> While I was doing a search recently, I came across this page:
> and was wondering what the electrically knowledgable members here thought
(warning, technospeak follows)
While that circuit may work well for building lighting and accessories, I see
problems for trains.
The most significant problem with that simple circuit is that you cannot
reduce the voltage below 1.2 volts with the throttle knob at zero. Its not
the fault of the circuit, its just the normal way the LM317 works: its a
(fixed) 1.2v to (programmable) 30v dc output device. Some of the new 5 pole
motor locomotives would still be moving right along at 1.2v dc.
Second, the lack of any meaningful input capacitor would leave rectified, but
unfiltered, DC going to the trains. Worse, the waveforms would have "flat
tops" not curved tops (i.e., pulse). While that might be an asset at scale
speeds of less than 10 miles per hour, you'd hear a clear "buzz" coming from
the motors at higher speeds and motor heating would be another concern.
Adding a large capacitor on the regulator's input causes the circuit to put
out pure DC. Problems gone.
Last, there are several diodes omitted which normally protect a LM317 from
Try this page:
Ignore the 3 amp title. The output of the circuit is determined solely by
the 3 terminal component you use (LM317 = 1.5 amp, LM350 = 3.0 amps). The
circuit is the same with either part.
As you can see from the excellent text, you can compute the resistance values
for any maximum voltage output you choose.
I've made several of these power supplies for the club I used to belong to.
That was 8 years ago, and they still work fine.
Hope this helps.