Using "other" powerpacks

Discussion in 'FAQs' started by MarioBarb, Oct 13, 2005.

  1. MarioBarb

    MarioBarb New Member

    I have a number of 12V .5 Amp powerpacks that are used for cordless phones etc. I would like to use them to power some of the 12V Lima locos that came with 12V, .25Amp powerpacks, but I would like to attach some kind of variable resistor to regulate voltage. Any ideas? Also, will the higher Amp rating have a negative affect on the locos.?
  2. ezdays

    ezdays Out AZ way

    Welcome to the Gauge, the friendliest and most informative train forums on this planet.

    You could use these power packs to run a train, but I wouldn't recommend it. To start with, they are designed to do a different job, and may have some current limiting circutry in them that will charge a battery OK, but could limit the amount of power delivered to the tracks. Most charging circuits provide a higher current initally, then as the battery charges, limit, or "float" the charge, or shut down entirely. The reason being if you provide a steady charge, the battery will overcharge and get very hot.

    If you could defeat this regulation, than you could add some sort of voltage control of your own, the simplest being the variable resistor you mentioned, but this will be an exercise in basic electronics. You could figure out what value resistor to use with one loco, and it may or may not be the same for another loco.

    The bottom line is that I don't think it's worth it. By the time you fool around you could spend more time and money than if you just went out and bought an MRC Tech power pack that is loaded with features.

    Someone else may dispute what I just said, but that's just not only my opinion, but it comes from having designed and built charging circuits in the equipment we manufactured.
  3. Clerk

    Clerk Active Member

    Take Dons advice on this. I have followed Dons advice to other modelers and he is right on.
  4. 60103

    60103 Pooh Bah

    On the other question, a higher amperage will not harm your locos. They will only draw what is required. When you look for replacement power packs, you probably won't find anything at a quarter amp.
    The only qualification on the above: stay under 5 amps. Above that you can do some inteeresting metalwork on track and fine details when you have a derailment and short.
  5. ezdays

    ezdays Out AZ way

    Yeah David, that's a good point. Many power packs that are available today do have circuitry that will detect a short and shut down until the short is removed, or reset. Older ones and basic supplies don't and you're right, you can draw a pretty good arc on the bigger ones when they short out.
  6. MarioBarb

    MarioBarb New Member

    Hi, thanks for the replies

    Could I assume that if a paired set of Identical locos are running OK (same speed, power etc) on the same line (i.e pulling the same train) and that the power pack does not get hot, everything should be OK?
  7. pgandw

    pgandw Active Member

    I could be wrong, but I believe the power packs (wall warts) you are talking about do not have any battery charge regulation built-in. Most devices have the battery charge regulation in the device itself, not in the charger. So you could use the power packs provided your motors don't draw more than .5 amps. Many older open frame HO motors can draw .6 amps; the newer small can motors draw .25 amps or less.

    The variable resistor will need to be about 0-100 ohms (rated for at least 10 watts) for can motors, but even then it will not work as well as you might like. The older, high current, open frame motors matched up much better with rheostats (variable resistors) because the current drawn was much more consistent. Say you want the can motor to see 4 volts at .25 amps. To lose 8 volts, the variable resistor must be set at 8/.25=32 ohms. However, if your motor is drawing only .1 amp (quite possible on a free-running engine with no cars) the variable resistor must be set at 8/.1=80 ohms. If using a linear 100 ohm rheostat, you would not reach 4 volts for the .25 amp motor until you rotated the knob over 2/3 of its throw.

    As others have said, the motor's internal resistance (varies somewhat with load) determines the current drawn. Voltage determines motor speed.

    Hope this helps.
  8. ezdays

    ezdays Out AZ way


    You are right, many generic plug-in power packs don't have any regulation in them, some proprietary ones do. There are many varieties, but the unregulated ones are just that. The DC voltage output is dependent on the load, and usually the AC to DC conversion is simply a single diode, or half-wave rectification. I use one for some lights on my layout, but I wouldn't use one for running engines. It most likely will work if you do, but I'd be afraid of the peak spikes that are higher than the "DC output" and could harm the motor over time. Some of these plug-ins are internally fused and if you get a derail and short the supply, that's all she wrote. You can take these apart to bypass the fusable links, but again, not a good idea.

    The bottom line is, even if it works and you can adjust the rail voltage, it isn't worth the effort. You can get a nice MRC Tech 4 with acceleration/decceleration, forward/reverse, stop/start, output limit, AC and DC for accessories, all in a neat package for around $40, and the output is electronically regulated.

    When I first got into this hobby, I was going to build my own power packs, after all, that's what I did for a living for years and I had most of the parts already. But I found a Tech4/350 on sale at my LHS and after testing it, I went back and bought a second one. I could build one cheaper with all those features as well, but I couldn't do it and have it look as nice plus I could spend that time working on my layout.


    That's a good assumption, but also be sure your engines aren't running hot either for the reasons I just stated.
  9. pgandw

    pgandw Active Member

    I agree with Don - you can do what you ask, and I've laid out parameters that will probably work - but it's better, and you'll be happier, to pick up a Tech 4 (or even a used Tech 2 or 3). Also, finding a suitable rheostat is not easy these days. Back when rheostat control was standard, MRC used custom-made taper-wound 40 watt rheostats to give the best control possible with the system. Current transistor controls do a much better job of voltage regulation for alltypes of motors.

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