Check your LEDs if in doubt

[jrock64]

I was putting together some LEDs for Halloween and went to calculate the resistor.
Supply voltage 5v. Voltage drop, did not know, but they suggested 4.5 for a white. desired current 15ma equals a 33ohm resistor.

Set it up on a board and am almost blinded when I power it up.
Switch to a 68 ohm and it is still as bright.
The 100 is still no dimmer.

Grab the meter and try to get a ma reading, not used to doing this.

The 100ohm is still reading 20ma.

Play with the calculator and discover the voltage drop is only 3.2v

To get to 10ma I need a 180 ohm resistor.

Oh yea, that 33 ohm I started out with.....

Was pumping out 50ma into a 20ma LED, no wonder it was so bright.

Joel

[P. Short]

Sounds like you should also check the voltage/current even if you're not in doubt. Especially if you purchased the LED from an Ebay vendor or one of the Internet surplus vendors. The accuracy of their specs are always in doubt as far as I'm concerned...

This is also one of the reasons to go with a constant-current source rather than just a dropping resistor. There may be more than a bit of variation from LED to LED as well as the 'average' V/I curve being off.

[Dennis Cherry]

Also the nominal current of 20 ma is for the published brightness. You will find that maybe fades are slow to react until the voltage drops low enough then the fade is faster and might not look right.

I recommend setting the nominal current slightly lower, about 17 ma and possibly using a maximum intensity of 75% in your program settings.

The maximum current of an LED is normally about 150-250% higher that the nominal forward current.

If you are using half wave LED's you can convert them to full wave, this has 2 advantages.

1. The perception of flicker is greatly reduced. Some people can see it better than others. The LED's on half wave are on 60 times a second while the full wave LED"s are on 120 times a second.

LED's are fast turn ON/OFF devices not like incandescence that have a filament that has to warm up and cool off.

2. Another issue is the reverse voltage of a LED which is about 5 volts. A voltage surge can blow or damage some LED's on a string. One way is to add a standard diode in series with each LED series string. Something like a 1N4004-4007 diodes, this will give you some added protection against a reverse power surge as this standard diode can handle up to 120 volts in the reverse polarity.


Just my 2 cents worth.

[budude]

Just one note - the OP was using a DC supply of 5v so there should be no issues with flicker but your points are all correct for AC based LED strings.

[Entropy]

I was putting together some LEDs for Halloween and went to calculate the resistor.
Supply voltage 5v. Voltage drop, did not know, but they suggested 4.5 for a white. desired current 15ma equals a 33ohm resistor.

Set it up on a board and am almost blinded when I power it up.
Switch to a 68 ohm and it is still as bright.
The 100 is still no dimmer.

Grab the meter and try to get a ma reading, not used to doing this.

The 100ohm is still reading 20ma.

Play with the calculator and discover the voltage drop is only 3.2v

To get to 10ma I need a 180 ohm resistor.

Oh yea, that 33 ohm I started out with.....

Was pumping out 50ma into a 20ma LED, no wonder it was so bright.

Joel

Who suggested 4.5 for white? Typical Vf for white LEDs is the same as blue/truegreen (in fact, white LEDs are blue LEDs with a phosphor coating), which is typically 3.2-3.6 volts.

Since human vision is logarithmic and LEDs drop significantly in efficiency once you start overdriving them, you won't see very much difference in brightness if you calculate assuming a worst case Vf of 3.2 and it turns out to be one of the outlier 3.6 volt LEDs.

[Dennis Cherry]

Just one note - the OP was using a DC supply of 5v so there should be no issues with flicker but your points are all correct for AC based LED strings.

Over looked that part about DC, but the current setting of 17 ma and using a maximum light intensity of 75% in your sequence software is still recommended.