What is the current draw at 12VDC or the operating resistance of the motor in the Bachmann HO PCC Trolley. #62931?

I am building a little power supply for it and I need to know its power requirements. I already have a source of 12VDC and it does not make sense to buy yet another power pak when the trolley will only be going back and forth at a constant speed on a straight track of about 8 feet. It is a Xmas display that will have the trolley going from end to end continuously. (And yes there will be a potentiometer to control the speed, in series with the track.)

LDBennett

OK, so I measured the DC resistance and got 12 ohms. That means at full throttle the order of magnitude of the current flowing is about 1 amp.

The big question is what resistance potentiometer should I use to limit the current flow? I'm thinking 1K (??). Anyone got better info? The trolley will be moving on this point to point layout (run automatically with a direction change at each end) at a pretty slow scale speed, like maybe 10% of full throttle operation or even less. Any insights on this?

If no one can help I guess I'll just have to wait until I get some track laid and the control system installed and do some testing with fixed resistors, later to be replaced with a potentiometer. My thoughts are that I'll use the pot as a current limiter and not as a voltage divider (pot not connected across the power supply but in series with the power supply).

LDBennett

You will burn up the pots used for electronics. You need a rheostat which is essentially a high wattage pot.
Buy a used power pack off of ebay or maybe a LHS if there is one near you. You can get used ones for under $5.00.
Use ohms law to calculate.

http://www.opamplabs.com/eirp.htm

Also, go to the below site and download the article. All kinds of converters for model railroaders.

http://issuu.com/mr-hobbyist/docs/mrh12-04-apr2012-ol/96?viewMode=presentation&mode=embed

Rich

Twelve volts applied to 12 ohms would indeed give a current of 1 amp if you were dealing with a resistor.  But this is a motor.  PM motors also work as generators when they are spinning.  This means they produce Back Electro Motive Force (BEMF) when they are running.  This BEMF opposes the applied voltage, lowering the effective voltage that is running the motor.  Depending on the motor and its mechanical load, the current could be anything from 1 amp (motor stalled) down to a few milliamps.  I assume you measured the motor's resistance with a meter.  Why not measure its current with a meter too?

Using a rheostat to control the motor's speed can cause you no end of problems.  The first problem is that a proper, high wattage, taper wound rheostat designed for this application will cost you more than a good used power pack would.  Or possibly more than a brand new power pack would.  You would be better off using a potentiometer and an adjustable voltage regulator.  A 1000 ohm, 1/2 watt linear taper potentiometer plus an LM317 regulator work very well.  Not only will this vary the voltage from about 1.2 to 12 volts, it will automatically limit the current to about 1.5 amp in case of a short circuit.  And if the short circuit lasts long enough to over heat the regulator, it will automatically shut down the current to protect itself.  I usually mount an LM317T on a 3" x 3" square of 16 gauge aluminum as a heat sink.  More details are available for the asking.

Jim   

The replies here and other reading convinced me to just buy another power pack from Bachmann. I could not build a solid state throttle for the cost of the power pack.

I was just trying to save a buck. I was in model railroading when in junior high over 50 years ago. In fact I slept under the layout during this years. My grandson wanted a model train so I set him up with a N gage little train layout. He is having fun with the scenery and structure building. I have no idea how far he will take it as he has a space problem. In the process of getting some trains for him I inadvertently order a HO gage Trolley instead of N gage. So I decided to put together a point to point little layout with buildings and urban scenery. I have no idea if once done then this is it. It was supposed to be just for a Xmas display but curiosity of where model railroading has gone to has got the better of me. I have to constantly throttle me and the investment.

Thanks for your inputs. I did consider them. I understand electrics but just wanted to save a buck but have re-considered and ordered the Bachmann power pack. If I ever went to a big layout it most certainly would be DCC controlled. I just want this trolley to go back and forth unaided. I got that figured out in a simple way, I think.

LDBennett