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Author Topic: 3.3 ohm resistor with led  (Read 3080 times)

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Offline JdogTopic starter

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3.3 ohm resistor with led
« 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?

Offline SmAsH

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Re: 3.3 ohm resistor with led
« 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 ;)
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Offline JdogTopic starter

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Re: 3.3 ohm resistor with led
« Reply #2 on: April 03, 2009, 10:01:23 PM »
I guess.

Offline SmAsH

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Re: 3.3 ohm resistor with led
« 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! >:( ;D lol not really but you can have it hooked up to a battery directly it will just burn out a bit faster than normal.
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Offline Razor Concepts

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Re: 3.3 ohm resistor with led
« Reply #4 on: April 03, 2009, 10:44:27 PM »
The soldering will probably have some resistance, so the resistor might not be needed.

Offline SmAsH

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Re: 3.3 ohm resistor with led
« Reply #5 on: April 03, 2009, 10:47:22 PM »
i thought solder had like .1R resistance?
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Offline Soeren

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Re: 3.3 ohm resistor with led
« Reply #6 on: April 04, 2009, 04:12:13 AM »
Hi,

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!   ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline SmAsH

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Re: 3.3 ohm resistor with led
« Reply #7 on: April 04, 2009, 04:18:08 AM »
wouldnt you have to use a tonne of solder to achieve more resistance?
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Offline Admin

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Re: 3.3 ohm resistor with led
« Reply #8 on: April 04, 2009, 04:22:26 AM »
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!

Offline SmAsH

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Re: 3.3 ohm resistor with led
« Reply #9 on: April 04, 2009, 04:33:51 AM »
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.
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Offline Soeren

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Re: 3.3 ohm resistor with led
« Reply #10 on: April 04, 2009, 04:34:35 AM »
Hi,

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?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: 3.3 ohm resistor with led
« Reply #11 on: April 04, 2009, 04:37:10 AM »
Hi,

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
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: 3.3 ohm resistor with led
« Reply #12 on: April 04, 2009, 04:39:47 AM »
Hi,

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.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline SmAsH

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Re: 3.3 ohm resistor with led
« 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)
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Offline JdogTopic starter

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Re: 3.3 ohm resistor with led
« Reply #14 on: April 04, 2009, 12:10:12 PM »
fine then, go. hook your led up directly to your battery and watch as it burns out faster than normal! >:( ;D 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.
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
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.

Offline Razor Concepts

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Re: 3.3 ohm resistor with led
« 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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