Ok, now it is a bit clear.

The IR Led specs say that when you apply around 2.5V it will consume
around 250mA, or by the other side, if you apply around 250mA, it should
generate a voltage drop around 2.5V.

It is somehow easy to control a current across the IR Led, but first
lets understand what you said, that you only get a Vdrop of only 0.6V
and 6mA.

First, no IR LED emitter drops only 0.6V, some can works with voltages
low as 1.3V, and they vary from 1.5 to 4V, but no 0.6V.  Problem also is
about how you managed to get the 0.6Vdrop and 6mA of current.  I guess
you are using a voltage source and some resistence in series.

To test it better, use any power supply you may have around with a
resistor in series, and measure everything again.
Follows possible voltages and resistors:

+5Vdc - Use a 100 or 50 Ohms resistor in series with the LED.
+12Vdc - Use a 240 or 120 Ohms resistor in series with the LED.

Measure again the Voltage across the LED and current flowing through it,
and report back here, so we can help you better.

By the way, the current in a IR led is not critical, so, if the specs
say 250mA (this is a bit high for a IR led), lets imagine that you have
+5Vdc to feed it, so, 5V - 2.5 (Vdrop) will result in 2.5V over a
current limiter resistor, 2.5V / 250mA = 10 Ohms.  So, connecting a 10
Ohms in series with the LED and connectig it directly to +5Vdc will
result in what you want.  Are you sure about the 250mA?

Wagner.



soon lee wrote:
>
> Ok the thing is that i need to drive a IR led . according to spec it need a
> constant current of 250maA and 2.5 V across it. But when i used normal
> supply i only manage to get 0.6V drop and current of 6mA
> Is it possible to drive the led at 250mV??
> ----- Original Message -----
> From: Wagner Lipnharski <wagner@USTR.NET>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Sunday, November 28, 1999 2:17 AM
> Subject: Re: constant voltage and current
>
> > Dear Soon Lee,
> >
> > Unfortunately nobody can help you unless you say what you want.
> >
> > You can not have a constant voltage and constant current at the same
> > time, because changing the load, at least current *or* voltage will
> > change too, this is pure physics based on electricity rules.
> >
> > Yes, you can have a constant current generator, within limits of
> > voltage, when then the output current will go down upon a high load,
> >
> > or,
> >
> > you can have a fixed voltage generator, with limits of current, when the
> > output voltage will go down upon a low load.
> >
> > This is quite easy to build, nothing special, but it is impossible to
> > help you unless you say exactly what you want.
> >
> > Reading your post, I can guess you want to build a battery recharger
> > unit, but... you need to explain better.
> >
> > Wagner.