Tag Archives: pwm

PWM Dimmer for LED Lighting

20160119_Projekte_046
Finished LED dimmer

I have recently moved to a new apartment and was looking for a PWM dimmer to control some 12V LED strips. I thought that should be easy enough nowadays but it proved more difficult than I thought. All I found either didn’t meet my requirements, were uggly or expensive. So I decided to build my own, tailor-made to my needs.

20160119_Projekte_044
Finished PCB mounted below a shelf

 The requirements

  • Handle 100W @ 12Volts comfortably
  • Controlled by a simple on-board pot (no remote control or the like)
  • Affordable
  • No acoustic noise
  • Fine-grained control down to very low brightness levels

I’ll go through these requirements one-by-one.

20160119_Projekte_040
Two LED strips below the shelf give a nice lighting on the desk

Handle 100W @ 12Volts comfortably

My LED strips suck up a bit more than 20 Watts per meter and there is a maximum of 4-5 meters of LED strips per dimmer so I need a power rating of around 100W. If you do the math you’ll find that there will be a maximum current of about 8.3 amps.

I don’t want this thing to get hot nor do I want to put a heat sink on it. So the total power dissipation in the dimmer should stay below, say, 1 watt. So if we use a single FET, we need a Rds-on of 14.5 milliohms. Thats not a lot but there are inexpensive MOSFETs that meet this requirement. And we can always parallel two or more of them if necessary.

And yes, there will also be some switching losses but they should be low given the modest switching frequency of an application like this.

20160119_Projekte_048
PIC microcontroller with its power supply

 Controlled by a simple on-board pot

This is most likely the simplest way of controlling a dimmer but it’s surprisingly hard to find. A lot of commercially available dimmers come with IR remote controls nowadays. And some of the higher quality models expect a 0-10V control signal which means that you have to use an external pot which you have to mechanically attach somewhere. I would like everything on a single PCB to keep things simple.

Affordable

I needed 3 of these things so cost was a factor, too. All the nicely-made dimmers I could find were priced at $50 and uppwards. Not that bad but I figured that I could make my own for a small fraction of this, perhaps $10.

No acoustic noise

We all know those dimmers that produce audible humming. Especially when dimmed somewhere half-way down. I hate it. Drives me crazy.

This proved to be more difficult to archieve than I thought. More on this later.

20160119_Projekte_054
Since the power supply has 4 output leads, my dimmer has a 8x connector at its input

 Fine-grained control down to very low brightness levels

This is where most products fail miserably. Most of those remote-controlled things only have 8 brightness levels. And just about everything I found works linearly which makes very little sense if you ask me. We humans perceive brightness logarithmically, rather than linearly. So going from 1% to 2% seems the same as going from 50% to 100%.

20160119_Projekte_050
Beefy mosfet and capacitor

Linear control will not give you fine control at the lower end. Ideally, you want to have an exponential transfer function from pot position to PWM duty cycle to compensate for the logarithmic nature of the human vision. I found the easiest way to do this was using a microcontroller. Furthermore, the ability to do all of this in software enables you to play around with it and find a transfer function that you’re happy with.

High granularity at the lower end also means that we need quite a bit of PWM resolution. The common 8-bit resolution translates to about 0.25% per step. Going from 0.5% (wich is about what I mean by very low brightness) to 0.75% is already quite a step. Many microcontrollers are capable of 10 bits which is 4 times better and probably good enough.

20160119_Projekte_045
Yet another view

The design

At the center of my design is a 8-bit PIC microcontroller, a PIC16F1936. There’s not much special about this particular model, it’s just a type I’ve used several times before and still had some on stock.

A LM2931 provides the PIC with 5 volts from the 12 volts input voltage. I use the LM2931 as my standard 5V regulator. It’s pin compatible with the legendary 7805 but survives input voltages in the range of -50 to +60 volts making it very robust against transients.

20160119_Projekte_051
A LM2931 generates 5V from 12V

The PIC controls a LM5111 dual FET driver that provides a powerful 12V gate drive to a pair of Infineon IPB136N08N3 N-channel MOSFETs. This is the same transistor that I’ve recently used for my Arduino Solar Charger Shield. Its an inexpensive (< $1), large SMD type with an exellent Rds-on of 11.5 mOhms.

There are several variants of the LM5111. It comes in inverting and non-inverting configurations as well as combinations of inverting and non-inverting. At Farnell, the the inverting ones were by far the cheapest so that’s what I’m using here. It doesn’t really matter since you can change the polarity in software as needed.

20160119_Projekte_053
Pot and one of the output drivers

Why am I using two FETs despite the fact that one could easily handle the entire current? First, I’m driving two LED strips with this dimmer and using two transistors simplifies the layout so I have two outputs exactly where I need them. Secondly, the LM5111 is a dual FET driver anyway so I get the second gate drive for free.

20160119_Projekte_049
Nice 2kOhm pot

I’ve provided each output with a generous 1.5mF capacitor in order to shield the supply from the ripple that is inevitably produced by the PWM. I’ve also taken care to use a cap with low serial resistance (ESR) and a high current rating. The Panasonic FR series fulfills both of these requirements while being good value for money. I thought this should be enough to avoid excessive ripple and therefore also acoustic noise.

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Top side of the long version

The input to the PIC comes from a quite nice 2 kOhms pot that I’ve recovered from some scrap. There is also a voltage divider to measure the 12V input voltage. The idea was to only enable the output once the input voltage has stabilized but I found this to be a quite unnecessary feature when programming the PIC.

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Bottom side of the long version

 The Layout

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Top side of the short version

I’ve built the two different versions of this dimmer. The schematic is exactly identical for both of them, they only differ in their physical layout and board dimensions. I’ve just tailor-made them to their specific application so the pots are located in a handy position and the outputs are exactly where I need them.

20151129_Projekte_009
Bottom side of the short verison

 Software and Testing

My first version of the software measured the voltage from the pot using the on-chip ADC and outputed an identical 2kHz PWM signal on both outputs. 2kHz should be enough to avoid visible flicker and seemed a reasonable choice. Everything worked but the power supply made quite a bit of noise over most of the brightness range. Worse than any commercial design. Even worse, there was an awful lot of flicker. Ouch.

Dimmer_2kHz_firstTry
Too much ripple producing lots of audible noise

Looking at the power supply output / dimmer input voltage on a scope if became clear that the two 1.5mF caps still allow too much ripple at this frequency.

The first thing I tried was running the two outputs out-of-phase. Since I’m using two FETs I have two independent outputs. So I can run them 180 degrees out-of-phase. Now, at  duty cycles below 50% it looks like I’m only driving a load half the size with a frequency and duty cycle twice as high. At precisely 50% duty the supply even sees a constant load at its output since exacly one LED strip is on at any point in time. At duty cycles above 50% the on-times overlap so the load only varies from 50% to 100% and with twice the frequency. As you can see from the scope screenshot below, this already helped a great deal but the problem was not yet resolved. So the natural thing to do was to increase the PWM frequency.

Dimmer_2kHz_phaseShift
Running the outputs out-of-phase helps

At 8kHz, things already looked (and sounded) much better. Ripple and acoustic noise were much reduced but the supply was still audible at least in a quiet environment.

Dimmer_8kHz
Increasing the PWM frequency to 8kHz almost solves the problem

So I moved the PWM frequency up as far as i could. Given the PIC’s 32MHz clock and a 10 bit resolution this was 31.25kHz. Now every last bit of audible noise was gone. Finally.

Dimmer_31kHz
At 31.25kHz all the noise is finally gone

I then noticed that the phase shift was 176 degrees as opposed to the intended 180 degrees.

Dimmer_PhaseShift_before
Phase shift is 4 degrees off

Not that this makes much of a difference in practice but I solved it anyway. I’ve implemented this phase shift by starting one PWM module at 128 and the other at 0 (we’re only talking about the 4 most-significant bits here, so the maximum is 255). The two instructions are on successive lines in my C code but 3 clock cycles are needed to process each of them so they are not enabled at the precisely same time. Starting the first PWM module at 131 has solved the problem as you can see below.

Dimmer_PhaseShift_after
Now the phase shift is fixed

With these changes in place the flicker mentioned previously was also much reduced but had not yet disappeared. Looking at the voltages on a scope for a while the problem became clear. I was measuring the voltage from the pot at fixed intervals that had no connection with the switching frequency. So I was effectively measuring at random points in time.

I said that the input voltage now showed much less ripple but some ripple is inivitable. Some of that ripple is likely to somehow feed through to the voltage from the pot. That introduced noise in the value measured by the ADC which lead to variations in the duty cycle which was noticable as flicker.

I did two things to resolve this. First, I’m generating an interrupt signal (from the same timer as I use for the PWM) every 64 PWM cycles. In the corresponding interrup service routine (ISR) I read (and save) the ADC value and start a new conversion. This way I’m always measuring at the same point during the PWM cycle. So the effect of the ripple should be similar every time. I’m also averaging 32 measurements which further helps to smooth the value I’m using to calculate the duty cycle. So flicker is gone as well as you can see below.

Dimmer_DutyCycle_stable
After averaging the ADC readings, the duty cycle stays rock solid

Now for the transfer function. My first try was exponential. The problem with that was that it gave away too much of the pot range for very low brightness levels. I played around with this for quite some time and finally settled for a combination of linear (at the very low end of the range) and exponential (for everything above that). Also, two of my dimmers can be fully turned off by turning the dimmer all the way to the left. Their power supply is always on and the light is only controlled by the pot so I need to be able to really turn them off (not only down). The third one has a slightly different transfer function that only allows to turn it down to 2% or so. That one has its power supply controlled by a conventional light switch so I don’t want the pot to completely turn it off.

Dimmer_DutyCycle_stable2
Same at lower duty cycles

 The result

After all, I’m very happy with the result. There is no noticable power dissipation on the board. There certainly is a bit of dissipation but the board doesn’t heat up noticably so I’d say its clearly below a watt.

The components have cost me around $10 per board. Some stuff like the connectors I have bought a flea markets, they can be surprisingly expensive through regular retail channels. The PCBs are home-made so they have cost me a considerable amount of time but not much in terms of cash.

20160119_Projekte_052
LM5111-2M is the inverting variant

They are, furthermore, controlled by a simple pot, produce no audible noise and can be finely dimmed just as planned. So I can proudly state that all the requirements have been met.

If you’re in need of a dimmer and have a soldering iron and a bit of spare time I can only encourage you to build your own. It’s not too hard, needs only few components and is very doable on a prototyping board if you don’t want to etch or mill your own board.

As always, attached is a zip file with all the eagle files, board layouts, schematic as well as the software.

Arduino MPPT Solar Charger Shield – Software

There have been two previous posts on this project: one on the concept and the hardware and one on hardware testing. You probably want to check them out first if you’re not yet familiar with this project. Or even better: Click here for an overview over this project.

20160212_Projekte_045
Maintaining an input voltage of 17 volts even if that means a lower-than-desirable voltage at the output

Now that we know that we have a functioning MPPT solar charger we are ready to talk about the software (or the sketch as the Arduino folks call it). It’s quite simple, really. So this will be a short post. And yes, you can download the sketch. There is a link at the end of this post. As always, I appreciate any feedback, comments and the like.

There is a number of basic tasks the arduino needs to perform in order for this shield to be useful. I’ll go through them one by one.

Controlling the DC-DC converter

At the heart of this project there is a synchronous step-down (or buck) DC-DC converter that is controlled by a PWM signal from the arduino. So one of the tasks is to set the frequency and duty cycle of that PWM signal.

We let the PWM signal run at the maximum frequency the arduino allows with an 8 bit resulution. Thats simply 16MHz (the Arduino’s frequency) divided by 256 (the 8 bit resolution), or 62.5 kHz. So the prescaler will be 1.

As you can see from the shields’s schematic, we need to output the PWM signal from Pin 6 (by the Arduino’s pin numbering, not Atmel’s). In order to do this kind of low-level stuff you’ll have to read the Atmega328’s data sheet. There is usually no Arduino-ish shortcut if you really need to controll what’s going on.

Luckily it’s just a few lines of code to set things up. All in the function buck_setup(). There are three more little functions to control the DC-DC controller once it’s set up:

buck_enable() and buck_disable() are very simple and just turn it on  and off, respectively. buck_duty(uint8_t duty) is only slightly more involved. It changes the duty cycle to the value you pass to it. Besides that it ensures that the duty cycle stays within certain limits.

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Test setup with resistor-based dummy load

You don’t want it to go to 100% since in order to keep the bootstrap capacitor C6 charged you need a little bit of off-time. In order to drive the upper FET you need a voltage higher than the panel’s voltage and that’s exactly what C6 is for. So we enforce an upper limit on the duty cycle.

Likewise, you don’t want your duty cycle to go below 50% because in that case you would be pumping energy from the battery to the pannel. A synchronous step-down converter is basically the same thing as a synchronous step-up (aka boost) converter with input and output confused. So we also want to enforce a lower limit on the duty cycle.

The upper and lower limits are set through the #defines DUTY_CYCLE_MINIMUM and DUTY_CYCLE_MAXIMUM.

Measuring voltage and current

The shield has all the hardware necessary to measure both voltage and current both at the input as well as on the output. We’ll just need to write some simple software to make good use of that hardware.

Unlike with the PWM singal where we had to do some low-level bit fiddling ourselfs we can just rely on convenient Arduino library functions to do the job. Basically, analogRead() is all we need here.

20160212_Projekte_043
Nicely regulating so that the input stays at 17 volts

I’ve written a function called read_values() that uses analogRead() to read all 4 values (input voltage, output voltage, input current and output current) 16 times each, averages the results and converts the ADC reading to proper voltages and currents.

The necessary multipliers are defined as floats in VIN_MULTIPLIER, VOUT_MULTIPLIER, IIN_MULTIPLIER and IOUT_MULTIPLIER. I’m doing all the voltage and current measurements in floating math. Yes, this is not at all efficient but we don’t need the Arduino’s computational power for anything else most of the time so this is fine here. Just keep in mind that you can save a lot of resources here if you ever need to do so.

Displaying voltage and current on the LCD

Our hardware also involves a 2 lines x 16 characters LCD so we can show the world what we are measuring. Again, we can rely on standard Arduino functionality to do the job. There is an LCD library that does everything we need.

So my function write_display() can focus entirely on formatting. The upper line shows the voltages in Volts, the lower line shows the currents in Milliamps. The input is on the left hand side of the display, the output on the right.

Deciding what to do

In the first section we’ve discussed the functions necessary to controll the DC-DC converter. But in order to use those functions, the Arduino needs to first decide what to do.

Vin21
66% duty cycle at 21V input voltage gives the desired 13.8V at the output

This is where the function buck_update() comes into play. You could consider this the heart of this sketch. This is where all the relevant decisions are made. When to turn the converter on, when to turn it off, when to increase the duty cycle, when to decrease it… You get the idea.

The behaviour of buck_update() is controlled by 8 #defines. I list them here together with the values I have used:

#define ENABLE_VOLTAGE 18.0
#define DISABLE_VOLTAGE 15.0
#define INPUT_TARGET_VOLTAGE 17.0
#define OUTPUT_TARGET_VOLTAGE_LOW 13.8
#define OUTPUT_TARGET_VOLTAGE_HIGH 13.9
#define INPUT_CURRENT_LIMIT 2000.0
#define OUTPUT_CURRENT_LIMIT 3000.0
#define INPUT_CURRENT_MINIMUM 0.0

I think they are quite self-explanatory, especially if you look at how they are used inside buck_update. It’s quite simple: If the panel’s voltage rises above 18V, turn the converter on. Once the converter is on, try to archieve a panel voltage of 17V without exceeding 13.9V at the output. If the panel’s voltage drops below 15V turn the converter off again.

Vin17
At 55% duty cycle with a 16.9V input voltage we’re getting only around 9.2V at the output

Besides that the function is also looking at the input and output current and makes sure certain limits are not exceeded. But with a 30W panel it should never be possible to reach those limits anyway.

Putting it all together

Now all we need to do in the loop() function is calling read_values(), buck_update() and write_display(). Since writing to the LCD is quite slow we are only doing it every 32nd time we read the values and update the PWM signal.

With this sketch I’ve hooked the MPPT Solar Charger up to my lab power supply. (a Keysight E3645A, my newest toy *g*) and my extremely simple but occasionally useful resistor-based dummy load.

The enable and disable voltages are simple and work as expected. Maximum output volage is also not tricky. If the voltage at the output goes too high, the duty cycle is decreased and everything is fine again.

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There’s not much to photograph when you’re writing and testing software

More interesting was to see how the shield would regulate when faced with a limited current budget at the input. For that the supply was set to a voltage of 21V (about a 12V solar panel’s open-circuit voltage) with a current limit of 100mA to 500mA. That’s quite a nasty supply, quite a bit trickier to handle than a real solar panel. Try to pull just a bit too much current and the voltage will drop to zero…

Also, the resistors at the output are not a realistic load for the converter. A car battery will pull no current at 12 volts or so (unless overly discharged) but will quickly start to sink large currents when the voltage goes just a bit higher and the battery is charging.

But I think the setup is good enough to test the sketch. And it handles the challenge quite well. With all resistors on (i.e a 100/6 ohms load) and a 300mA current limit, the input voltage sits at 17V (our target input voltage) while 9.25V appear at the output. At 400mA, the output voltage rises to 10.7V with the input still at 17V. At 600mA the input is still at 17V but with the output now at 13.15V. If I take the current limit even higher, the output voltage rises to 13.82V but not any higher, just as we want. The input voltage rises to 21V (since this is a lab supply and not a panel) with a corresponding drop in current to 530mA.

20160212_Projekte_044
Quite realistic: The charger is pulling as much current as it can with the current limit at 530mA and reaches an output voltage just above 12 volts

I’m honestly quite happy with the project as it is now. The idea definitely works and I’m motivated to design a new, deployable version with some fancy features that will use much less power at the same time. I’ve already done quite some work on that new version but it will take another few weeks until I get to describe that project here.

Until then I will show you some other, smaller projects that I’ve already finished but didn’t have time to document yet. So you will first see a number of smaller, simpler projects over the next few weeks.

Before I forget: There’s the Arduino sketch for download. And click here for an overview over this project.