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Project Help and Ideas » Could I use a NerdKit to read and control Amps and voltages please? (High power LED torch idea)

May 22, 2010
by Wobbler
Wobbler's Avatar

Question: Can I, and how would I, read and control current and voltage using a NerdKit please?

The "source" voltage will be something like a a ~12v 10Ah NiMH battery pack. I will be driving an LED to a constant 3 Amps, later 5A and ultimately 9A! I want to read the battery state and I want to read the Amps across the LED to keep it constant. So I need to "do something" to maintain the constant current within certain parameters.

Is this possible using the MCU please and what other bits would I be likely to need please?

............................................................................

Background info, if you're interested!

I bought the NerdKit with a couple of aims in mind, one of which was to make a diving computer/controller. A simpler project, I hope, before I get there is to create a great diving torch using the new high power LEDs available.

LEDs require constant current, for example 2.8A, 5A or 9A depending on the package. They also take rather low voltages. This means you normally use a driver board (NerdKit Application #1) to take the voltage, convert it to something manageable by the LED and make sure the constant remains current across the LED as the voltage varies/lowers.

Heat is a big issue with LEDs and the driver boards - too hot and things die a horribly expensive death! So monitoring heat would be useful too (NerdKit Application #2).

Finally, if you need a torch when diving, you really need two. However, dive torches are unbelievably expensive. It would be nice to have some method of measuring and displaying how much time left the batteries could run at 100% (before and during the dive) (NerdKit Application #3). It would also be nice if the torch could react to preset battery levels so that it could start dimming or flashing the light to extend the time left and to let the world know it's time to go home (NerdKit Application #4).

(Batteries are sensitive creatures. All require careful charging. Ultimately, it would be nice to include a battery charger control system in the torch, although this may be guilding the lily somewhat. (NerdKit Application #5, one day). Battery packs are usually 12v, occasionally 7.2v, sometimes 14.4v. I want to re-use my 12v torches and "upgrade" them using the NerdKit and LED systems. This means I don't need to worry about making anything, just swapping out the bulbs and reflectors and sticking the NerdKit somewhere)

9A + water = !!!

So, what do you think? Here is an idea of the sort of LEDs I'm playing with:

http://www.luminus.com/stuff/contentmgr/files/0/8bdf01102a2326a6cddaecf20c8e4d4c/misc/pds_001345_rev_02_sst_50_w_product_datasheet_illumination.pdf

http://www.dealextreme.com/details.dx/sku.11809

June 02, 2010
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi Wobbler,

Not sure if you've made any progress on this since your post, but hopefully this information will still be useful:

Because of the signficant voltage difference between your battery pack and your LED, and the fact that you probably want your battery to last as long as reasonably possible, you probably want to use a "buck converter" to efficiently supply current to the LED. This is a type of switching power supply that uses inductors and capacitors and rapidly-switching voltages to convert from a higher DC voltage to a lower one. We built a boost converter in one of our videos, which is the opposite (going from lower DC voltage to higher). A buck converter is quite similar in design. You can try to design your own buck converter if you wish, although be aware that your LED is significantly more sensitive to being over-driven than our electroluminescent backlight was!

Measuring the current is most easily accomplished with a "sense resistor" -- just a low value resistor that's in series with the LED. Sometimes, an amplifier is used so that it's easier to get an accurate current measurement.

Another option is off-the-shelf LED driver chips. For example, some from Maxim or Linear, although I haven't used any of these. They might also only be available in high-pin-count surface-mount packages, which can be tough to work with.

I agree that heat is a big issue, and your mechanical/thermal design will have to reflect that! Temperature monitoring is possible (like in the tempsensor project), but probably not in itself fast enough to prevent the LEDs from being destroyed by a circuit issue.

Sounds like a cool project with lots of interesting angles -- efficiency, charging, switched mode power supplies, thermal management, etc. Keep us posted!

Mike

June 08, 2010
by Wobbler
Wobbler's Avatar

Hi Mike,

Well, I read your post, did some digging and now have a really good Buck module! Still looking to measure battery life, charging etc and looking forward to getting stuck in.

Many thanks!

June 08, 2010
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Excellent. Post some photos / etc as you go!

Mike

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