Hi people thanks for your help

I think i know what when wrong.
As the IR led is transmitting, the value that i get on the meter is the
average.
Therefore after some convertion , i manage to compute the value and it is
correct.

thanks anyway
----- Original Message -----
From: Wagner Lipnharski <wagner@USTR.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, November 28, 1999 9:09 AM
Subject: Re: constant voltage and current


> 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.