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led info needed

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spicydeath82

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hi, i have a dim green led, i wish to power it from a 3.3 volt source. What size resistor will i need for this? thanks in advanced.

ps. i realise this is a rather basic question but that's how we all started right? :)
 

ericgibbs

Well-Known Member
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hi, i have a dim green led, i wish to power it from a 3.3 volt source. What size resistor will i need for this? thanks in advanced.

ps. i realise this is a rather basic question but that's how we all started right? :)
hi,
The Vforward of GREEN LED's varies, look at this chart, which voltage is yours.?
 

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spicydeath82

New Member
not sure, but a computer system battery powers it fine, which is 3V. it also came from either a cd rom drive or a pc power indicator light.
 
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spicydeath82

New Member
also i just discovered that the power supply also puts out 18a on the 3.3v line, i'm guessing that 18 amps is it's max rating, but still that could be bad for running an led off of it huh? also it has a 5volt line with 25a and a 12v @ 15A. also has a 5V @ 2A(i guess this would be the one to use?)
 

Reloadron

Well-Known Member
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also i just discovered that the power supply also puts out 18a on the 3.3v line, i'm guessing that 18 amps is it's max rating, but still that could be bad for running an led off of it huh? also it has a 5volt line with 25a and a 12v @ 15A. also has a 5V @ 2A(i guess this would be the one to use?)

OK, first and foremost the 5V @ 2A line (rail) you see on a computer PSU is the 5 Volt SB (Stand By) line and don't screw with it. Even when the PC is shutdown as long as there is power to it and the rear power switch (if one exist) is on that line is always active.

Next, the remaining current ratings you see for the PSU are maximum ratings per rail but that does not mean that all those rails can be loaded to their max and the PSU will deliver.

The best rail to work with for messing with LEDs is the 5 Volt rail and not the 5V SB as I mentioned.

The reason you need to know the normal working voltage of the LED and the normal forward current it uses is so you can determine the value of a series resistor to place in series with the LED and its power source. That is done to limit the current through the LED.

I don't deny that you can place the LED across say the 3.3 volt rail and it will illuminate but if the LED is rated for 3 volts it will have a short life.

Ron
 

vlad777

Member
The PSU always gives same voltage but the current(amps) depends on what you connect to it.
Very important info (other then forward voltage) is LED's current and it is about 5 mA (0.005A).
This is how you calculate resistor:

Rled = (Vpsu - Vforward)/ Iled

Edit:
You can determine the parameters like this:
Slowly increase current trough LED until it is bright enough.
LED current ranges from 1-10 mA.
Then measure LED current and voltage.
 
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spicydeath82

New Member
yeah, i figured i wouldn't use the 5VSB line since i knew it was for something else.

Rled = (Vpsu - Vforward)/ Iled
ok i think i get this part(Vpsu - Vforward) this is saying voltage from the psu minus the led's forward voltage(?) but not entirely sure what Rled and Iled are. i know it's refering to a led but the I & R are throwing me off... lol.

as a side note lets just assume that the green led in question is 2.5 volts since a 3 volt lithium button cell runs it fine. i say 2.5 rather then 3 because i've read on here before that button cells have built in resistance or that they can't drain juice faster then a certain amount.
 

Reloadron

Well-Known Member
Most Helpful Member
R LED is the resistance you would place in series with the LED and I LED is the current for the LED. I LED is the current for the LED based on a data sheet of manufacturers info.

Ron
 

spicydeath82

New Member
koo. thanks. to bad i don't have a data sheet on it since it's salvaged from an old cdrom drive(or mobo, can't remember which)
so basically your giving me ohms law then? R=V/I or Resistance= (supply volts - led volts)/Led amps
well apparently i'd need roughly a 300Ohm resistor but any thing higher with in reason should work right? such as a 470 Ohm resistor?

here's my calculations then (5-2)/0.01= 300 this is assuming that the led is 2V and is 10Ma. does this seem right?
 

Reloadron

Well-Known Member
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Good starting point.

Ron
 

spicydeath82

New Member
My first working circuit

sweet thanks guys. now my power indicator light is working. the only thing left to do is secure it with some hot glue(and to insulate some things a bit).
This is an extra pc power supply i had but had no use for. It's new purpose will be my bench top power supply. it has several 12V, 5V and a couple of 3.3V supplies.
i imagine with a couple other small circuits attached i could make adapters to change 12V to 6V or 5V to 4.5V.

the resistor is a 100Ω. seems a bit small but the led seems fine and it's supply voltage is only 3.3V. and if it end up burning up then oh well lesson learned.

PS. the resistor and led and jumper are salvaged(recycled) parts.
SANY1610.JPGSANY1612.JPG
 

ericgibbs

Well-Known Member
Most Helpful Member
hi s82,
Unless I am going colour blind that dim Green LED is Orange.?:rolleyes:

If its orange, thats about 2Vfwd, so with a100R and 5V thats 30mA, which is a bit on the high side, I would suggest another 100R in series to make it 200R [15mA]
 

spicydeath82

New Member
@ Vlad777: lol, yeah i'm aware of how to start it. the led is being powered by an extra 3.3v line and ground. Till i get a proper switch i simply twist the green power_on line to another extra ground line. works fine. so the led is hovering above the piece of paper in the second photo.

@ Ericgibbs: nope not color blind. i decided that that i have too many orange LEDs laying around so decided to use one of them instead. Also it's being powered by 3.3V instead of 5V. from the calculations i made i needed at least a 130 Ohm resistor or greater but most of my salvaged resistors are way to high so the 100 Ohm one is the smallest one i had. i have a 33 ohm resistor laying around, maybe i can splice it in to bump up the resistance.


EDIT: hey guys, could i replace the jumper wire on the negative rail on my circuit with that 33 ohm resistor i have thus bringing the resistance up to 133ohms? or do the two resistors have to be on the positive side to accomplish it?

examples:
+ ----[100Ohms] ---|
( ignore this } ( ) led | will this work to bring the total resistance up to 133ohms prolonging the life of the LED?
- -----[33ohms]-----|

or does it have to be this way?

+ ------[100]--[33]---|
{ format spacer } ( ) led | will the above work? or is this one my only viable option?
- -----------------------|
 
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spicydeath82

New Member
koo, just got done swapping the jumper wire for a 33 ohm resistor and the led is a little bit dimmer, but to me it seems more appropriate. before it seemed a bit to bright. thanks folks.
 
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