On Sat, 30 Sep 2006 14:12:59 GMT, Al wrote:

In article , jasen
wrote:

On 2006-09-29, Al wrote:

If your LED is specified at 10mA, then get two diodes whose forward
voltage is 0.75V at 10ma. Put them in series with your LED. If the
supply voltage goes up and the current goes up, the forward voltage of
the diodes will go up also thereby dropping more voltage across them.

yeah, if the current doubles the voltage may increase maybe 10%
how's that going to help?

Thus the dynamic resistance of the dropping diodes serves as a voltage
regulator. Really good diodes with a low dynamic resistance won't do.

it seems to me that regular resistors would work better,

If your current doubled for some reason, you have other big problems.

---
I'm not trying to be rude, but the problem would rear its ugly head
if your suggestion was followed. Take a look at what it takes to
double the current through pretty much _any_ diode once it's on the
far side of the Vf knee and you should be able to see that your
suggestion, if followed, would be inviting disaster. The LED(s)
need to be fed from a constant current source. Period. End of
story.


--
John Fields
Professional Circuit Designer