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Running a LED below Vf

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StudentSA

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Hi,

I have built a simple LED lamp using two 1.5V batteries in series with a 1W white LED which has a specified min Vf of 3.0V and max Vf of 3.6V.

What are the implications of running this LED without a resistor and at Vf or lower voltages?

Kind Regards,
StudentSA
 
Not much as long as your sure the total applied voltage is less then 3.6V - the LED will do its own current limiting
 
It will be very difficult to predict the current, and the current will vary wildly with temperature and battery state. Different LEDs to the same spec could have very different brightnesses and battery lives.

Do you have some way of monitoring the current?
 
Hi,

When you run an LED at lower current levels (assuming you do it right) the life of the LED goes up more than the ratio of nominal current to lowered current levels. For example, if you run a 20ma nominal LED at 10ma, you'll get something like 3 or more times the life out of the LED. Life here is defined as the time to the point where the LED becomes lower in brightness by a factor of 0.707 or approximately 70 percent of its brand new brightness.

The other 'advantage' is that you get more brightness per milliamp. In other words, if you run two 20ma LEDs at 10ma you'll get more than twice the brightness of one LED running at 20ma. That's because of the curve of the LED.

The other advantage is of course less heat generated.

The disadvantage is less brightness ie running at 10ma instead of 20ma gives less overall brightness of course, although it is more than half as bright.

I run one of my LEDs at 100ua average although the peak is more like 20ma. The advantage there is battery life although this is an indicator application not a general lighting application.
 
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If the LED specifies a minimum of 3.0 V; in a few minutes of operation, the pair of 1.5V new cells will be discharged to under 3.0V and the led would be very dim or not turn on at all, while the cells will still be nearly 'new'
No harm will be done to the LED nor to the cells.
 
Not much as long as your sure the total applied voltage is less then 3.6V - the LED will do its own current limiting
Please do not make wrong statements like this. LEDs limit voltage, not current.

If a 3V LED is connected to a 3.5V battery without a current-limiting resistor then its current will be extremely high and it will quickly burn out.
 
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