Lighting an LED when I don't know it's voltage rating..

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Lighting an LED when I don't know it's voltage rating..

I'm in the process of converting an XArcade control panel into a test bench panel. I've done all the rewiring, but I noticed there is a small red LED mounted in one corner set flush with the top of the CP. Apparently it was a power LED to show the MAME box's IPAC-clone had power.

I'd like to hook it up to the +5 of my switcher. My switcher is an older style that doesn't have an LED. I know the +5 is fused internally, and that it can shut down from an overload, and be restarted later (sometimes) after a few minutes. An LED would let me know whether the +5 was active or not.

The problem is - the LED looks to be epoxied into place. No removing it to look for markings. I know I can but a resistor in to drop the +5 down to whatever the LED needs (maybe 3v?) but I need an idea of the voltage requirements of the LED before I can figure out which resistor to use (already found an online calculator that will tell me the resistor size).

What do y'all think?
 
Start at 2.1 to 2.3 volts for operating voltage and 20ma current on a standard red LED and adjust your resistance as needed to darken or brighten it. I've never burned one out doing that.

R = (Vsource - Vled) / Iled

Vsource = 5.0v
Vled = 2.3v
Iled = .020 (20ma)

R = (5.0-2.3)/.020 = 2.7/.020 = 135 ohms

Or just fudge it to the next closest standard value.
 
Thanks. I looked around and only had a 100-ohm resistor handy, but I used it and it seems to be working just fine.

+5.10--->100-ohm resistor--->+1.85--->LED--->Ground

Nice and bright, too, but not too bright....
 
Put the LED between your finger and thumb. If it's not burning your finger then it'll last for decades.
 
I touched it for a bit, and didn't feel anything.

Sweet!

Thanks...
 
The way I've done it many times in the past is to put a pot in series with it, turn the pot to full resistance then apply voltage and dial it down to the right intensity, then remove the pot and Ohm it out with a meter.
 
Thanks. I looked around and only had a 100-ohm resistor handy, but I used it and it seems to be working just fine.

+5.10--->100-ohm resistor--->+1.85--->LED--->Ground

Nice and bright, too, but not too bright....

Might be too much current thru the LED.
Based on what you have above -- the LED has a forward voltage drop of 1.85V.
So - based on the 5V source and the 3.15V drop across the resistor (5 - 1.85 = 3.15+ -- then you have 31.5mA going thru the resistor... and the LED.
Most LED's are rated for 20mA continuous current and 30mA absolute maximum current.
I normally shoot for between 10mA and 20mA.

Based on 20mA LED current - you want a minimum of 157 ohms. Closest standard value with out going down in value would be 160 ohms.

Ed
 
Led forward drops

Almost all modern LEDs have a forward voltage drop of between 1.5 and 1.8 Volts and average currents of 5 to 40 mA. This includes most 3mm and 5mm LEDs in all colors EXCEPT white and blue, made in the last 20 years or so. White LEDs are actually blue LEDs with a phosphor coated cap which re-emits broad spectrum light. White and blue LEDS typically have forward voltage drops of 4 Volts and higher current requirements.

The formula for calculating the resistor from a DC source is: (Supply Voltage - Forward Voltage Drop) / Desired Current. So if you have a 5 Volt DC supply, a 5mm red LED and want it to be reasonably bright you would want approximately 20 mA of current. (5 V - 1.7V)/20mA = 165 Ohms [3.3v/.020 = 165 Ohms] (adjust for desired brightness higher resistance=dimmer light)

The calculation works for AC RMS voltages too but is not terribly accurate. The LED is a fancy diode and will half-wave rectify the AC and still work, although it flickers at the line frequency. (ie 60Hz)
1.7 Volts is the 'ballpark' that most designers use for regular LEDs and 4 Volts is the 'ballpark' for blue/white LEDs forward drops. There are huge high output diodes available now, with exotic forward drops and current requirements, but you would probably be able to find a datasheet on those. Multicolor LEDs are another story.
Hope that helps!
Kenneth
 
The way I've done it many times in the past is to put a pot in series with it, turn the pot to full resistance then apply voltage and dial it down to the right intensity, then remove the pot and Ohm it out with a meter.

That's my approach as well.
 
It's usually a bad idea to reverse-bias LEDs.... the reverse breakdown voltage isn't well specified, so if you're going to be applying full-wave AC, put a standard diode in series to block reverse currents (and add .6V or so to the 'rated voltage'.
 
I'm running +5DC to it, through a resistor, to get 1.85DC.

It seems to be working fine, and I barely ever have it on for more than 10 minutes at a time...
 
I'm running +5DC to it, through a resistor, to get 1.85DC.

It seems to be working fine, and I barely ever have it on for more than 10 minutes at a time...

Not sure how you are calculating that. Assuming the LED is rated for 20ma at 2 volts puts its resistance at 100. In series with a 100ohm resistor on 5 volts gives 2.5 volts accross the LED. Maybe close enough to not worry. Ideally you would have a 150ohm resistor.
 
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