slightly OT LED question

[matg]

can i wire , say, 4 LED's rated working voltage of 3-3.5VDC in series to create a 12-14VDC load? or does an LED rated 3-3.5VDC still work on 12 VDC with a shorter life or am i just gonna blow it? TIA, mat g.

[steve_m]

An LED will have a constant voltage drop across it...in your case, 3 volts. You have to use a series resistor to limit the current to 20 mA.

But yes, if you wanted to create an LED array or soemthing and had 12 volts available, use a series resistor and 4 series LEDs.

[thespeakerguy]

Provided the LEDs are same mfr. part, yes they can be put in series.

LEDs have different voltages. It has been a while, but reds are about 2V, yellow and green about 3V, and blue (white) are closer to 4V. Moral of the story is check what you have and bias to desired intensity.

Do not connect an LED directly to any voltage source. The current must be limited with a resistor, otherwise you let the smoke out.

[mikec]

Can we do the series thing with lightbulbs because the filiment is the resisitor which is self-limiting?

So treat LED's as if they were circuits that happen to emit light and protect them accordingly?

Learn something while here again!

________
LIVE *** WEBSHOWS

[envisionelec]

> Can we do the series thing with lightbulbs
> because the filiment is the resisitor which
> is self-limiting?

> So treat LED's as if they were circuits that
> happen to emit light and protect them
> accordingly?

> Learn something while here again!

Yuppers. I use a lot of very high power LEDs here at work. The regular line of thought is to vary the voltage to modify the brightness. However, the proper method of control is to vary the current delivered because the LED voltage doesn't appreciably change over a range of currents.

[matg]

o.k., so i need to put a series resistor in the circut to protect the LED's,no problem, but how do i determin the resistor size or ohm. I know that the current is supposed to be limited to around 20-30 Ma depending on the number of LED's in the array, but for a 12VDC power source (basicly a car battery with a few solar panels charging it) how do i figure the resistor size?

> Yuppers. I use a lot of very high power LEDs
> here at work. The regular line of thought is
> to vary the voltage to modify the
> brightness. However, the proper method of
> control is to vary the current delivered
> because the LED voltage doesn't appreciably
> change over a range of currents.

[matg]

Am i just looking at a simple ohm's law calculation to figure the size of the series resistor for the LED protection, knowing my supply voltage 912VDC), knowing the current needed to run the LED's (20-30mA), apply ohms law to get the resistance?

> Yuppers. I use a lot of very high power LEDs
> here at work. The regular line of thought is
> to vary the voltage to modify the
> brightness. However, the proper method of
> control is to vary the current delivered
> because the LED voltage doesn't appreciably
> change over a range of currents.

[envisionelec]

> Am i just looking at a simple ohm's law
> calculation to figure the size of the series
> resistor for the LED protection, knowing my
> supply voltage 912VDC), knowing the current
> needed to run the LED's (20-30mA), apply
> ohms law to get the resistance?

You got it.

Fer instance.

3 Red LEDs on 12V wired in series:

1.8V x 3 = 5.4V
12V - 5.4V = 6.6V
20mA per LED
6.6V/.020A = 330 ohms

So, you'd wire a 330 ohm resistor in series with the string of LEDs.

[philip]

20ma is maximum current, usually provides very brite LED ilumination. Try using 10ma (.010A) or even 5ma (.005A) in your calculation for a not so brite LED ilumination.

[matg]