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Author Topic: LED Resistor value  (Read 1287 times)

craggle

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LED Resistor value
« on: November 10, 2015, 10:41:02 AM »

Hello all

Are there any electronics geniuses here that can help me out please?

I am replacing some surface mount LED's on a circuit board and I need to change some of them from orange to red.
The specs on the orange LED's are 3.4V forward voltage, 30mA forward current.
The Red LEDS are 2V forward voltage and 50mA forward current.

Can I assume that the supply voltage is already 3.4v to the LED as the orange ones currently work fine. If I can then an on-line LED resistor calculator suggests I need to add a 33ohm 1/4W resistor to my red led to get that to work correctly.

I have tried to follow the tracks back to find the resistor that is in place already but it's a multi layer board and it gets impossible to follow very quickly.

Thanks

Craig.
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Stavros

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Re: LED Resistor value
« Reply #1 on: November 10, 2015, 10:42:51 AM »

Have you tried using a meter to determine the voltage to the led's....this will then be your base figure

Dave
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craggle

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Re: LED Resistor value
« Reply #2 on: November 10, 2015, 11:36:23 AM »

Hi Dave

I haven't yet but may well try to do that tonight. I have a digital meter so should be able to get a good reading.
It just means plugging the item in without the plastic case around it so I'll be careful where I stick my fingers! :-)

Craig.
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Brian60

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Re: LED Resistor value
« Reply #3 on: November 10, 2015, 03:51:54 PM »

This link should help......

http://www.hobby-hour.com/electronics/smdcalc.php

Determine which resistors pull down the voltage for the orange led's by comparing what is on the board to this calculator, be careful they could be run in pairs! with some juggling of figures it should be easy to work out. Then change out those resistors for the ones you want in place.

By the way, have you tried soldering surface mount resistors or leds before? If not, good luck you'll need it!

malcolmfrary

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Re: LED Resistor value
« Reply #4 on: November 10, 2015, 04:20:03 PM »

What the previous posters said plus the actual supply voltage for the LEDs appears at the other end of whatever is limiting the current that the LED is allowed to draw. 
If it is just a resistor, it should be simple enough to change (double plus on the wishes of good luck dealing with SMD is you don't have the right tools).  You MIGHT get away with just changing the LEDs - the red ones SHOULD just glow a bit fainter and still be acceptable without cooking the limiting device.
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craggle

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Re: LED Resistor value
« Reply #5 on: November 11, 2015, 08:54:55 AM »

Thanks all for your help

I had a look again at the circuit last night and it's pretty complicated. Quickly applying a 1.5V battery across the LED in question make it glow but it also makes about 5 others glow faintly too so I assume they are connected in series.

I took a chance anyway and took it off the board and soldered in the red one and it seems to work just fine. Doesn't look too bright or too faint so I'm hoping the voltage difference is kind of absorbed in the other LED's and the resistors or the other LED's won't fail.

Time will tell I guess. The original LED came of the board okay so I can replace it again if the red one gives up.

Yes, They are small things to solder! I have a variable temp soldering station with a very small tip and 0.5mm soldering wire but it's still kind of fiddly!

Thanks again

Craig.
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malcolmfrary

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Re: LED Resistor value
« Reply #6 on: November 11, 2015, 09:48:03 AM »

Thanks all for your help

I had a look again at the circuit last night and it's pretty complicated. Quickly applying a 1.5V battery across the LED in question make it glow but it also makes about 5 others glow faintly too so I assume they are connected in series.

I took a chance anyway and took it off the board and soldered in the red one and it seems to work just fine. Doesn't look too bright or too faint so I'm hoping the voltage difference is kind of absorbed in the other LED's and the resistors or the other LED's won't fail.

Time will tell I guess. The original LED came of the board okay so I can replace it again if the red one gives up.

Yes, They are small things to solder! I have a variable temp soldering station with a very small tip and 0.5mm soldering wire but it's still kind of fiddly!

Thanks again

Craig.
On the circuit board they and their associated resistors would have been in parallel.  When the 1.5 volt cell was connected across just one of them, it would have glowed, not too brightly because of the internal resistance of the cell, but there would have been a path for current the wrong way through that LEDs resistor, and a series path the right way through all the others which would form a parallel network and be working at a much reduced current.
A lot of places give specs for LEDs that leave you a bit short on information - sometimes the current quoted is the max, above which the smoke comes out, other times it is a current that causes a given intensity of glow.  In this case, if the LED is glowing in a satisfactory manner, and its resistor isn't cooking, then the jobs a good 'un.  There was always a good chance that the resistance required to limit the current to one LED and result in 3 and a bit volts would be the right-ish value to limit a slightly higher current to the slightly lower voltage that the new LED wanted, the only uncertainty being the power dissipation while doing it.
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