LED resistor value


lmackattack

lmackattack

old school
Hi Guys.

I bought a 12 pac of dual color (red/green) 3mm LED.to make signals to note switch positions. I need to figure out the resistor value needed to light them properly.I have a 12V input The paperwork I have is a little tricky for me to follow it says the LED has the following...
------------------------------------
Forward current
Red 20 mA
Green 25mA
(peak 60 mA)
----------------------------------
Forward voltage
Red 2.1v -10mA
Green 2.0 - 10mA

Reverse Voltage
3V

What value resistor do I need?

thank you!
 
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I wouldn't trust the values at the Halloween link since they are based on a 9 volt power supply. There's an easy calculator at http://www.muzique.com/schem/led.htm. Just enter the voltage supply (presumably always at least 12 volts in our case), forward voltage, and LED current. Press the Calculate button and you'll have your answer. I always go about 10% above the calculated value to account for fluctuations in the power supply. You can ignore the reverse voltage when calculating resistor values.

Edit: Since these are bicolor LED's, use the higher current and forward voltage to calculate the resistor value.
 
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UP2CSX said:
I wouldn't trust the values at the Halloween link....
Click to expand...

I don't mean to be argumentative, but why not? The LED calculator lets you enter the power supply voltage. What's not to trust? After that, physics is physics and math is math regardless of what holiday is coming up.
Doc
 
thanks guys.

I figure I need a 396 ohm Resistor for both red and green.
this was calculated with a 12V power supply with a 2.1V forward voltage and 25ma current.

does this sound correct?

Thanks again
 
I just checked my tortoise doc's and they say no resistor is needed when wired in series with the motor.
I still have to wire mine for indication so I'll see how you do with yours before I blow anything up myself!
 
Trent, I'd go a little bigger, say 500 ohms, again to account for power variations. You can hook up the LED's temporarily to see if they are still bright enough, but they should be fairly bright even with an 800 ohm resistor. Since resistors get hot, I like to make mine as large a value as possible and still have the lights be bright enough.

Rico, are you saying that the Tortoise documentation says you can wire LED's in series with the motor and not use a resistor? This doesn't sound right. First, it's never a good idea to wire LED's in series. Second, the SPDT contacts output 12 volts, so I can't see how they could power an LED without a resistor. :confused:
 
Jim, what they are saying is the Tortoise will only draw 15-16ma at stall which is all the current that will be in that leg. The voltage will be dropped approximately 2 volts at the LED and 10 volts at the Tortoise. This setup is only good if the turnout toggle remains "on" supplying voltage and not a momentary. The built in switches are dry contact and do not supply anything. (A power source must be connected, either the turnout power or external.)
 
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Ah, Rex, that makes more sense. Still, sounds like a tricky way to wire LED's, leaving the power to the Tortoise on at all times. Since it's a stall type motor, I guess there's no reason you couldn't leave the power on all the time, but I still like resistors, just in case something goes haywire.
 
Yeah Jim, me too! I wire my LED indicators off the built in switches using an external 12volt bus for supply, but my stationary decoder has a timed output. Oh yeah...and a resistor. :D
 
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