Large Scale Central

How to size a pot?

Electronics
#1

Hello-

I know this is very basic but I don’t think I have all of the information I need so I’m hoping you can help. How do I size a potentiometer to control my track voltage? I have a solid industrial power supply rated at 24VDC/5A. The plan is to run that through a voltage regulator (Digikey LT1084CT-12#PBF-ND) and then through a pot. Here’s where I get lost. I don’t know the draw range for locomotives. It seems very unlikely that it would be five amps. Using the max values I’m at 60 Watts which seems excessive. Any suggestions?

#2

Mark - If I understand your email you have a fixed 24 volt DC power supply that you want to use to deliver a variable voltage between 0 and 24 volts to your track to power a train.

If that is the case I would not use the regulator that you specify and I would not use a variable resistor (pot) - a much more elegant solution would be to use a PWM motor speed controller such as this one from SunTekStore.com

http://www.suntekstore.com/goods-14002158-12v-40v_10a_pwm_dc_motor_speed_controller_w_knob.html

It costs all of $10.55 delivered - I have used several of these as motor controls and they work very well - add a DPDT toggle switch to reverse the polarity, stick it in a box and you are done.

Hope that helps and that I read your intent correctly!

dave

#3

Another solution for some would be the “Simple Critter Control”. 5amps, 7-20V, and a bit more costly, but a much smaller package specifically designed for large scale trains. (But won’t help in this case, because it can’t handle the 24V.) http://1stclass.mylargescale.com/DelTapparo/gscalegraphics_1_020.htm

On another note, I have done a few special projects using my new Pocket RailBoss control as a trackside receiver. 5 amps, 7-25V, 2.4GHZ with a keyfob transmitter.

#4

I should have specified, output from the regulator will be 12VDC. It just happens to be a 24VDC PS. As far as the PWM style controller goes, that is part of the plan. Just in a future portion of the plan. I think what I really need to know are the ranges of power requirements for the average large scale traction unit.

#5

Typically, less than 1 amp.

#6

But 12 volts may not get many locos up to prototypical speed.

#7

And if you want to use a pot, find a variable voltage regulator that will allow you to put the pot in the voltage adjust circuit, not the output. There are many.

#8

0-24 volts depending on the loco.
What sort of locos will you be using?

LGB locos with sound and lights need at least 18 volts to overcome the first 6 volts being used to turn on lights and smoke before moving.

Big AC and USAT locos also need at least 18 volts to get motoring.

Beware of PWM output being used if the locos have DCC decoders. The decoders will get “confused”.

I would make a linear output that can be used safely with anything.

A simple Darlington pair with small pot for speed control and a biggish transistor as the driver output. You will likely need a heat sink for the output transistor.