March 6th, 2015

VoltLog #7 – In The Mail

A list of the items I show on this InTheMail video:
2.4 inch 320 x 240 px TFT display module based on the ILI9341 lcd controller, complete with touchscreen, uSD card slot, 128Mbit flash, independent font chip and the required power circuitry. ESP8266 modules (3 different variations), a small solar panel, waterproof enclosure and Li-ion battery cells. uSD card holder thing. 13W LED 5050 E27 lightbulb. Breadboad + breadboard power supply + jumper cables. 0-30V 0-2A Adjustable lab power supply kit, unassembled. A bunch of components I ordered from Comet Electronics.

In this article I continue my project where I needed a constant current source. In the previous article I talked about the first approach the LM317 constant current source which didn’t worked out that well but could of been improved as suggested by some of my readers in the comments. So I decided to use a dc-dc converter and turn it into a constant current source. Since I’m using the TPS54232 in another project and I have it around I decided to use it. This technique can also be applied to other dc-dc converters with a bit of reading on the subject.

I’ve started by reading SLVA374 app report from TI which is a Step-Down LED Driver Design Guide based on the TPS54160 dc-dc converter, normally used as a buck voltage regulator. The schematic from the app report can be easily adapted to our device with a few changes like adding a sense resistor and an output capacitor:

TPS24232 constant current source schematicTPS24232 constant current source board

If you remember from my last post I needed 1.12 A to power the two led packs that I’ve  build connected in parallel. We have a simple equation that we use to find out the feedback resistor value: R=Vref/Io In our case Vref is 0.8V taken from TPS54232 datasheet, Io is our desired current of 1.12 A. Doing the math we get the R value = 0.71 ohms. This is not a standard value, but we can use two 1.5 ohms resistors in parallel(R1 and R2) to get 0.75 ohms which is pretty close. With 0.75 ohms feedback resistor I have 1.06A at the output which divided by 56(the number of white LED’s) means 18.9 mA for each white led.

We must consider the power dissipation for this resistor and we can calculate it : Pdis = Vref^2/R In our case we have Pdis = 0.8^2/0.75 = 0.85W. The only 1.5 ohms resistors that I could find locally were 5 W so I had to work with a bigger package. The EN pin is used to enable and adjust the Undervoltage Lockout but I’m not going to use that feature since my input voltage is always gonna be high enough not to cause any problems. The resistors are placed on the pcb anyways so you could use those pads and soldere the necessary resistors.

We also must consider the power dissipation in the low side diode. During the converter on time, the output current is provided by the internal switching FET. During the off time, the output current flows through the catch diode. The average power in the diode is given by: Pdiode = (1-Voled/Vin) * Vfd * Io , where: Vfd is the led forward voltage, Voled is the supplied output voltage and is approximated by: Voled = Nled * VLed + Vref where : Nled = number of LEDs, Vled = forward voltage drop of each LED. In my case Pdiode = (1-(4.2/12))*0.75*1.06=0.51W so I used a 2A 40V schottky diode part number CDBA240-G.

You would also have to consider the inductor and the input and output capacitors, but you can read all about that in the datasheet of the TPS54232 or in the app report mentioned above. After I had all of my circuit figured out, I routed the board and etched it using my photo etching technique. For such a small board its not worth taking out the solder paste so I soldered it using the soldering iron, as you can see its a mix of through hole and surface mount parts but they fit together quite nice.

TPS54232 LED constant current source pcb assembledTPS54232 constant current source pcb back side

The solder drops that you can see on the back of the board are a sort of DIY thermal vias. I’m not sure how efficient they are but I gave them a try with this board, probably because I had too much time available :-). First when I designed the board I included a area of copper in the top layer right beside the resistors. Next I drilled some 0.7 mm holes and I placed 0.7 copper wire in the wholes securing it by soldering on both sides. The trick is to make the solder joint as small as possible so it doesn’t spaces your component from the board. As I’ve said I don’t know their efficiency but I think they work, I can feel the heat transferring from one side to the other faster. If you like them you can try them.

The testing went smooth, the LED’s light up perfectly, no problems at all. During the testing I noticed one bug though: if the input wires are not firmly attached or secured and there is an imperfect contact the converter will tend to output less current than the programmed 1.02A. I’m not sure why this is happening, it might have something to do with the Undervoltage Lockout feature that I skipped on but I’m not sure. Anyway its not that much of a problem since once in place the power source will have the cables firmly attached so no worries. That’s why I used the cable connectors on the PCB in the first place, I knew it would save me some trouble later.

56 white LED

Now I had one last thing to do. Since this circuit is going to be operated in the outdoors the corrosion would set in pretty quickly so I improvised once again. I had this idea for quite a long time but never actually tried it. So I used my Bison universal hobby glue, which is transparent, and covered the copper traces on the PCB in glue. After it hardened, it looks like you could dip these board into water and nothing would happen to them. Well, except from the terminals which stick out of the PCB :-). Nonetheless I think these boards will run no problems even after a few years. If you’re wondering where the bubbles come from, they form when the glue dries out :-).

TPS54232 constant current source covered in gluewhite led packs pcb's covered in glue

I’m pretty happy how this project ended up and I feel like I know more about LED’s and ways to power them. I’ll definitely need to experiment more with some high power LED’s, maybe use them to light my workbench. As for the efficiency of this circuit I don’t know if I’m calculating the right way because I’m using the equations from page 16 of TPS54232 datasheet, and those are clearly stated to be used only under continuous conduction mode. Since the circuit has been modified to act as a constant current source I’m not sure the same equations apply. But I did the math anyway and I got an efficiency of 75.36% and according to the same equation if I would connect the two led packs in series I would get an efficiency of 79.01%. Once again I’m not sure these calculations are correct and I ask the readers to comment on these.

There is more that you can do to improve the efficiency of this circuit. It turns out you can reduce the power losses in the current sense resistor by lowering the voltage across the resistor. The solution is to inject a bias voltage, but I’m not going to try this solution since I’m already happy with my design. You can read more about it in SLEA004 app report from TI.

Downloads:

October 2nd, 2010

LM317 constant current source

Recently a friend of mine asked my help about replacing some 12V light bulbs with LED’s for longer battery life. After doing some searches the cheapest solution was to use a bunch of 5 mm white LED’s powered from a constant current source. A constant current source is not cheap to buy and since were in the spirit of making why not make that also. But where to start ? there are voltage regulators that you can turn into constant current sources, dc-dc converters that you can turn into constant current sources or you can choose a specialized circuit designed exactly for powering LED’s. Obviously the first option is the cheapest and simplest to implement but it comes with its drawbacks.

Anyway, I had everything needed on hand so I proceeded and designed a PCB in Eagle. I used two LM317 voltage regulators because I want to power two sets of LED’s. Each set of LED’s is composed of 28 LED’s tied in parallel. I’m gonna set the current on each LED to 20 mA so that is 560mA for each set of LED’s. I wanted to spread all that load on two LM317 so I used two.

28 x 5mm white LEDs wired in parallel Dual LM317 constant current source schematic

The PCB were easy to route and ended up like this:

28 x white LEDs in parallel PCBDual LM317 constant current source PCB

Kind of small and nice looking. The LED pack could of been smaller but since LED’s don’t spread light on a wide angle I decided to spread the LED’s a bit to spread the light. The LM317 board was designed so that it would fit perfectly under a heatsink I had around. Actually the heatsink is from an old video card I had in my junk box; its nice to reuse these things, its like recycling.

old video card ripped out of heatsink

I proceeded and used my DIY photo etching method but something went wrong and I messed up all of the PCB’s, they ended up looking like this:

messed up PCB

The thing is I stopped using ferric chloride a while ago. Instead I turned to ammonium persulfate which comes as a white powder that you need to dissolve in watter. The watter stays clear, it doesn’t smell and it doesn’t stain (not checked yet). It only catches a blueish color after you’ve used it and copper accumulates in it. The only disadvantage for using ammonium persulfate is that it etches slower than ferric chloride, other than that is great. The problem with mine is that it was mixed with water more than 6 months ago and it turns out it doesn’t last that long when mixed with water, so keep it as powder and only mix it when you need it. Unfortunately I couldn’t get more of it locally so I had to return to the good old ferric chloride. I did all the process again and the boards came out great this time.

The assembled LED packs look like this:

Assembled 28 led packsAssembled 28 led packs back

and the assembled LM317 board look like this:

dual LM317 constant current source assembleddual LM317 constant current source assembled

Notice how I designed the board to fit just over the raised part of the heatsink? its the little things that count 🙂 (and I’m not referring to the heatsink as you will read next). The LM317 pair was mounted on the back so that it sits firmly attached to the heatsink transferring all that heat.

here’s a photo between the heatsink and the pcb:

LM317 constant current source - between heatsink and pcbLM317 constant current source - between heatsink and pcb

I was pretty happy with the overall result until I actually run it. The heatsink got very hot pretty fast and it was clear it was not dissipating enough heat. With 12 V in, LED Vf of 3.4V , 560mA and an ambient temperature of 28 degrees C, the heatsink reached almost 80 degrees C. It turns out The LM317 is not very efficient in this setup and together with the sense resistor has to dissipate allot of heat. And a simple calculation would of shown this from the beginning if I paid attention to it.  (Vin-Vf.Led-Vadj)xIo => (12V-3.4V-1.25V)x0.56A= 4.116 W for each regulator, double that and the heatsink is just too small to dissipate that kind of heat. Not to mention the poor efficiency of the circuit. Its the little things that count , I would of done that calculation earlier I wouldn’t of made that circuit just to realize that is not good for my setup.

At this point I knew this option is not good and I had to find something else to power the LED’s. I could use a specialized circuit like the LM3406 which would give me the best efficiency but I would have to wait for delivery and get some more parts, or I could use a dc-dc converter modified as constant current source which would give me decent efficiency and I could build it with parts that I already have. I decided to go with the dc-dc converter modified as constant current source. I already have the TPS54232 from another project of mine; the TPS54232 is a 2A, 28V, 1MHz, step down SWIFT™ dc-dc converter. Now its time to design a new PCB for the new power source but more about this in the next article.

Downloads:

December 31st, 2009

Happy new year 2010 POV

You need to a flashplayer enabled browser to view this YouTube video

I would like to wish my readers a Happy New Year and may all your wishes come true. As you may have experienced, during the holidays,  hobbyists get some free time and they like spending it building stuff. In this case the author, Infernoz, build this 8 LED POV display, which seems like a fun way of wishing a happy new year. The device is based on the Attiny26 from ATMEL and from the video we can see it works quite nice, but unfortunately I was unable to locate any source code posted on the project page, but this is not that bad as you can find POV source code from others projects(POV1 POV2 POV3) if you’re interested in building one.

Happy new year 2010 POV: [via]

August 27th, 2009

Bicycle Tail Light

Bicycle tail light

You want to learn more about DC to DC boost converters? Here is a good and practical application of such converters, an LED tail light for your bicycle. Why do you need a boost converter for that you might ask, when you can simply have a battery power up some LEDs. Well, this way you greatly increase efficiency, you increase the number of LEDs you can drive and thus increase the perceived power of light. And yes, you will learn about them.

This project makes use of the National Semiconductor LM3410 chip. This chip provides a current regulation rather than a voltage regulation and makes it suitable when working with LEDs. This current regulation is done by switching a NMOS gate at constant frequency of 525Khz ( you can also chose 1.6Mhz) and varying the duty cycle, the so called pulse width modulation. Given a constant frequency varying the pulse width results in varying the mean value in time. The regulation is done by providing a feedback current that causes a voltage drop on a resistor which in turn is fed into a comparator.

The output of this comparator goes to the PWM comparator where it faces the output of the amplifier that reads the switched current through an inductor. The result will be a PWM signal attacking the NMOS gate and thus providing a steady mean output current value. The current sensing amplifier also provides an output for switched current limiting. Some nice features of LM3410 makes things easier, it’s internally compensated,  has a very low stand-by current, has dimming possibilities and thermal shutdown.

As the author states in his article you have to be careful when choosing components. Since the operation frequency is pretty high the diodes through which the inductor discharges must be high speed or you risk blowing the chip. Taking things a step forward he added a PIC12F683 microcontroller to provide some light effects selected by a pressing a button.

If we look closer at this project we can see it can be further developed, using the dimming possibility of LM3410 you can also turn it into a stop light. Whenever you break light intensity goes higher. Another development is to adapt it to be used with a dynamo. Have fun.

Bicycle Tail Light: [Link]



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