Making a power pack...

Discussion in 'FAQs' started by steamhead, Aug 26, 2008.

  1. steamhead

    steamhead Active Member

    Hi...I have a 12v. computer power supply which I want to use as a power source for a DC controller. Can anybody out there come up with the "rating" for the potentiometer I need to use to get good current response over a wide range of the pot "travel"..? I have one, but I get response only over 1/2 of its travel.

    Any suggestions will be most welcome.
  2. nachoman

    nachoman Guest

    I can tell you that you are better off using a transitorized control. I built a few years ago based upon plans in model railroader, and were quite simple. Perhaps a google search could provide you a circuit.

  3. pgandw

    pgandw Active Member

    It is almost impossible to size a pot properly due to the wide variety of motors and currents drawn in present and past locomotives. For example (I'll use HO locomotives for illustration purposes), an Athearn with open frame motor might draw 0.6 amps, while a new locomotive with a good can motor might draw 0.2 amps.

    If you use a 60 ohm pot set at half its range (30 ohms), the Athearn with open frame motor will not receive enough voltage/current to start moving - 0.6 x 30 = 18 volt drop at the pot. At the same pot setting, the can motor will be running at 6 volts - 0.2 x 30 = 6 volts drop at the pot.

    Unless your motors draw pretty much the same current in situations encountered on your layout, a simple pot is a poor controller. As Kevin said, you need a voltage controller. This is usually done with a few transistors. Google DC throttles or similar terms for circuits.

    "Pulse power" is often used to overcome motor cogging and assist in smoother slow speed running. Some circuits will use the AC ripple or 1/2 wave input from the rectifier to generate the pulses. Your computer power supply carefully eliminates all the ripple so that if you want pulse power, you will have to use a circuit with its own pulse generator/oscillator.

    hope this helps
  4. Russ Bellinis

    Russ Bellinis Active Member

    Even can motors vary from one to another. I have a gp9 powered by an Alco can motor. Before the club went to dcc, I couldn't run it with any other locomotive because the motor was so efficient that it made a Kato or Atlas look like an Athearn in comparison. If I moved the throttle at all, that locomotive would start to crawl. A throttle setting high enough to get a Kato or Atlas moving would have that Gp9 running as fast as I would want it to go. When our layout was set up with typically 4 blocks on each main, a passenger train would run slightly above 1/2 throttle, an Athearn might require 3/4 throttle, a Kato or Atlas would run freight speed at about 1/2 throttle and my Gp9 would seldom need more than 1/4 throttle.
  5. railohio

    railohio Active Member

    Suggestions? Buy a good power pack from MRC and leave the tinkering to the ham radio guys.
  6. steamhead

    steamhead Active Member

    Thanks for the input....I can't use an MRC pack (not the "modern" ones anyway,,) since I'll be using the pack as a "jump" throttle in conjunction with my Digitrax Zephyr. The pulsed power packs make loco operation "jerky", and D'trax recommends they not be used...

    I did turn to an "expert"...I found a schematic diagram for a transitorized throttle online, and gave it to my son to make. He's an electronics engineer - digital controls are his thing....:mrgreen:

    He's rounding up the parts and I should have it in a couple of weeks when he comes home....:thumb:

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