Author Topic: 3.3 ohm resistor with led (Read 2893 times)

Jdog · « **on:** April 03, 2009, 08:51:16 PM »

I'm powering an infrared led with a forward voltage of 1.2 volts and a forward current of 100mA from a AAA battery. The LED calculator said I should use a resistor of 3.3 ohms. Do I really need a resistor if the resistance I'm supposed to use is so low?

SmAsH · « **Reply #1 on:** April 03, 2009, 09:09:51 PM »

you should always use a resistor even if it is 1ohm. it just helps protect the led from burning out

Jdog · « **Reply #2 on:** April 03, 2009, 10:01:23 PM »

I guess.

SmAsH · « **Reply #3 on:** April 03, 2009, 10:16:04 PM »

fine then, go. hook your led up directly to your battery and watch as it burns out faster than normal!

lol not really but you can have it hooked up to a battery directly it will just burn out a bit faster than normal.

Razor Concepts · « **Reply #4 on:** April 03, 2009, 10:44:27 PM »

The soldering will probably have some resistance, so the resistor might not be needed.

SmAsH · « **Reply #5 on:** April 03, 2009, 10:47:22 PM »

i thought solder had like .1R resistance?

Soeren · « **Reply #6 on:** April 04, 2009, 04:12:13 AM »

Hi,

Quote from: Razor Concepts on April 03, 2009, 10:44:27 PM

The soldering will probably have some resistance, so the resistor might not be needed.

If your solderings are that high a resistance that it can protect the IRLED, you better practice... A lot!

SmAsH · « **Reply #7 on:** April 04, 2009, 04:18:08 AM »

wouldnt you have to use a tonne of solder to achieve more resistance?

Admin · « **Reply #8 on:** April 04, 2009, 04:22:26 AM »

Quote from: Jdog on April 03, 2009, 08:51:16 PM

I'm powering an infrared led with a forward voltage of 1.2 volts and a forward current of 100mA from a AAA battery. The LED calculator said I should use a resistor of 3.3 ohms. Do I really need a resistor if the resistance I'm supposed to use is so low?

Your math is wrong. Assuming 6V, I calculated 48 ohms.

What voltage are you powering the LED with? Are you sure its 100mA, because that's a lot for an LED!

SmAsH · « **Reply #9 on:** April 04, 2009, 04:33:51 AM »

Quote from: Admin on April 04, 2009, 04:22:26 AM

Quote from: Jdog on April 03, 2009, 08:51:16 PM
I'm powering an infrared led with a forward voltage of 1.2 volts and a forward current of 100mA from a AAA battery. The LED calculator said I should use a resistor of 3.3 ohms. Do I really need a resistor if the resistance I'm supposed to use is so low?
Your math is wrong. Assuming 6V, I calculated 48 ohms.

What voltage are you powering the LED with? Are you sure its 100mA, because that's a lot for an LED!

i do agree on the 100ma being a bit much for an led tho.

Soeren · « **Reply #10 on:** April 04, 2009, 04:34:35 AM »

Hi,

Quote from: Jdog on April 03, 2009, 08:51:16 PM

I'm powering an infrared led with a forward voltage of 1.2 volts and a forward current of 100mA from a AAA battery. The LED calculator said I should use a resistor of 3.3 ohms. Do I really need a resistor if the resistance I'm supposed to use is so low?

If you don't use a resistor, the LED will die, perhaps not instantly, but it won't take long. A fresh AAA has got an internal resistance of between 100 mOhm and 300 mOhm (that's not MOhm in case you wondered) so with 0.3V to spare (with a new cell it would actually be up to 0.45 for a short time), a rough estimate of the current flowing is 1A to 3A!

While the battery wears down, current will go down too and you will soon have no power to the LED. A cell starts at around 1.65V and is considered empty at 0.9V, but your LED won't do a thing below 1.2V, so you will not be able to use up the cell.

If you instead use two cells and a Constant Current circuit, you will have a controlled current for the entire life time of the battery.

Btw. Why do you want to just have an IRLED light?

Soeren · « **Reply #11 on:** April 04, 2009, 04:37:10 AM »

Hi,

Quote from: Admin on April 04, 2009, 04:22:26 AM

Quote from: Jdog on April 03, 2009, 08:51:16 PM
I'm powering an infrared led with a forward voltage of 1.2 volts and a forward current of 100mA from a AAA battery. The LED calculator said I should use a resistor of 3.3 ohms. Do I really need a resistor if the resistance I'm supposed to use is so low?
Your math is wrong. Assuming 6V, I calculated 48 ohms.

What voltage are you powering the LED with? Are you sure its 100mA, because that's a lot for an LED!

As he wrote... An AAA cell.

It leaves the math to: ~0.3V/3.3 Ohm = 91mA

Soeren · « **Reply #12 on:** April 04, 2009, 04:39:47 AM »

Hi,

Quote from: SmAsH on April 04, 2009, 04:33:51 AM

i do agree on the 100ma being a bit much for an led tho.

I have some IRLEDs that are good for ~1A and I have seen them loads beefier.

SmAsH · « **Reply #13 on:** April 04, 2009, 04:42:17 AM »

triple post much? please try to modify your posts instead of double or triple posting

just makes scrolling through topics easier if we only have to go through 10 posts instead of 20-30

no offense intended (we've all done it once)

Jdog · « **Reply #14 on:** April 04, 2009, 12:10:12 PM »

Quote from: SmAsH on April 03, 2009, 10:16:04 PM

fine then, go. hook your led up directly to your battery and watch as it burns out faster than normal! lol not really but you can have it hooked up to a battery directly it will just burn out a bit faster than normal.

I didn't say I wasn't going to use one.

Quote from: Admin on April 04, 2009, 04:22:26 AM

Your math is wrong. Assuming 6V, I calculated 48 ohms.

What voltage are you powering the LED with? Are you sure its 100mA, because that's a lot for an LED!

I said a AAA battery which is 1.5 volts.
Yes' I'm sure it's 100 mA. Here's the specs if you don't believe me.http://www.radioshack.com/product/index.jsp?productId=2062565&tab=summary

Quote from: Soeren on April 04, 2009, 04:34:35 AM

If you instead use two cells and a Constant Current circuit, you will have a controlled current for the entire life time of the battery.

Btw. Why do you want to just have an IRLED light?

I can't use two cells because I need it to fit inside a highlighter. I'm doing Johnny Chung Lee's multi-touch wii whiteboard project, and I need to make an infrared pen.

Razor Concepts · « **Reply #15 on:** April 04, 2009, 12:42:01 PM »

Dang that thing is cool... like a tablet for $40.

Just a thought, maybe open up a 9v battery and get one of the AAAA batteries inside? Those would fit a lot better in a pen.

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Author Topic: 3.3 ohm resistor with led (Read 2893 times)

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