I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks
You will have no problem using the 16V wall wart with the LM317. The
current rating on the wall wart tells you want it is CAPABLE of. In
other words, it tells you the maximum power output of the device. In
your case, you'll never need that much. Of course, the LM317 will draw
a SMALL amount of extra power, and so you want a power supply that can
provide GREATER than 0.5A. So you're in good shape with your supply
and your IC.
*) On a related note, it is easy to increase the current carrying
ability of the LM317. If you look at the application examples at the
end of the datasheet, you'll see how you can add external pass
transistors around the LM317. As the LM317 draws more current, it will
signal to the external power transistors that they should provide that
current. So the LM317 becomes a control circuit driving pass
transistors that actually carry most of the current.
*) On another related note, if you look at the LM317 schematic, you'll
find that internally it uses a Darlington pair as a set of pass
transistors that do exactly the same as what I just mentioned. It has
a bandgap reference to hold the 1.25V, and that reference drives a
buffer which drives the base of the internal pass transistor which
delivers all of the power to the output.
--Ted
