Kathy Wrote:
> I had a similar problem with a 7805 running from a 24V source, the load
was
> a 555 timer, 4020, 2716 eprom and a 74ls373  driving 3 opto triacs IIRC.
not
> much load, but the 7805 had to be mounted on a 1.0oC/W heatsink to keep it
> working, since then I have mad a design rule, all 7805's are fed from 8
Vdc.
> I know they have a wide input voltage (I have stack (not sure of PN) that
> were 7805 compatible that had 120VDC input rating).

I see problems like this all the time and it bears going back and
understanding what is happening.

As a rough guess, I would think the circuit was drawing 300 to 350 mA
(assuming the 555 is bipolar and 2716 is NMOS) - according to the
datasheets, well within the output range of the 7805.

But, remembering that the formula for power dissipated is:

Power = Voltage Across the device * Current

In your application, the 7805 has 19 Volts across it (24 Volts in - 5 Volts
out) so the total power dissipated in the 7805 is:

Power = 19 Volts * 300 mA
      = 5.7 Watts

This power level is going to put the 7805 into thermal shutdown (crowbar)
pretty quickly.

> These days unless cost is an issue I either use a multi tap transformer,
or
> I use switching regulators.

Maxim has some nice regulator chips that are pretty inexpensive - on a
production board, a power supply using these chips would cost you about as
much as a 7805 and you don't have the heat issues (and if you are running
from batteries, the circuit will run a *lot* longer).

myke

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