New Classic Finds and etc. (Long)

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Jan 26 17:20:39 2004

> Woah! Stop right there! The voltage _might_ or _might not_ be critical
> depending on the nature of the circuit just to the left of the plug (i.e.,
> a 7805 can take a wide range of voltages), but I have seen other devices

However, rememebr that the dissipation of a linear regulator (like the
7805) increases with the input voltage. IIRC, the 7805 is specified to
work on a 7V to 35V input, and will supply a maximum current of 1A. The
output, of course, is 5V.

Now, if we give it 7V on the input, the power dissipated (neglecting the
small amount used by the 7805's control circuitry) is (7-5)/1 = 2W

If wie give it 35V, the power dissipated is 30W.

The former may well be OK with a fairly small heat sink, the latter will
almost certainly cause things to overheat.

So please don't think a 35V AC adapter is safe just because there's a
7805 in there.


> blow from being just a volt or two off. Some things work at *only* the
> rated voltage.

In gneneral, such devices use AC adapters with internal regulator
circuits. You _must_ use a regulated adapter with them.


-tony
Received on Mon Jan 26 2004 - 17:20:39 GMT