As my sig line suggest, Im a n00b...

Ive got a 3w LED that I want to drive with an LM317 in current regulation mode. I need 800ma to drive the LED per the specs I received.

Running the requirements though the calculations, I came up with needing a 1.6 ohm resistor with 1w of power handling.

Now, where the HECK do I get that??

Im in the US, and my most local source of parts is Radio Shack. I dont see where they have any 1.5ohm resistors (refer to sig line again, ha)

I am still trying to understand resistors in series and parallel. Can someone take the time to help me learn and calculate, based on the availability of what is at the Radio Shack website, what resistors I need, how to wire them and why, so I can calculate this for myself (and possibly help someone else) in the future?

Many thanks-
"Lost and Confused" Andrew

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Firstly, 1.5 Ω is probably close enough.

If not, two 3.3 Ω resistors in parallel would be 1.65 Ω

Resistors in series simply add the resistance.

For resistors in parallel, take the inverse of each resistor, add up the inverses and invert the sum to give the total resistance.

The power in each resistor is V*I, or V^2/R or I^2*R. They are all the same. You must keep the power less than the rating of the resistor.

It is a good idea to keep the power a lot below the rating of the resistors, as resistors running at their rating get alarmingly hot.

 

Here is a good link to understanding the math of resistors in parallel.
Resistors in Parallel

Just take a standard value like 12 or 24 ohm and plug in the values until you get the correct number of resistors for an answer.

 

An excellent resistance calculator program I use is ResCAD. Available at :
Directory of /~rstevew/Public/Software ....download Rescad.zip and extract. Allows you select different % accuracy resistor ranges then gives you possible resistor combinations for needed series, parallel, or dividers...and the % error.

Ken

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10 ohm 1/4W 5% Carbon Film Resistor pk/5 - RadioShack.com

Check my math please:

Qty:6 10ohm resistors in parallel will give 1.6ohm of resistance.


These are 1/4w each, would that mean I will have 1.5w of power handling then??

Thanks!!
Andrew

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Yep, that's how it works

PARALLEL RESISTOR CALCULATOR

Lefty

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16' of #30 AWG = 1.6Ω.

 

1.5 and 0.1Ω in series also get you there. Check out wired 2W resistors.

You might as well use a 1.5Ω/1W carbon film resistor. It will gain more resistance with increasing temperature.

Besides that, the LM317 reference voltage differs slightly from the nominal value.

Best advise: Try the sample of the regulator you have and check what current it produces at 1.5Ω. For that value the reference voltage must be 1.2V (instead of 1.25)

Boncuk

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I just got back from the shack, the setup works perfect! However, I think 120 deg viewing angle of the LED is not giving the focus I would like to have seen.

BTW, I also have a 600lm 10w LED which when I applied power, was so bright I think I still see a spot from it!

Andrew

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and very temperature sensative.

 

That sounds expensive!

 

What the device or the medical treatment?

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It was $20 with free shipping from China, Ill post the seller's info and a video here soon, the daughter is on my 'main' computer right now, Im on the ham bench laptop.

I bought it to try it out, damn the LED is bright.

Andrew

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Here is the video of my 10w LED:
http://www.youtube.com/watch?v=FAMXmYMPAwc

I bought it from 'luckzdl2008' for $20 and free shipping from China on ebay.

HTH,
Andrew

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A very bright LED can easily blind you.
Didn't your LED melt with 12W and no heatsink?

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