Ignoramus8558 wrote:

We were playing with my son last night and made a toy railroad
gate. It would open and close based on a relay signal from a momentary
switch that is activated when a train comes near the railroad
intersection.

It worked, kind of.

My problem is that the DC motor (pulled from a demolished HP X-Y
recorder) would overheat from being under power constantly.

I would like to find a little 12 VDC motor that could survive a locked
rotor condition indefinitely, or some other simple solution. A little
solenoid with a dual coil (pulling and holding) also comes to mind. I
need something that can be bought for a few bucks at most or made
easily, and I am out of ideas.

i

Put a resistor in series with the motor windings. The unloaded speed
won't be affected much, but the stall current will be. You could get
fancy and order a polyswitch from Digi-Key -- they are little resistors
made of a polymer material with a strong positive temperature
coefficient; they work like little resettable circuit breakers. The
nice thing in this application is that they're slow to turn off and they
hold their value automatically. Their mechanism is entirely thermal so
if you tape the thing to the motor case it'll tend to go off sooner when
things are toasty.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com