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Powering LED

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Rickmo

New Member
I am building a project that requires a small, low voltage LED powered by a 9 volt battery. How do I connect this? Do I need a resistor? The LED spec 2.2V 25 mA. I only want a "normal" glowing LED, not the super bright ones you see in flashlights. Like a "power is on" light.
Thanks.

Rickmo
 

Rickmo

New Member
Thanks.
So does this mean the output of the resistor is 1/4 watt? Regardless of the input voltage, in this case 9V?
Also.. how long do you think a 9volt battery would run this light?
 

Rickmo

New Member
Looking at Fry's... they list a 1/4 watt 5% carbon film resistor (seems expensive. .95)
Also a Power Resistor
and a SIP single in line resistor.
Which is the one I need?
 

MikeMl

Well-Known Member
Most Helpful Member
If all you are trying to do is make an "on" indicator, there is nothing that says you need to run the LED at 25mA. I have run some "ultrabrite red LEDs" on as little as 500uA, and they are bright enough to see in sunlight.

If the goal is long battery run times, then try 1/2 to 2mA, and see if you like it.

The formula for figuring the resistor value has already been given to you. Just reduce the LED current by making the resistor bigger: R = E/I = (9V-2.2V)/0.001mA = 6800Ω. (Resistor range 3K to 12KΩ).

The power rating of the resistor just has to be more than the resistor is actually dissipating which can be calculated as P= I*E = 0.001mA*(9V-2.2V) = 6.8mW, so a 1/4W carbon or metal film resistor is more than good enough.
 
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Rickmo

New Member
Thanks, Mike. But you're way over my head I'm afraid. I know nothing about this and certainly don't follow your very excellent formulas!
I don't need a super bright. Just bright enough to see and I want to last as long as possible on a 9v battery.
More thoughts?
 

MikeMl

Well-Known Member
Most Helpful Member
I don't need a super bright. Just bright enough to see and I want to last as long as possible on a 9v battery.
More thoughts?

Yes you do! The Superbright puts out more light at very low operating currents, therefore makes the battery run time much longer.
 
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Sceadwian

Banned
LED's can provide useful light (because the human eye is so sensitive) with very little current if just basic vision is needed. Doing this efficienctly can be a problem though as you need a method to control the current to the LED that doesn't result in a lot of loss.
 
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Rickmo

New Member
Ok, so... if you wanted to make a light that is bright enough to see... like a power on light... and you wanted to power it with a 9v battery... and you wanted it to last a long time... what would you use?
 

audioguru

Well-Known Member
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Ok, so... if you wanted to make a light that is bright enough to see... like a power on light... and you wanted to power it with a 9v battery... and you wanted it to last a long time... what would you use?
You have already been told to use 6800 ohms. 3300 ohms will be a little brighter.
370 ohms is not a standard resistor value and will kill the 9V battery very soon.

Maybe you should make a circuit that blinks an LED. Then it is turned off for most of the time so the battery lasts for a very long time.
 

audioguru

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Most Helpful Member
I designed two chaser circuits that light 10 very bright LEDs in a sequence. It runs around a few times then stops. Then it chases around again then stops again.
Each LED is lighted for a very short time then all the LEDs are turned off when it stops. So the 3V or 6V average battery current is very low and the battery lasts for months.
A pot adjusts how fast it spins and another pot controls the brightness with pulse-width-modulation but they are always set to max brightness.

I made about 12 of them in different colours. I gave some away, sold some and kept some.
The 3V ones keep blinking when the two AA alkaline cells drop to a total of 1.5V but then the red LEDs are fairly dim.

The parts cost about $5.00 ($100.00 at RadioShack).
 

BrownOut

Banned
Looking at Fry's... they list a 1/4 watt 5% carbon film resistor (seems expensive. .95)
Also a Power Resistor
and a SIP single in line resistor.
Which is the one I need?

Fry's??? You must be on the West Coast.

You want the 1/4Watt carbon film resistor. You can typically get a package of 5 or so for a couple bucks.
 
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MikeMl

Well-Known Member
Most Helpful Member

audioguru

Well-Known Member
Most Helpful Member
Carbon film 5% 1/4W resistors are $.05 each, aren't they?
I bought an assortment of about 200 for $4.00.
 

BrownOut

Banned
What's your point? He doesn't need 200. Either way, it's just a few cents per part.
 
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