5V (for the LVDS receiver) and 0V
  • High side PWM + and –
  • Low side PWM + and –

There’s a ground-loop I need to be wary of between the 12V and 5V rails, really it’s the same 0V rail for both.  I suspect they’ll still need to be connected at both ends.  Add in more of those screw terminals to take the input and output power off-board, and I think we should be set.

Similarly, the INA219 should probably be a separate board, with scope for having a chassis-mounted current shunt.  The connection to the current shunt’s sense output is a low-power connection, so no issue there.  You want to keep it short for accuracy reasons, but a simple twisted pair will work fine.

Sep 272018

So, the last few days it’s been overcast.  Monday I had a firmware glitch that caused the mains supply to be brought in almost constantly, so I’d disregard that result.

Basically, the moment the battery dropped below ~12.8V for even a brief second, the mains got brought in.  We were just teetering on the edge of 12.8V all day.  I realised that I really did need a delay on firing off the timer, so I’ve re-worked the logic:

  • If battery drops below V_L, start a 1-hour timer
  • If battery rises above V_L, reset the 1-hour timer
  • If the battery drops below V_CL or the timer expires, turn on the mains charger

That got me better results.  It means V_CL can be quite low, without endangering the battery supply, and V_L can be at 12.8V where it basically ensures that the battery is at a good level for everything to operate.

I managed to get through most of Tuesday until about 4PM, there was a bit of a hump which I think was the solar controller trying to extract some power from the panels.  I really need a good sunny day like the previous week to test properly.

This is leading me to consider my monitoring device.  At the moment, it just monitors voltage (crudely) and controls the logic-level enable input on the mains charger.  Nothing more.  It has done that well.

A thought is that maybe I should re-build this as a Modbus-enabled energy meter with control.  This idea has evolved a bit, enough to be its own project actually.  The thought I have now is a more modular design.

If I take the INA219B and a surface-mount current shunt, I have a means to accurately measure input voltage and current.  Two of these, and I can measure the board’s output too.  Stick a small microcontroller in between, some MOSFETs and other parts, and I can have a switchmode power supply module which can report on its input and output power and vary the PWM of the power supply to achieve any desired input or output voltage or current.

The MCU could be the ATTiny24As I’m using, or a ATTiny861.  The latter is attractive as it can do high-speed PWM, but I’m not sure that’s necessary in this application, and I have loads of SOIC ATTiny24As.  (Then again, I also have loads of PDIP ATTiny861s.)

The board would expose the ICSP pins plus two more for interrupt and chip select, allowing for a simple jig for reprogramming.  I haven’t decided on a topology yet, but the split-pi is looking attractive.  I might start with a buck converter first though.

This would talk to a “master” microcontroller which would provide the UI and Modbus interface.  If the brains of the PSU MCU aren’t sufficient, this could do the more grunty calculations too.

This would allow me to swap out the PSU boards to try out different designs.

Sep 232018

Well, I’ve now had the controller working for a week or so now… the solar output has never been quite what I’d call, “great”, but it seems it’s really been on the underwhelming side.

One of the problems I had earlier before moving to this particular charger was that the Redarc wouldn’t reliably switch between boosting from 12V to MPPT from solar.  It would get “stuck” and not do anything.  Coupled with the fact that there’s no discharge protection, and well, the results were not a delight to the olfactory nerves at 2AM on a Sunday morning!

It did okay as a MPPT charger, but I needed both functions.  Since the thinking was I could put a SSR between the 12V PSU and the Redarc charger, we tried going the route of buying the Powertech MP3735 solar charge controller to handle the solar side.

When it wants to work, it can put over 14A in.  The system can run on solar exclusively.  But it’s as if the solar controller “hesitates”.

I thought maybe the other charger was confusing it, but having now set up a little controller to “turn off” the other charger, I think I can safely put that theory to bed.  This was the battery voltage yesterday, where there was pretty decent sunshine.

There’s an odd blip at about 5:40AM, I don’t know what that is, but the mains charger drops its output by a fraction for about 50 seconds.  At 6:37AM, the solar voltage rises above 14V and the little ATTiny24A decides to turn off the mains charger.

The spikes indicate that something is active, but it’s intermittent.  Ultimately, the voltage winds up slipping below the low voltage threshold at 11:29AM and the mains charger is brought in to give the batteries a boost.  I actually made a decision to tweak the thresholds to make things a little less fussy and to reduce the boost time to 30 minutes.

The charge controller re-booted and turned off the mains charger at that point, and left it off until sunset, but the solar controller really didn’t get off its butt to keep the voltages up.

At the moment, the single 120W panel and 20A controller on my father’s car is outperforming my 3-panel set-up by a big margin!

Today, no changes to the hardware or firmware, but still a similar story:

The battery must’ve been sitting just on the threshold, which tripped the charger for the 30 minutes I configured yesterday.  It was pretty much sunny all day, but just look at that moving average trend!  It’s barely keeping up.

A bit of searching suggests this is not a reliable piece of kit, with one thread in particular suggesting that this is not MPPT at all, and many people having problems.

Now, I could roll the dice and buy another.

I could throw another panel on the roof and see if that helps, we’re considering doing that actually, and may do so regardless of whether I fix this problem or not.

There’s several MPPT charger projects on this very site.  DIY is a real possibility.  A thought in the back of my mind is to rip the Powertech MP3735 apart and re-purpose its guts, and make it a real MPPT charger.

Perhaps one with Modbus RTU/RS-485 reporting so that I can poll it from the battery monitor computer and plot graphs up like I’m doing now for the battery voltage itself.  There’s a real empty spot for 12V DC energy meters that speak Modbus.

If I want a 240V mains energy meter, I only have to poke my head into the office of one of my colleagues (who works for the sister company selling this sort of kit) and I could pick up a little CET PMC-220 which with the addition of some terminating resistors (or just run comms at 4800 baud), work just fine.  Soon as you want DC, yeah, sure there’s some for solar set-ups that do 300V DC, but not humble 12V DC.

Mains energy meters often have extra features like digital inputs/outputs, so this could replace my little charge controller too.  This would be a separate project.

But that would leave me without a solar controller, which is not ideal, and I need to shut everything down before I can extract the existing one.  So for now, I’ve left the Powertech one in-place, disconnected its solar input so that now it just works as a glorified VSR and voltmeter/ammeter, as that bit works fine.

The Redarc is now hooked up to solar, with its output going into a spare socket going to the batteries.  This will cost me nothing to see if it’s the solar controller or not.  If it is, then I think some money on a VSR to provide the low-voltage protection, and re-instating the Redarc charger for solar duty will be the next step.  Then I can tear down the Powertech one at my leisure and figure out what they did wrong, or if it can be re-programmed.

The Meanwell charger is taking care of things as I type this, but tomorrow morning, we should hopefully see the solar set-up actually do some work…

… maybe. 🙂

Sep 222018

So, I’ve gotten back to this project having spent a lot of my time on work, the Yarraman to Wulkuraka bike ride and the charging controller #Solar-powered cloud computing — just to name 3 things vying for my attention.

In the test board, I had wired up some LEDs for debugging, dead-bugged 0805s, which were hooked between the output of the octal latch and 0V.  I omitted the series resistor, as I presumed that, given the output was PWMed with a maximum duty cycle of ⅛, the LEDs shouldn’t burn out.

Turns out I had forgotten a property that all diodes exhibit, that is the desire to clamp the voltage across them.  Today I was testing the board, and wondering why some channels were dim, others didn’t work at all, but one worked so much better.  Did I accidentally put the wrong current limiting resistor in series with the drain?  No, all checked out as about 12 ohms.

I put a program on the MCU that just turned a channel on when the button was pressed.  No music, no fancy PWM stuff, just turn on a LED when the corresponding button was pressed.  Measuring the gate voltage showed about 2V.

Even with the PWM output forced low, the output was still 2V.  Moreover, I was using my new bench supply, and with nothing running, the circuit was drawing ~300mA!  Why?

Turns out, the LEDs I had dead-bugged in, were trying, and succeeding, in clamping the output voltage.  2V was just barely enough to trigger the output MOSFETs, but clearly this was borderline as some worked better than others.  I was likely in the linear region.

Snip out the common connection for the LEDs to 0V, and the problems disappeared.  I’ve dead-bugged a 1kOhm resistor in series with the lot, and that’s got my debug LEDs back and working again.  The MOSFET outputs now work properly.

The bigger chunkier MOSFETs I bought by mistake could have worked just fine: maybe I was just driving them wrong!

Two prospects have crossed my mind:

  • Getting the MOSFET board made professionally
  • Getting a board that combines all components onto one PCB made professionally

The version that is shown was really designed for the home PCB maker to be able to produce.  The traces are wide and the board is fundamentally single-sided: when etching, you just etch one side of the board and leave the other side unetched.  When drilling the holes, you just countersink the holes a bit on all pins not connected to 0V.

A smaller board with everything in one would be worth making now that I’ve proven the concept.  Not sure there’s a good reason to go to SMT at this stage: I still want to make assembly simple.  The thinking is the all-in-one would have some headers so you can conceivably break things out for other projects and just omit parts as required.

This could theoretically be entered into the #The 2018 Hackaday Prize as part of the musical instrument contest, as that’s what it is: it’s a musical instrument for the severely physically handicapped.  There is a video of a slightly earlier prototype in this post .

Code wise, I’ve done little.  The basic functionality is there, it makes noises, it flashes LEDs, that’s about what it needs to do for now.  I did have to increase the start-up delay so that the buttons were detected properly, as without this, if I used my bench-top supply, it would fail to see any inputs.  People aren’t going to notice 100ms boot-up delay vs 1ms, but it makes a difference if the power supply is a little slow.

Apr 282018

So, I’m learning now to read datasheets a little more carefully.

Finally got around to testing that board, and at first, I had some shorted connections to deal with and one dry joint.  No problem, a bit of scoring with the knife and a tap with the soldering iron, and we’re in business.

Apply 9V to the GPIO pin, and ~3V appears at the output.  Good enough, the AVR will recognise that.

Now for the MOSFET, applying 5V to the input, switches the MOSFET.  Great.  Try it on the actual synth… sure enough I can make noise, but nothing lights up.  What gives?

In this prototype I’m using Fairchild FQI4N80s, which I bought a tube of 50 of them.  Turns out these MOSFETs have 40ns turn-on delay, and a 100ns rise time.

Doesn’t seem like much, but then consider this: our PWM clock is 64MHz.  That means the pulse time can be as low as 15ns.

They also can have a gate threshold as high as 5V… so borderline for this application.  Never mind, I’m sure they’ll be useful in other projects.

I have a couple of options in mind, both by Infineon: the IPS70R900P7SAKMA1 and the IRLU120NPBF .  Out of those two, the latter sounds like the better bet, I seriously don’t need 700V Vds!

It is tempting though to design a board based on the prototype though, as I think if I make room for a TO-220 or TO-251 (IPAK; not to be confused with the larger I²PAK), there are actually a lot of choices out there, and much of the time, the choice is not critical.

If I do that, I’ll be spacing out the KK connectors a bit, as that’s another point: the plugs are just a wee bit wider than the sockets, so I need to make room for that.

Jul 152017

So… last weekend I got to trying out the I/O modules. I rigged up wiring harnesses for all five push-buttons and their associated LED strings.

I used CAT5e since I’ve got loads of it around… and ran four strands up to each button, carrying: +12V, MOSFET drain, GPIO and 0V. I ran a resistor between +12V and GPIO to pull it high, the switch NO and common connected between GPIO and 0V. The button’s illumination LEDs and the LED string both connected in parallel between +12V and MOSFET drain.

So far so good. I have just a 3-pin connection on the I/O module, carrying all but the +12V. That remaining wire I hooked to +12V directly on the input feed. Not having a 0V feed going direct to the power supply though was my mistake.

If this happens, the 0V reference on the I/O module is open-circuit, and the zeners, meant to protect the MCU from +12V, don’t do anything. So I suspect a MC14066 copped a belt of +12V by mistake!

Not sure, but I thought I’d play it safe and build a new one anyway. I started on it earlier this week and finished it this afternoon. This time around I opted to put LEDs on the outputs of the 74HC574. No current-limiting resistors, since they’re meant to be ⅛ duty cycle anyway, and if they smoke, well, who cares?

This highlighted a glitch on the GPIO_EN signal during programming, LEDs would illuminate during MCU programming. A pull-up helps here.

I tested using a 9V supply from my electronics kit, which has AA cells in it (somewhat stale ones) this time around. Handily, the plugs for the LED strings have LEDs in them themselves, and they work at 9V, so we can use those to see what the LED string would do.

On the software side, I finally fixed polyphonics and a key-sticking issue. One of my I/O modules has stopped working for whatever reason, but the others are working fine, as can be seen here:

The other issue I have to chase is some leakage on the blue and white buttons: the plugs for them are not meant to be glowing unless pressed, but you note they are glowing significantly, and get brighter when those buttons are pressed. I might have to re-visit those connections. Otherwise though, very good progress today.

May 282017

So… after some cursing at me being dimwitted enough to get the amplifier around the wrong way… and luckily the amplifier is a tough little beast… I managed to get some basic functions going.

It is supposed to be polyphonic, the synthesizer supports it, and it does actually work, but it seems my GPIO arrangement isn’t picking up multiple buttons correctly.

Ahh well, we’ll get there. 🙂 At least it is making noises and flashing lights on cue, so in that sense, a minimum viable product has been achieved.

Ohh, and I was able to bump the sample rate to 16kHz. 32kHz was a bit much for the little MCU though, it couldn’t quite keep up.

Code size at this point is 3.6kB… so still lots of room left. The answer might be to implement an I²C slave mode so I can interrogate it from the host.

May 272017

So… a bit of head scratching this afternoon. I had working PWM on the LEDs… which was fantastic.

With the trade-off of LEDs being ⅛ brightness maximum, which is fine since the LED strings that I’ve been given for this project would make Manfred Mann proud…

I was a little stumped with my audio PWM. I could not get a peep out of it, not via the amplifier or anything else. Tracing back, I was getting nothing out of the PWM pin. Real strange. Looked up the data sheets, couldn’t see what I was doing wrong.

My code for initialising Timer1 looked like this:

	/* Timer 1 configuration for PWM */
	OCR1B = 128;			/* Initial PWM value: audio */
	OCR1D = 32;			/* Initial PWM value: light */
	OCR1C = 255;			/* Maximum PWM value */
	TCCR1A = (2 << COM1B0)		/* Clear OC1B on match,
					   nOC1B not used */
		| (1 << PWM1B);		/* Enable PWM channel B */
	TCCR1B = (1 << CS10);		/* No prescaling, max speed */
	TCCR1C = (3 << COM1D0)		/* Set OC1D on match,
					   nOC1D not used */
		| (1 << PWM1D);		/* Enable PWM channel D */
	TCCR1D = (0 << WGM10); /* Fast PWM mode */

It looked fine… even checked the header files to see there wasn’t some cock up in the headers. Never had an issue with avr-libc, but you never know, and of course, no problem there.

In exasperation, I swapped setting up TCCR1A and TCCR1C. Bingo, I had PWM both channels. Why? Turns out there are some bits in TCCR1C that shadow TCCR1A, and I was inadvertently setting those back to 0 when I set up TCCR1C. So my code now:

	/* Timer 1 configuration for PWM */
	OCR1C = 255;			/* Maximum PWM value */
	TC1H = 0;			/* Reset counter high bits */
	TCNT1 = 0;			/* Reset counter low bits */
	OCR1A = 0;			/* Initial PWM value: spare */
	OCR1B = 127;			/* Initial PWM value: audio */
	OCR1D = 127;			/* Initial PWM value: light */
	TCCR1A = (1 << PWM1B);		/*
					 * Clear TCCR1A except for PWM1B,
					 * we'll configure OC1B
					 * via the shadow bits in TCCR1C
	TCCR1B = (1 << CS10);		/* No prescaling, max speed */
	TCCR1C = (2 << COM1B0S)		/* Clear OC1B on match,
					   nOC1B not used */
		| (3 << COM1D0)		/* Set OC1D on match,
					   nOC1D not used */
		| (1 << PWM1D);		/* Enable PWM channel D */
	TCCR1D = (0 << WGM10);		/* Fast PWM mode */
	TCCR1E = 0;			/* Not used: PWM6 mode */

We still need to turn PWM1B on via TCCR1A… but we set the output control mode via TCCR1C to avoid clobbering our settings.

Having fixed that, I now have a second problem, in my dead-bugging of the amp, I got my left and right arse-about, which would explain why my amp is making no noise. That’ll be a tomorrow job methinks, at 22:30 UTC+10:00 it is too dark to work on PCBs. I’ll just keep myself amused looking at the waveform on the oscilloscope.

May 072017

So, I’ve done some tweaks to the prototype… namely fixing some wire assignments. I also had to drop my 1kΩ resistor on the input to a 100Ω as the voltage drop of the 1k was too high.

At the moment, I am seeing ~3V at the MCU, which is good enough.

Beneath the afro of wires, is the prototype.

… and this is the PWM LED control in action (animated GIF). So PWMing the output enable of a 74LS374 works, and doing the same on a 74HC574 should too.