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# LED help for a newby.

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#### monkey_boy

##### New Member
Hi,
I'm new to the world of electronics and also to this forum. I have a number of questions about LED'S that I'd like answered if possible and after trawling the net for hours I decided to ask the experts (you people). I have no doubts that my questions are so basic your all probably gonna laugh :lol: but hey we all start somewhere

Right here we go.

1. I have an LED which I'd like to run at 9v. This is the technical bit: -

SOURCE MATERIAL:InGaN
EMITTING COLOUR:BLUE
LENS TYPE:WATER CLEAR
LUMINOUS INTENSITY-MCD:2000 TYPICAL/3000MAX
REVERSE VOLTAGE:5.0 V
DC FORWARD VOLTAGE:3.3V TYPICAL
DC FORWARD CURRENT:20~30mA
VIEWING ANGLE:± 10°
LEAD SOLDERING TEMP:260oC for 5 seconds

I'd like to get the brightest from the LED without putting to much pressure on it. What resistor would I use?

2. If you place one LED after the other would the resistor have to be changed or would you use 2.

3. What resistors would you use for a 12v set up.

Sorry if these are really simple questions but as I say I'm new (3 days into electronics) and could do with a little bit of help.

Thanks

The answer is 285 ohms for 9 volts or 435 ohms for 12 volts. More important is how I got those numbers. I took the supply voltage (9), subtracted the running voltage (3.3), and divided by the current (0.020 A) used by the LED for 285 ohms. If you substituted 30ma (0.030 A), you would get 190 ohms. Since these are odd values, you might like 220 ohms (in the middle) or 330 ohms (my first choice). You will find that the blue LEDs are prety bright either way- oh yea, they are neat too.

Question #2. It depends. Are you putting them in series or parallel? Either way, recalculate the voltage and current changes based on your configuration and use the method for resistor calculation above. You know. For instance, if you put them in series, the voltage goes to 6.6 etc.

Enjoy.

Thank you very much for your reply

As much as you've put it in into basic form I'm still struggling with certain points.

How do you get the running voltage. I know the supply is 9v but what makes it run at 3.3

Also how do you know what current would be used by the LED.

Bit of a mine field at the moment all this electrical stuff, exciting though so I'll stick with it.

Could you recommend a book or site that may help me with these questions rather than bugging yourselves with such simple questions.

Also one last thing (I'm going on a bit now) What is the best web site to order component parts, resistors, solder, books, etc etc.

Thanks once again.
Paul

As to the 3.3 and the current, LEDs are different. These numbers came from the data sheet that you supplied for your LEDs in your initial post.

No question is simple - when you learn something new, jot it down (like a formula) . . . that's what I do.

As to books, there was a post recently about a 5 - 6 book series free on-line. Do a search right here. I downloaded the whole thing and put it on one CD. It's really quite good. The key words are free and good :wink:

Parts. jameco, digikey - I'm sure everyone here has some favorites. I'm not sure why I'm the only one responding here but there are a ton of very smart people on this forum who are always willing to help.

This is the link I always break out for questions on leds and resistors. It does a good job of explaining ohms law, and placing things in series and parallel.

I searched but couldn't find said book series

Do you have a link to it, or remember the name?

Try:

that's what I have in my notes.

Excellent. Thank you!

Just a personal observation regarding LEDs. I don't like hooking them up in parallel (with one limiting resistor in series) for 2 reasons. First is that if one of the LEDs dies, the full current load goes through the remaining LEDs. If you only have 2, this could quite easily and quickly blow the other. Second reason is more cosmetic. The LEDs often do not have exactly the same resistances, and so currents will vary - and consequently you will get one LED slightly brighter or dimmer than the others. This is not serious, but to me is a painful irritation :?

Rather put the LEDs in series with one limiting resistor, or for the best results, in parallel with one resistor per LED. With the LEDs all in series, one failure will result in all the other LEDs going out (not blowing, just not getting current). So my personal preference with lighting several LEDs at once is to put them in parallel with a separate resistor for each LED - you could also use a resistor pack.

WHAT CAN I SAY OTHER THAN "THANKS". I'VE SEARCHED THE NET FOR HOURS LOOKING FOR THIS ADVICE AND BY CHANCE I FOUND THIS SITE, HOW GLAD AM I THAT I DID.

NO DOUBT I'LL BE BACK SOON ASKING WHAT A SCREWDRIVER DOES!!!

PAUL :wink:

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