Need coreless motor protection circuit for DC-only running on a DCC track

[mmagliaro]

Here's the situation.  I am wiring up a coreless motor in a utility car for someone.
It will run on his DC layout of straight track power.
It's a natty Maxon coreless for ultra slow, smooth turning.

If he ever puts this on someone's DCC track, the DCC pulses will eventually destroy the coreless motor.
Although I've never experimentally done it long enough to prove it, I believe that the heat build-up
from letting a coreless sit there buzzing away on the DCC pulses will destroy it because it has
no mass to dissipate  the heat like a conventional motor.

I would like to tack a simple circuit onto the coreless motor that will simply block the DCC pulses.
It won't work on a DCC track, but it won't be destroyed either.

I think I need some sort of LC circuit to choke off the DCC pulses from the motor, but I have no idea
what parameters I need.   Anybody know of some existing schematics or other examples
of a circuit like this?

Thanks

[victor miranda]

I think peteski has been given a time-out....

I think I'd try a pair of back to back diodes
and a high performance capacitor (decrease in size mostly)
across the ends
4 diodes total for about a volt drop
to give the cap something to use as a PD
to smooth the tiny spikes.

ETA in series with the motor.

victor

[mmagliaro]

I must be dense.
What is "PD" ?
What is "ETA" ?

I think you are trying to use pairs of diodes to create some voltage drop, and also to
isolate the capacitor's filtering effect from the track (because it needs to be a filter
only on the supply to this one motor, not a cap just slapped across the track).

I don't understand how this will block the DCC track voltage from the motor.

Can you draw a little diagram? 

I would have thought the way to do this was to wire a choke coil in series with the motor
to quash certain frequencies.  Knowing that there are older lower-frequency DCC systems and
higher frequency ones (that operate at something over 20 kHz?), one might need two
chokes, but I think that the higher frequency pulses would not be a problem anyway.

(Of course, one could just be careful and not ever put this thing on a DCC track until one
puts a decoder in it, but that's not always as easy as it sounds).

[Ken Rice]

The DCC track signal is the same frequency it's always been (around 9KHz).  What you're thinking of with lower and higher frequency is the motor drive, which is a signal the decoder generates - it has nothing to do with the track signal frequency.

I don't quite follow what Victor is suggesting either, but I have a more basic question.  Say you find something that someone thinks should work.  How will you know if it's good enough to give you confidence that you won't fry the motor anyway?  Personally I'd think I'd opt for the "just be careful" approach.  I believe the failure mode is not instant annihilation, it takes a little time.  I think you'll have at least a few seconds to notice the high pitch whine and remove the loco.

[victor miranda]

PD  is potential difference ... voltage.
caps need  some voltage across them to capacitate

ETA is Edit To Add...  or estimated time of arrival

diagram....

 #-->|--*-->|--#  two diodes
 #--|<--*--|<--#   the backs
 #-----(cap)----#  the hash marks are tied together

at one point I think you called it a diode ladder.

the first thing I thought was a resistor/capacitor pair tuned to eat the spikes...
buuuut that may have problems
this is a signal rounder.... I think the cap will eat the spikes
if it is big enough.

[mmagliaro]

Ken... THANKS!  I was confused about that.
So it's 9 kHz.  Yeah, that's not good for a coreless motor.   Much higher would be safer, I would think.

Actually, I got an email from Peteski that suggested, I think, quite a simple and complete solution.

Just put a bridge rectifier in place inside the car.   Anything that comes off the DCC rails
will all come out as DC, and I could even add a small electrolytic capacitor across the rectifier
for extra smoothing if I want.   The coreless motor will see 12 volts (or maybe a little more).
It will run, and it might be a little fast, but it will be safe.

Victor, I am having trouble following that diagram.  Where would the track AC connections go?  On the "*" ?

[Iain]

The bridge rectifier would mean it would only ever go in one direction 

Go ahead and put a good decoder in it, then you can run it anywhere without having to worry about the motor exploding and it will go forwards and backwards.

Some clubs that are DCC will not let you run in 'analog mode'.

[victor miranda]

a bridge rectifier...

you may want to ask  him to diagram it
cause I have no idea how he wants you to set it up...

just keep in mind a bridge rectifier is a pair of back to back  diodes
with out the two centers tied together.

luck.

victor

[mmagliaro]

This thing is not for an engine.  It is for a utility car - actually one of those Dimi snow plow cars.
I am just putting a motor in it to make the plow blade go 'round.  So yes, it only needs to go in one direction, and I don't even care which direction it spins.

The power would come in off the rails and connect to the AC inputs on the bridge rectifier.
This trick has been used many times before to force DC to always be one polarity.
If you run this on DC, you will still get DC out of the rectifier and it will only be one polarity regardless of
the track polarity, which is fine with me.

If you put it on a DCC track, the AC signal will get rectified so that the motor will only see DC, so once again,
it should be just fine.  I will put a filter cap across the output of the rectifier to smooth out the pulses, but I do not think even that is necessary.

Victor, if your drawing is supposed to be like a full wave bridge, I apologize.  I just had trouble following the
drawing.

[victor miranda]

Max,

you have used back to back diodes.
you called it a diode ladder

in the same way people add constant brightness lamps using the 1.5 volt bulbs.
you can use a bridge rectifier,  it has 4 diodes in one package.

where you put the lamp.... add a capacitor.
or add the cap across the motor the caps will... smooth out some of the DCC signal

the ac part.... you will need at least one diode to rectify
the bridge will get the motor always going the same direction...

this is intended to be fun.  I am allowed to be frustrated that my posts are not clear.
I will get better at it.

I still want you to have luck.

victor

[lyled1117]

Do you have a target voltage you'd like to have across the motor when running? For what you are describing, a constant voltage (thus constant speed) seems logical. For example, if you decide 5V applied gives a good speed, then use a 5V regulator. Put a rectifier in front of it and it'll run on AC or DC. On DCC it'll be constant speed, on DC it'll varying until track voltage reaches a certain level then it will be constant as well. As low current as the motor probably is, a simple zener diode regulator may do the trick, perhaps something like 3.3V. You'll just need to use components that can handle the power when DCC voltage is applied.

Lyle

[mmagliaro]

Lyle,
Thanks, and yes, I have been considering all this - a possible regulator or just a zener.  The motor
being a coreless will only draw about 20 mA tops, even at 12 v.  Thats only 0.24 watt, so a
1 watt zener, which is still quite small, will easily handle this.

A 3.3v zener wired across the motor leads, with a small resistor in series with the whole thing to
limit the current, will do the trick for keeping everything constant.  I can test it on 16 volts to make sure
that it doesn't fail even with a "hot" DCC track.
The added benefit is that I can also drive an SMD LED off the same supply to get constant headlight
brightness.  Since this is a snowplow car, it does have a headlight on the front.

The Maxon 16:1 gearmotor is just perfect for this.  Too big for a loco, and too slow with the 16:1, but for
a plow blade that is only supposed to spin at 90 rpm maximum, it's just perfect.

[mmagliaro]

Well, after some experimentation, I'm not going with the rectifier.  I am not happy with losing the voltage through it.
This thing is going to live on a plain DC system.  The DCC protection was just a "just in case" thing.
Without the rectifier, the plow blade starts turning down near a volt, which is really nice because
it will run even before most engines start moving. 
4 little 1N14849 glass diodes in series neatly give me a clamped voltage at around 3v, so I just put those
in series with a resistor, and the blade will move at about 2.5v, and it's top speed is hard limited by
the diodes.   (And I was able to solder up a little package of 4 of those tiny glass diodes that is very small
and fits in the car, and in fact, I put two 4-diode packs back to back so the whole thing works in forward or reverse).

As for what will happen if this is ever placed accidentally in a DCC system, well, the short answer is "don't do that."  But in reality, with the diodes clamping the maximum voltage across the motor, and the dropping resistor in there, I think that the current will be so limited that the motor won't be harmed.