Archive for December, 2005
Here are a few pictures of my boost-mode converter, originally posted in my thread on linear1.
breadboard with 10 led series string, and boost converter circuit
just the boost converter circuit – here you see the output capacitors, the rectifier (buried under the heatsink), the switch (on the other side of the heatsink) and the inductor
this is the pwm pulsetrain from the PIC … only 10khz and already not quite square – it looks a lot worse at 20khz
output current measured across a 10 ohm resistor, with a basic choke filter on the output from the converter.
Popularity: 12% [?]
Well well well – time is running short, and only one of three outstanding orders has been filled.
The boost converter is coming along – I’ve made refinements to the software that offer some good features and better performance. However, I think my hardware design needs more work… and for that, I need parts!
Out of desperation, I visited Satan’s Lair aka Radio Shack and was pleasantly surprised to find some of the items I had been missing. I managed to get a 2a 100uH inductor and (only) two 1uF tantalum capacitors. Having a proper inductor really made a big difference in the output of my boost converter – it sails to the limit I set for 25mA without any problems now, the old hand wound torrid would struggle to get past 20, begrudgingly giving me 25mA along with a lot of HEAT. The real test will be tonight – I’m going to wire up some power resistors as a test load and see if my boost converter can give me half an amp. Why 0.5A you ask? I need half an amp to drive my Moon LEDs at their fully rated 2w power.
The boost converter was originally intended to be a power supply for two parallel strings of LEDs, installed as under cabinet lights.
The lights themselves will be made of aluminum C channel and plexiglas, using 2 watt IO Moon led modules, spaced evenly along the length. I’m not sure what the spacing should be – there is probably some math that would tell me, based on viewing angle and what not. Since these are a prototype, I’m going to ballpark the spacing, so the LEDs I have (4) are spaced evenly across the length of the countertop. I need to take a few covert measurements to figure out how long the channel needs to be. I managed to find a local source, and have secured sixteen feet of aluminum C channel. This stuff has a chunky 1/8″ wall thickness and 1.5″ x 0.5″ outside measurements… combined with a little Arctic Silver, this should be an awesome heat sink for the LEDs.
Pictures to come shortly!
Popularity: 11% [?]
I have a running thread over at the linear1 forums, chronicling my experiences with building a digital switch mode power supply.
Here is a re-cap of where I’m at now.
The boost converter is a coil, a few capacitors, a diode and a darlington transistor. The PIC manages switching the transistor. Switching is done autonomously by a PWM generator inside the PIC. Constant current regulation is achieved by measuring the voltage drop across a 100 ohm resistor. The voltage is measured approximately 5000 times a second, and the pwm duty cycle is adjusted up or down, depending on the read voltage’s deviation from the set voltage. With a 100 ohm resistor, the math works out real easy – 100mV is 1 mA. Of course, 100 ohms will not work for a heavy current load – so I will have to reduce it to 1 ohm or 0.5 ohms, which changes my math a little, but no big deal.
I’m having problems with noise in the output current corrupting the A2D input to the pic. Better capacitors and a real ‘power’ inductor are on order and should arrive wed or thur. When they get here, I will rebuild this project so I don’t have, for example, a 12″ piece of wire connecting my switch to my pwm output.
Popularity: 16% [?]
The idea of using a PIC microcontroller as a replacement for the analog circuitry of a smps controller was originally suggested to me by SurJector over on the linear1.org forums.
Originally I felt the topic was over my head. I also dismissed it because I thought it added unnessecary complexity to an already difficult to understand process. However, as I read more about how switch mode power supplies and dc/dc converrts worked, I realized that using a PIC as the brain was indeed a good idea, and very cutting edge. ‘Digital’ switch mode power supplies are just now starting to ‘pop up’ in the industry, with all the current “all in one” controllers still relying completely on analog.
As I understand it, a digital controller offers the potentional for great efficiency, by adjusting the operating frequency of the switch dynamicly with the load. I’m not sure how that all works yet, but I have a few other things in mind, related to using these converters in the field of solid state lighting. Firstly, I like the idea of being able to digitally controll the output current to the load. Instead of having to dim a bulb using PWM, which is very non linear, I can instead dim a bulb using current mode control… using whatever interface suits me. Secondly, a digital controller could be integerated into a larger project, an illumination manager. For example, combine the dc/dc conversion routine with a routine reading a thermistor – monitor your bulb temp and dynamicly vary the current to keep it at a safe level. Another possibility – with a digital processor monitoring the current draw of the load, a faults such as shorted or open diode could be detected and an alarm condition set – in addition to reducing the current to compensate, a small indicator led could light up – “Check Lights”.
I have some pictures and schematics to share when I’m back home – more to come!
Popularity: 13% [?]
In addition to my new moons, I also received some sample DC/DC converters from Texas Instruments. In order to experiement successfully with this new switch-mode power supply, I need to be able to use it on a breadboard.
I could have drawn up and made my own ‘breakout board’ pcbs. I could have bought commerically made ‘experimentor’ boards. But I decided to use the cheap and fast “dead bug” method instead. I used a little dab ‘o hot glue and a 6-pin DIP socket to convert my SOT-23 packaged device into something I can now use with a bread board.
The jumper wires I used are two strands of wire for a 24ga stranded wire. I twisted the two strands together for a little more strenght. I also used some 24 ga solder wire to extend up from each spring in the socket, rather than soldering to the spring itself. The hard part with this process, and I think I’ll look to epoxy instead of hot glue next time – as I heated the part during soldering, it would soften the glue, and the part would move! In the last picture, it looks like there are some solder bridges, but there are not – that crap between the pins is just debris from my parts cleaning brush … looks like its time for a new brush!
More on the TPS61040 and what it does a bit later.
Enjoy:
6 pin dip socket
cut tape string of four converters
single converter sitting on an 8 pin DIP package IC
converter glued to ic socket
starting the soldering process
finished soldering
Popularity: 9% [?]