Green LED power consumption

[Cirkit]

I have replaced a red LED (Vf = 1.9V) in a circuit powered by a 3V battery supply with a green LED (Vf = 3.3V). Both LEDs have a specified If of 20mA. As the batteries are Lithium, the voltage starts off slightly higher at approximately 3.4V when new.

The current through the green LED is much lower than the red one using the same current limit resistor. Is the green LED likely to cause the batteries to run flat faster? I assume that the lower current doesn't necessarily mean the overall power consumption is less since Vf is higher?

I needed the higher brightness hence, chose that particular green LED instead of one with a lower Vf.

 

[dorke]

You can calculate it very simply.
Since you haven't noted the resistor value(assuming there is one).

and assuming the original red led is driven at 20ma with a lithium battery voltage of 3.7V we get:
R=(3.7-1.9)/20ma=90 ohm

With the smae resistor for the green led we get:
igreen=(3.7-3.3)/90=4.5mA

So,
the current with the green led is 4 times less.
that is the battery will last about 4 times more.

With that lower current, the intensity of the green led is very poor...
you should adjust lower the value of the resistor.

BTW,
Where did you get the 3.3V for the green led from?
They usually are 2.2v

 

[Cirkit]

The current limit resistor is 1.2k. If overall power consumption is V*I and the forward voltage is higher but the current lower, the power drawn from the battery will be the same or greater? Or does the rate at which the battery will run flat only depend on the current through the LED and resistor?

 

[dorke]

Something doesn't compute:
With a 1.2K resistor that red led current is:
(3.7-1.9)/1.2k=1.5ma
much less than the led 20ma "advertised current"

To your questions:
With the same resistor,
the higher the led Vf ,the lower the led current(= battery current).
The time the battery will be drained(run flat) is dependent on it's current.

 

[Cirkit]

I did originally think that the resistor value was very high however, I presume it is to keep current consumption low and the LED is really bright anyway.

 

[dorke]

I did originally think that the resistor value was very high however, I presume it is to keep current consumption low and the LED is really bright anyway.

In this case,
try using a high efficiency 2ma LED

 

[BobK]

A green LED with a Vf of 3.3V is likely a high power LED. Typical 20mA green LEDs run around 2.2V. If the battery starts at 3.4V and drops from there, the green LED will grow much dimmer as the battery depletes. Get a normal 2.2V green LED, a high brightness one if you want more brightness.

In answer to your question about power:

The power is the battery voltage x the current. It is split between the LED and the resistor. For the same current, it does not matter what the Vf of the LED is.

Bob