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Hi ive attached a led to a couple of rechargeable batteries it begins with a bright light and starts fading and then reach a point where it doesn't fade out any more and stay like that for days. what shall i do to make the illumination of the led constant?
P.S. No capacitors were used

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## 7 Replies

seandogue (author)2010-04-01

It starts with a bright light because the voltage on a freshly charged battery falls off very quickly in time to it's steady state value, so if using a simple resistive regulator (resistor in series with LED) the voltage across the diode junction is high, then drops to its steady state value, and a corresponding change in brightness is observed.

To make the brightness constant over the !life of the battery charge (until the voltage across the cell terminals is too low to be regulated), one would build a constant current regulator to drive the LED, rather than using a simple resistive regulator

steveastrouk (author)2010-04-01

Have you got a resistor with the LED ?

electrod (author)2010-04-01

no and if you could tell me whats the use of the resistiors in this case i would be grearfull.

steveastrouk (author)2010-04-01

Like Rory says, look at the link, BUT if you can find a figure called "Forward volts drop"  and "typical forward current" you can design the resistor by
Resistor = (Supply volts - forward volts)/ forward current.

Otherwise, for old fashioned little LEDs, recking Vf = 1.2 V and If= 20mA.

Steve

electrod (author)2010-04-01

the led is not burned and if i turn the current down then start it again in around 15 m it goes bright again and start fading after a while again.

steveastrouk (author)2010-04-01

The resistor limits the current that can flow in the LED. You've been very lucky that the battery and your LED are pretty well matched, and the LED is still alive. It won't be though, unless you add the resistor.

Steve

Rorymi6 (author)2010-04-01

Ok what you have burned out your LED, this is because the current passing though the led was too high. You need a resistor in series with the LED.
www.doctronics.co.uk/resistor.htm

Also the LED will not work again
Rory