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# Some basic Power LED question for building a simple LED board Answered

This question is coming from a total rookie in the field of electronics, please keep that in mind :)
I need to build a simple multi-LED board, but I lack some essential knowledge about electronics.

The LEDs I need to use are 5W IR High Power LEDs:
DC Forward Voltage (VF): 1.4V
DC Forward Current (IF): 1400mA
Maximum Pulse Voltage: 1.7V
Maximum Pulse Current: 2000mA

I would like to build on the board three pieces of this LED, with a momentary switch. And I just realised I will probably need some kind of "driver" circuit - whatever it is - to use the Power LEDs. I'd rather buy one from eBay, than make myself one :) Just need to know what type I need.

The input voltage is a 12V DC battery.

So long story short: could someone help me out with this simple LED board?
How many "mAh" should the battery have  to power these three power LEDs for about 1,5 - 2 hours?
I am aware of a 5W probably need some cooling, I will use a heatsink.
If someone could draw me a circuit for this project I would be amazed :)

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## 5 Replies

LEDs want constant current, and that is the function of the magic LED driver module.  It watches (or senses) its own output current, and keeps this value constant.

For your three IR LEDs there, I recommend wiring them in series, so that they all share the same current.  Then connect this string of three of them to the output of the driver module.  Just to be clear, a diagram of this is attached. Large version here:
https://www.instructables.com/file/FGD0539GT6ZDZIS/

Assuming the voltage drop across each LED is 1.4 V, the drop across a string of three of them is 3*1.4 = 4.2 V.

Conveniently, that number is a lot less than 12, so a good choice for driving this string would be a buck-style switching regulator.    "Buck" is one of several switching regulator styles.Some others include "boost" and "inverting".  Anyway, there's a rule for buck-style which says that its output voltage is always less than its input voltage.

If you needed the output voltage to always be greater than the input voltage you'd want a boost-style switcher.

http://en.wikipedia.org/wiki/Buck_converter
http://en.wikipedia.org/wiki/Boost_converter
http://en.wikipedia.org/wiki/Buck-boost_converter

One of the sort of annoying things about shopping for a regulator module is that they often don't mention whether it is buck,  or boost, or what.  Also there might be certain other assumptions.  Like maybe they assume you are going to be powering exactly one white LED, the forward drop for which is about 4 or 5 volts, so that their assumptions about voltage get converted into assumptions about the number of some particular color of LEDs in series.

Anyway you specifically mentioned eBay, and you know that's a good way to browse into a wide selection of stuff; i.e. see what's out there.  One warning about the Hong Kong merchants is that if you live in the Former US like I do, that stuff takes forever , well more like a month or so, to arrive, and that's kind of annoying.  Also the documentation can be kind of crappy, or non-existent, or in Chinese... But aside from those issues, they make good electronics, and cheap!

I don't where pulled that picture of a driver from, but if it's this:
http://www.ebay.com/itm/Constant-Current-LED-Driver-1-x-3W-LED-12V-700mA-MR16-/220865269073
Then that looks like it will work, and could be wired up same the one in the diagram I drew.  The output current through your string of 3 LEDs should be a constant 700mA,  or 0.7 A, something close to that.

If I were doing this I might be tempted to get a regulator with an adjustable current output, so that I could adjust the current/brightness level.  This looks like a good one:
But the drawbacks are that it is more expensive, and it looks like it might be a little tricky to adjust.  For something like that, I would test it first with three big 1-ohm resistors in series, instead of my three expensive LEDs, just to make sure I actually got it set so it is regulating the current properly, before trusting it enough to connect actual LEDs to it.

Final note:  I'm not sure why you, or whoever is selling those LEDs, is calling them 5W, since 1.4V*1.4A = 1.96 W.  The power quote might just be a little white lie, or a little infrared lie. Ha! ;-) Or maybe you need to go back and check the spec., just to make sure.  But those numbers look believable for an IR led with 4 emitters on the die like that.

arraxis (author)2011-10-02

Wow, thank you very much! :)
One last question: I need to be able to switch off the lights separately, so this way probably I will not be able to wire the LEDs in series. Would it be problematic if I would use the circuit you drawn but with parallel LEDs?

Jack A Lopez (author)2011-10-02

The circuit I drew would not work the same way with the LEDs connected in parallel.

If you want to be able to turn on/off each LED independently, then you'll need a current regulator for each LED.

If you're wondering why this is the case, I suppose I could draw you another picture.  But it would probably be better if you drew it yourself.  I mean:  draw three LEDs in parallel on the output of the driver, and then ask how the driver can "see" or "sense" its way into that network and control the the current through each LED?

The only thing the driver sees is its own output current.

frollard (author)2011-09-30

The linked picture for a driver circuit is exactly what you need -- that particular one (gu10?) is for higher 110 or 220 volt input -- great if you want to wire it to a wall plug.

Just make sure to match
a) the output wattage
b) the output polarity.

iceng (author)2011-09-30

I don't know every translator driver circuit out there.
The inductor indicates a change of voltage ,  and / or current levels.
The wires are probably for the led and pins for input power