Small flat LED replacement

[Fuganater]

So I got a temp probe for my WC setup and the display is blue... boo. I want it to be red but it has this weeeeee little flat LED. I've never seen these to buy.

http://i11.photobucket.com/albums/a190/Fuganater987/Project%20Rage/DSC05934.jpg

Can anyone tell me where to get a red one? and how to figure out how much power the current one has and what the new one needs?

Big thanks in advance!
-Fuga

[xr4man]

you can get them from digikey, mouser, or newark. just look for a size 1206, 0805, or 0603 depending on how big that one is.

you will also have to change the resistor that is inline with the led. since blue leds drop more voltage than red leds (usually) you will need a bigger resistor to drop the voltage for the red led.

[Fuganater]

there is no resistor....

[SXRguyinMA]

there has to be one somewhere in the circuit

[Fuganater]

I'll take pictures of the circuit board tomorrow so you can see. Unless its some small crazy one I don't see it.

[slaveofconvention]

It's probably a standard SMD LED - you can measure voltage across it with a multimeter like any other LED - just use the metallic pads on each side as contact point.

SMD LEDs are pretty cheap to buy so you could always buy a few of each possible size and play trial and error. You might want to try soldering a couple to something else though, for practice as they can be pretty fiddly...

[xr4man]

that's good advice. it is really easy to over heat them when soldering and destroy them. i've done that more than once.

if there is no resistor at all in the circuit with the led, then that means the board is putting out the exact voltage the blue led is rated at and you will have to add in a resistor for a red led.

[Fuganater]

Its 3.10v across the LED.

No resistors that I can see.
http://i11.photobucket.com/albums/a190/Fuganater987/Project%20Rage/DSC06204.jpg
http://i11.photobucket.com/albums/a190/Fuganater987/Project%20Rage/DSC06205.jpg

[xr4man]

i see 8 resistors on that board. they are the small black rectangular parts with white numbers on them.

it appears that one lead from the led is going to the resistor on the far right in the top picture.

[Fuganater]

That resistor says 101. How do I find its rating and where do I find a new one that would work with a red led... which i haven't found yet...

[xr4man]

the first two numbers are the first numbers in the rating. the last number is the number of zeros after the first two numbers. so that 101 is 1 and 0 and 1 more zero. so it is a 100 ohm resistor. it is more than likely a 0603 size, judging by the size compared to the wires soldered on.

you can go here http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator to figure out what resistor you need.

usually the blue leds drop about 3v and the red leds drop about 2v the current rating is usually about 20mA.

so if you put in 3.1v for the supply and 2v for the voltage drop and 20mA for the led current, then click calculate. it gives you a resistor of 55 ohms. now you don't have to use exactly a 55 ohm resistor, the closest common size resistor is 56 ohm and you can always go a few more ohms higher if that's what you happen to have or can find. the required wattage of the resistor is .022W. 0603 resistors are usually about .1W, so that is fine. now, since you can't just go to radio shack and get 0603 surface mount resistors, i would just get a standard 1/4 watt leaded resistor and solder it in place of the surface mount resistor. or an easier thing to do would be to solder a wire across the top of the 0603 making it 0 ohms and then splice in the 1/4W resistor to the wire itself.

[xr4man]

something i just thought of. did you take your voltage measurement across the two wire going to the led? if so, then you want to leave the 100 ohm resistor alone and add in the 56 ohm resistor to the wire. therefore having both resistors in series for a total of 156 ohms.

[Fuganater]

I took the measurment across the LED leads.

I've been looking at mouser and still can't find a soutable LED that looks the same as the current one.

I found these on a diff site.. these right?
http://www.lc-led.com/View/itemNumber/124
http://www.lc-led.com/View/itemNumber/54
http://www.lc-led.com/View/itemNumber/35

[xr4man]

i would go with the 0805 leds. they are a touch smaller than the 1206 and a bit bigger than the 0603, but they are still big enough to solder without a microscope if you have a steady hand. also the 0805 will fit on a 1206 pad if the original led happens to be a 1206 and with a little finagling it can fit on a 0603 pad as well.

without getting into a bunch of detail, it is sometimes difficult to see which end of the led is the cathode and which is the anode. so don't be surprised if you solder it in and it doesn't work. that just means it's in backwards. take it off, throw it away and solder a new on on in the opposite direction. it's a very strong possibility of damaging those small leds when you try and remove them, so it's best just to grab a new one if you have to remove it.

[Fuganater]

Maybe I'll just buy 5 of the 0805 and the 1206.

But what resistors? For the 0805, using your calc, i think i need 106 Ohms And for the 1206 I need 132 Ohms.... I think I'm doing the math right. 5V supply, 0805 is 1.8v and the 1206 is 1.7v, the 0805 is 30mA and the 1206 is 25mA...

That all right?

[xr4man]

yeah, that sounds right.

they're cheap enough i'd buy a few of each too.

since you are ordering parts anyway, i'd go ahead and get the resistors too. get 10 0805 and 10 1206. that'll probably come to about $0.50 total. chip resistors are cheap.

[Fuganater]

So...

this (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=99996119&uq=634408148790792421) and this (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=99996363&uq=634408148790792421)

or

this (http://www.mouser.com/Search/ProductDetail.aspx?R=CR0805-JW-103ELFvirtualkey65210000virtualkey652-CR0805-JW-103ELF) and this (http://www.mouser.com/ProductDetail/Bourns/CR1206-FX-1330ELF/?qs=sGAEpiMZZMu61qfTUdNhG6LqqFhkr6pUiAhtUQpxKI0%3d )

[Fuganater]

http://www.mouser.com/ProductDetail/Vishay-Dale/CRCW0805107RFKEA/?qs=sGAEpiMZZMvdGkrng054tygjBeyq%2fOAOxAkgNvFlDFE% 3d

for the 0805 mouser resistor. The original one I posted is 10kOhms... oups

[xr4man]

the ones from digikey didn't show up. but the ones from mouser look right.

i would buy from mouser because digikey has a $25 limit on orders. if you don't go over that, they charge a handling fee. i don't think mouser does.

also since they are so cheap, i'd buy a few different values. like 100, 130, 150, 180 ohms.

[Fuganater]

On a electronics forum they said I need a 180 ohms resistor....

[xr4man]

that works too. you probably wouldn't be able to tell the difference in light output between using a 156 or a 180 ohm resistor. the 180 would be "safer" to use probably.

[Fuganater]

Ok so I bought 25 of each LED (couldn't get less) and 10 of each of the resistors. We will see how it goes.