At 01:36 PM 3/27/02 -0500, M. Adam Davis wrote: >>Using forward biased diodes as voltage references is shoddy >>design. It isn't just cheap, it's skanky. >It's not just cheap, it's /very/ cheap. You can get a dozen red leds >for the cost of your inexpensive precision reference, and if you're >building the thing and sourcing the parts then it's a completely valid >design. When you make a million of them, the 19 cents per LED you save >ends up being nearly $200,000. You can get red leds from hundreds of >suppliers, and lead time is zero. > >So from a design perspective I can see it isn't ideal, but from a >manufacturing, profit perspective it is /much/ better than a voltage >reference. Remember that this is a value added troubleshooting feature, >and (IIRC when I built mine) it only gave the chip the ability to >monitor the voltages used - it isn't part of any feedback loop. And it >gives you a power on indicator that you might've added anyway, so you >are killing a few birds with a 1 cent part. One other thing to add fuel to the fire: most *red* LEDs have a pretty darn reasonable tempco - better than some cheap zener diodes! Sorry to disagree with Dave but an experienced designer working with a parts purchaser who understands and obeys a 'Do Not Substitute' label can effectively use LEDs as voltage references. But I agree on one thing: its not a good hobby technique just because of the variety of LEDs available. dwayne Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu