6 amp LED Driver Project

Hi, I usually either buy regulators as needed or use a resistor to knock down the current flow for LED use, but I thought it would be fun to try to actually build one from scratch. This seemed like as good of place to post this as any.

I have never actually built a working circuit that contained more than a battery, diode, resistor and LED, so this will be a real test for me.

Constant current drivers require a bit more finess, so I thought I would start with a constant voltage one instead. I realize that they are some very sophisticated ones out there (Alan B versions) - this is just a simple version as a learning experience and for use in a project light I am thinking about. There is nothing "intelligent" or programmable in this project - think on/ off - maybe some dimming.

I am targeting a fairly wide voltage input range (about 5 - 30 volts) and approx 4 amps or so output at 3.3 volts. This is in the range where you can just about "cheat" and run a power LED direct drive. :naughty:

There are an amazing number of companies that offer controller chips - so it is almost a daunting task to pick from them all. I looked mostly at IRC (irf.com), TI, ON Semi, and National. They all have advantages, and I mostly wanted very simple and few parts, so I ended up at National. I don't know if it is the "best" decision, just a decision.

The main values their product brought to the table are
- 4.5 V to 42 volt input
- 5 amp output
- adjustable voltage output
- Built in mosfet (the on / off large power transistor)
- Very few parts needed to build (at least in theory)
- A very strong web design center that does all of the calculations and part selection for you.

Re: Hobby voltage regulator

I attended an LED lighting seminar in Sunnyvale CA recently and National gave a demosntration of how to use their on-line tools. I put in the values I was after and it recommended a design based on the "LM22679TJ-5.0", so I picked that one. The link is here http://www.national.com/pf/LM/LM22679.html

I assume they know what they are doing so I didn't try to guess against the web site.

I tried then to buy some of these parts:
- National - out of stock - 16 week lead time
- Future - on order
- Newark - non stock item
- Digikey - not in stock

I guess the part is either "too new" or something ? Suggestions ?

Re: Hobby voltage regulator

Inductors Question

The National Web Bench design software recommends an inductor / coil with about a 5 mH inductance. I was curious, with resistors and capacitors, you can just add them up to get the right resistance (yes, I know the parallel rule). Can you more or less do this with inductors as well ? The reason I ask, is that it would be handy to split the inductor size in 1/2 so they would be shorter.

Any idea if this works or not ?

Thanks

Harry

Re: Hobby voltage regulator

In theory, you can put inductors in series and the values add up. In practice, there is so much more to the specification of inductors than just the inductance value that this is unlikely to work very well. You are much better off finding a recommended part of the right specification and using that.

Re: Hobby voltage regulator

Hi Mr. Happy - Thank you for that information. I guess for some reason, I had intuitively thought that inductors in parallel would add up to a single larger inductor - I guess not.

I found more on the part - you can get them in the UK via Farnell. Here is the info from the Newark web site:

Farnell UK Direct Ship - Non-Cancelable/Non-Returnable
Handling Fee $20 per order
(Delivery within 7 business days)

Basically, to get one part is $ 10 for the part + $ 20 more for shipping and handling and I am still just getting started. Now I know how the guys in the EU normally feel about parts costs.

That internal mosfet feature is a dang expensive feature.

Re: Hobby voltage regulator

HarryN said:

Hi Mr. Happy - Thank you for that information. I guess for some reason, I had intuitively thought that inductors in parallel would add up to a single larger inductor - I guess not.

Click to expand...

Here's a rule of thumb to help with the intuition:

Devices that work on voltage should go in parallel so they all see the same voltage.

Devices that work on current should go in series so they all see the same current.

Capacitors resist changes in voltage, so for more capacitance you put them in parallel.

Inductors resist changes in current, so for more inductance you put them in series.

Resistors resist the flow of current, so for more resistance you put them in series.

Re: Hobby voltage regulator

Thanks Mr. Happy - that is a good rule of thumb.

Re: Hobby voltage regulator

Mr Happy said:

In theory, you can put inductors in series and the values add up. In practice, there is so much more to the specification of inductors than just the inductance value that this is unlikely to work very well. You are much better off finding a recommended part of the right specification and using that.

Click to expand...

I have to admit that I've never considered using two power inductors instead of one. My guess is that it could be made to work, but wouldn't be ideal. The biggest risk might be that the flux from one coil might couple into the other coil in the wrong direction, and effectively reduce the total inductance.

Other than that... I'd worry most about the current that the inductor is rated for (don't want to saturate the core), winding resistance, core losses, and the frequency that the inductor is rated for. I'd bet that the inductor manufacturer might have some useful application info on their website that would help the user understand the various qualities of a inductor, and how the user should select the appropriate inductor.

best wishes,
Steve K.

Re: Hobby voltage regulator

I think by using one inductor instead of two should make the switching regulator more stable too.

Re: Hobby voltage regulator

Hi - Any comments on the actual National switcher part ? Does this seem like a good selection or am I missing something in the big picture here ?

Re: Hobby voltage regulator

LED's are current driven devices. If your regulator puts out 3.3V, it will barely light up a high-power white LED as their forward voltage is typically in the 3.6-3.7V range. Look into CC - constant current drivers.

Re: Hobby voltage regulator

rmteo said:

LED's are current driven devices. If your regulator puts out 3.3V, it will barely light up a high-power white LED as their forward voltage is typically in the 3.6-3.7V range. Look into CC - constant current drivers.

Click to expand...

Hi - Thanks for the CC comment. I agree that CC drivers are ideal in general and I already own / have used a variety from taskled, the shoppe and leddynamics. They all work fine for what they are designed to do.

I would not pretend to even attempt a real CC driver as the next step in playing with electronics. I have watched experienced EEs with lots of diagnostic gear scratch their heads over CC circuits and fancy dimming methods. That is way beyond what I am trying to do.

This design is actually variable output voltage so I can tune to meet the needs of the LED (s) within my happiness level. It will certainly be no worse than my normal route of batteries + resistor (which works fine for me in general).

I am not too worried about driving the LEDs at max current rating - the light for this project is planned for "bright" but mostly longer running time than my other lights.

Re: Hobby voltage regulator

HarryN said:

It will certainly be no worse than my normal route of batteries + resistor (which works fine for me in general).

Click to expand...

Actually it could be worse than a resistor. A resistor in series with batteries approximates a constant current supply. Whereas a voltage regulator is a true constant voltage supply. One reason constant voltage is not good for LEDs is that as they warm up they tend to draw more current. Which naturally makes them warmer still, and in the limit you can get a thermal runaway that destroys the LED.

A regulated current supply is not necessarily a whole lot different from a regulated voltage supply, though. To regulate current you can pass the current through a low value resistor, measure the voltage drop, and regulate that voltage.

Re: Hobby voltage regulator

I built a constant current regulator of that design (linear) to charge a 12V SLA. It is current limited to 300ma but also tops out at 13.8V to nicely float the battery. Such a design could be adapted to drive a high flux LED easily. I'd be willing to provide information on how to do this if useful to the OP. There is a lot of information on the internet about building simple regulators like that

Re: Hobby voltage regulator

Hi, Thanks for the offers about constant current supply designs. I really appreciate the offers for constant current designs but perhaps a bit later in my skill level development.

I probably should tell you a little more about what I am up to and that will make it more clear why I chose this design path.

I am working on a home built light with at least 2 K2s and maybe some other LEDs in it. Since the various LEDs have different current demands, I cannot really power it with just one constant current supply. Instead, I am planning to use a constant voltage supply (approx 3 - 4 volts ) and very lightly resistor as needed for each LED.

It is relatively easy to set this up to dim using a rotary switch, which I much prefer over push button UI based setups. I could not find any CC supplies which:
- Fit inside this light body
- Had dimming without a push button UI
- Had a whole bunch of software code to drive them
- Could deal with the current demands without being massive

If this works out (which is of course questionable) then I can also integrate some more parts onto the board, making the overall light easier to build.

I did not set out to build this thing, I just could not find a driver that was going to work for this project.

If it is ok, I would like to ask everyone to help me focus on building the actual voltage regulator, not get into a discussion of the benefits of PWM vs other dimming methods or which LED is the "best".

I will see if I can find a way to post the output of National's web output.

Thanks

Harry

Re: Hobby voltage regulator

I just checked Digikey's website and they do have them in stock:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2556570&keywords=LM22679

Re: Hobby voltage regulator

ctom said:

I just checked Digikey's website and they do have them in stock:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2556570&keywords=LM22679

Click to expand...

Thank you very much - I am not sure how I missed that one. Maybe I over specified or something.

I have had some boards fabricated by advanced circuits before ( www.4pcb.com) so I was playing with their cad software - trying to learn I guess. The last time I did a board, I designed in in excel draw, then Georges80 ( taskled.com ) helped me make it into a real gerber files board. I had hoped to avoid the two step process, but after several hours on their cad program, I am still not very far.

It would help if they actually had more parts or packages in their library. AFAIK, they don't even have Cree or Lumileds LEDs as a standard part. It also does not seem to run that well in Vista.

Re: Hobby voltage regulator

Have you tried Pad2Pad (http://www.pad2pad.com/) for board layout?

I have used these guys before and found the process quite straight forward. They provide their own free software which is very easy to pick up and use - no need to mess around with Gerber files. Once your design is done, just submit it electronically and you're done......

Re: Hobby voltage regulator

Hi Billy - Thanks for the link to pad2pad. Do you happen to know where they make their boards ? I sort of ended up at advanced circuits because the boards were fabricated in North America vs China.

I have not used them (only did one board in my life and that was with 4pcb. Both firms offer similar setups - free software that ties you to them, which I guess is ok.

I watched the video at pad2pad which actually helped me with the s/w at 4pcb, so that was helpful. I have kind of a strange shaped board, so that is atually a challenge for both firms, but I can work through that.

I am still debating about using a "new program" vs just drawing in in excel and doing a fully "home made" board. I am not too thrilled with having to learn how to draw up my own components, esp. for something as common as a K2 or a stock National Semi part.

BTW - I would like to put add in a way to help reduce the likelihood of "voltage spikes" on Vin. I think there are some parts that short these to ground?

Any suggestions on where to find a part that will short to ground spikes over about 40 volts ?

Thanks

Harry

Re: Hobby voltage regulator

HarryN said:

Hi Billy - Thanks for the link to pad2pad. Do you happen to know where they make their boards ? I sort of ended up at advanced circuits because the boards were fabricated in North America vs China.

Click to expand...

I'm based in UK and the boards I did were shipped from USA. I now use PCBPool in Ireland which works out slightly cheaper for me.

I'm pretty sure that Pad2Pad offer a routing service to get your board any shape you want.

I know it is a pain learn new software and to do all the component footprints from scratch, but it will be worth it - especically if your board gets quite complicated. These schematic/layout tools can cross check which can help avoid any embarrassing errors.......