Simple Power Supply Stuff

[R.G. Keen]

>The questions are:
>
>1.  When running like this, each supply is dropping about 4.3 Volts (1.3V
>over the minimum) at .75A.  After a few minutes, the heatsinks seem to get
>too hot to touch, but the regulators do not shut down!  How can I tell if
>this is too much heat for normal operation?
"too hot to touch" translates to 115-130F. This is about 49C, so you're
probably getting a 25C rise over ambient. 25C/3.2W = 7.8C/W, a very small 
sink. Moto says that the internal thermal resistance is 5C/W, so the junctions
will rise about 13C for each watt, or 41.6C with the sink. At room temperature,
the junctions are sitting at about 66C. I can't find a Tmax spec, but Moto 
says that the max junction temperature is 125C for the TO220 package, so you
have another 59C to go - which translates to 4.5W or another amp or so of
current. The current limit will stop it before the temp cutout does. 

If you stick it inside a box with little airflow, you may be able to make it 
temperature limit by raising the effective ambient.

If the sinks immediately burned or blistered your finger, the actual
temperature would be much higher of course... :-)

>
>2.  The power resistors also get very hot and actually put off an odor from
>the heat.  Is this normal?
It depends on how new and what technology. Resistor ratings are based on about
a 100C temperature. I'm assuming you are using two 10 ohms in series to 
get 0.75 (0.73) amps from 15V. Each resistor is dissipating 5.6W - so yes, 
they will be getting too hot to touch, too, probably. 

It's all reasonable, and probably not near failure. I like things cooler, too,
though.

R.G.