Electrical schoolwork...

Huw Powell audi at humanspeakers.com
Tue Feb 10 02:31:49 EST 2004

> Although the current wasn't given, this really the only way to
> calculate anything for an LED.

So why aren't you guys asking him for the current rating for the LED?

Wihtout it, you're just shooting for the hip...

> Oh, and the 15mA is just a usual figure for red LED's...

Since this was an assumption, it should have been stated (ie, "for a 
15mA LED...")

> Basic electronics 101 here.  I thought you'd know that well, but
> apparently not.

I'm holding my own with Ohm's Law, thanks...

> An LED is not a resistive device, thus you cannot calculate based on
> that assumption.  An LED is a light emitting *DIODE*. It has a
> relatively FIXED forward bias voltage.  That 150K ohm "LED
> resistance" is an irrelevant spec.  What *is* important is the
> forward bias voltage and current requirement of the LED, and the
> total supply voltage.  Then the required resistor value can be
> calculated from that.

Yes.  If he had stated the current rating, your calculations would have 
had some merit.  As it was, you introduced an unstated assumption.  As I 
said above, without the current rating of the LED, you're just winging 
it.  They do range, after all, over at least an order of magnitude, right?

And, of course, I should also have noticed that the basic spec was 
missing at the start.  The dust will settle while the rest of the list 

Huw Powell



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