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

Ignoramus8558 wrote:
On Thu, 15 Dec 2005 08:47:14 -0800, Tim Wescott wrote:

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.


What about torque, it needs some torque to turn the gate (something
that I can help with by use of a little rubber band).

I will definitely play with this, as this seems to be the easiest
solution. I will try to measure DC resistance of this motor and add
perhaps 3x as much resistance.

It seems that I found something that my 4.5 year old takes long term
interest in (control of his electric railroad) and I want to keep it
going.


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.


Very nice. I will try to use resistors as you suggested.

i


Motor torque goes with current, so it will definitely go down with the
resistor. The polyswitch should be better in this regard. Are you
driving the gate directly from the motor? If so you're really not
matching the motor's capabilities to the problem. If you have an old RC
servo lying around you could strip out the electronics and just use the
geartrain and arm from that, with an appropriate resistor to limit the
current, of course.

--

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

Ignoramus8558 wrote:

On Thu, 15 Dec 2005 09:06:21 -0800, Tim Wescott wrote:
snip
Are you driving the gate directly from the motor?


Yes, it is just a little stick on the shaft.


If so you're really not matching the motor's capabilities to the
problem. If you have an old RC servo lying around you could strip
out the electronics and just use the geartrain and arm from that,
with an appropriate resistor to limit the current, of course.


You know, I may be able to just use a little counterweight and reduce
torque requirements to next to nothing. Then this motor and a resistor
would just be perfect. Torque is a non issue, as I am realizing. Sorry
to have brought it up. As long as I limit current and overheating, I
will be perfectly fine.

i

You mentioned using a solenoid earlier. They're terribly inefficient,
but they're generally built for this kind of load. They have lots of
windings that bulk them up and would slow them down if they should go
fast, but give lots of actuating force and a high resistance to limit
current. You may even want to use separate "open" one and "close" ones;
this will let you use two single-acting solenoids and would simplify
your switching.

If you don't already know about it you want to check out Herbach and
Rademan: http://www.herbach.com. They have this kind of stuff cheap.

--

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

In article ,
lid says...


I checked them out and did not find 12 vdc solenoids. McMaster has
them, see item 70155K2, for $8.82. I may use them on railroad
switches. I hope that they can stop in an intermittent position (if
the part being moved does not want to move further) without
trouble. But that will come later.

Most solenoids will overheat if they remain energized with
the armature prevented from fully seating in the coil.

Ned Simmons

Ned Simmons wrote:

In article ,
lid says...


I checked them out and did not find 12 vdc solenoids. McMaster has
them, see item 70155K2, for $8.82. I may use them on railroad
switches. I hope that they can stop in an intermittent position (if
the part being moved does not want to move further) without
trouble. But that will come later.


Most solenoids will overheat if they remain energized with
the armature prevented from fully seating in the coil.

Ned Simmons

Unless the armature provides a thermal path that cannot be the case in a
DC solenoid. AC, yes -- the inductance would go up, and the current
down. DC, no -- after you get past the R/L time constant the inductance
doesn't mean anything.

--

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

On Thu, 15 Dec, Tim Wescott wrote:
Motor torque goes with current, so it will definitely go down with the
resistor. The polyswitch should be better in this regard. Are you
driving the gate directly from the motor? If so you're really not
matching the motor's capabilities to the problem. If you have an old RC
servo lying around you could strip out the electronics and just use the
geartrain and arm from that, with an appropriate resistor to limit the
current, of course.

Or build a small servo controller and just use the servo as it was
intended. I once built a small circuit that used a potentiometer to let me
"control" a servo. I can't imagine it's a hard circuit to find, nor would
it require any expensive components (nor very many components). Then just
find an RC servo "big enough" to handle the size of the gate, which also
shouldn't be hard to find (and I'm sure used ones are dirt cheap on eBay).

The circuit I found used a potentiometer, but it wouldn't be hard to hack
it to have a switch change the resistance appropriately for the circuit to
move the servo to the right place (hint, use a pot to find the proper
resistance values for "open" and "closed" gate positions and then use a
switch with NO and NC terminals and wire the proper resistance to each
one).

You can likely also drive multiple small servos off the same signal from
such a device, though I never tried it. That way you could use a gate on
each side of the crossing, just like real life.

I mean if Iggy can build his own 17.5HP RPC, this is child's play. Pun
intended. ;-)


--Donnie

--
Donnie Barnes http://www.donniebarnes.com 879. V.

On Thu, 15 Dec, Tim Wescott wrote:
Unless the armature provides a thermal path that cannot be the case in a
DC solenoid. AC, yes -- the inductance would go up, and the current
down. DC, no -- after you get past the R/L time constant the inductance
doesn't mean anything.

Use pinball solenoid parts. Fairly cheap and built to last. Pinball
flipper solenoids have two sets of windings...one to "hit" and the other to
"hold". There's lots of pinball information on that great world wide web
thingamabob.

As for track switches, what scale are we talking? I can't imagine a
scenario where hacking your own is cheaper than buying the canned stuff
that exists. I do G-scale railroading and those little solenoids they have
aren't really cheap, but damn if they aren't very well packaged for the
application and just plain work.

I suppose if it's just for one or two and you just want to DIY...


--Donnie

--
Donnie Barnes http://www.donniebarnes.com 879. V.

In article ,
Ignoramus8558 wrote:

On 16 Dec 2005 02:20:15 GMT, Donnie Barnes
wrote:
On Thu, 15 Dec, Tim Wescott wrote:
Unless the armature provides a thermal path that cannot be the case in a
DC solenoid. AC, yes -- the inductance would go up, and the current
down. DC, no -- after you get past the R/L time constant the inductance
doesn't mean anything.

Use pinball solenoid parts. Fairly cheap and built to last. Pinball
flipper solenoids have two sets of windings...one to "hit" and the other to
"hold". There's lots of pinball information on that great world wide web
thingamabob.

As for track switches, what scale are we talking? I can't imagine a
scenario where hacking your own is cheaper than buying the canned stuff
that exists. I do G-scale railroading and those little solenoids they have
aren't really cheap, but damn if they aren't very well packaged for the
application and just plain work.

I suppose if it's just for one or two and you just want to DIY...

If I can buy an electrically switchable switch, it would be great. I
have a HO scale, I believe. Right now I run it at 12 volts, although
it could go to 16.

i


Have you got any powered remote-operated track switches to spare?

I'd need to ponder the details of the wiring, but it seems to me that
one could put a sensor/switch package on each side of the crossing, much
like real RR tracks, to operate the switch, then tap the solenoid that
normallly switches the track position to move the gate to the up or down
position, as approriate.

Early pondering suggests maybe a series of "whisker" or "feeler" type
switches wired in parallel poking up through the roadbed leading up to
the crossing, so that when the train goes over, the feeler(s) get
deflected to make a circuit. Circuit powers track switch solenoid, gate
goes down, solenoid holds until train is gone. Once the train is gone
and the last of the parallel-wired feeler circuits breaks, the solenoid
releases, and the gate goes back to the up position.

--
Don Bruder - - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
ever knowing it arrived. Sorry... http://www.sonic.net/~dakidd for more info

Ned Simmons wrote:

In article ,
lid says...


I checked them out and did not find 12 vdc solenoids. McMaster has
them, see item 70155K2, for $8.82. I may use them on railroad
switches. I hope that they can stop in an intermittent position (if
the part being moved does not want to move further) without
trouble. But that will come later.


Most solenoids will overheat if they remain energized with
the armature prevented from fully seating in the coil.

Ned Simmons
Shouldn't matter with DC ones.
...lew...

In article ,
says...



Most solenoids will overheat if they remain energized with
the armature prevented from fully seating in the coil.

Ned Simmons

Unless the armature provides a thermal path that cannot be the case in a
DC solenoid. AC, yes -- the inductance would go up, and the current
down. DC, no -- after you get past the R/L time constant the inductance
doesn't mean anything.



Yeah, I knew that, or should have, or once did and forgot g. In any
case, any day that you learn (or relearn) something is a good one.

Ned Simmons