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Variable Power

Discussion in 'Electronic Design' started by Kevin R, Jun 25, 2007.

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  1. Kevin R

    Kevin R Guest

    Good Morning, I have a 13.8VDC 10amp power supply that I would like to
    make variable. I will be powering a slot car track with this and
    would like to adjust the voltage to each lane by individually. Any
    help would be great.
  2. Jim Thompson

    Jim Thompson Guest

    ...Jim Thompson
  3. Charles

    Charles Guest

    Rheostats are not out of the question for slot cars.
  4. Kevin R

    Kevin R Guest

    Whatsize Rheostat would I need lets say to make this variable from
    5VDC to max 13.8VDC keeping the amps around 4-5?

  5. Meat Plow

    Meat Plow Guest

    What scale slot cars? How many tracks? Is that 10 amp supply going to be
    enough? Appears to me that 10 amps is going to be woefully insufficient
    with motors and lane rheostats, controller rheostats, and track losses
    sucking up all those amps.
  6. Winfield

    Winfield Guest

  7. Kevin R

    Kevin R Guest

    10 Amps should be enough, I will only be powering 2 lanes of 1/32 size
    cars. From my reading it will only take about 2amps to power each
    lane, so I should have plenty of reserve in each lane. The other
    option I've been considering is converting a PC PSU, but that would be
    later. The main thing I need to complete is to vary the voltage to
    the track, as I want my kids to be able to race on the track too but
    with lower voltage so the cars don't go flying off the table. By
    looking at rheostats it seems a fairly simply way to put something
    inline to reduce voltage, the other option somebody passed by me was
    to use rectifier diodes on a 12 way switch, but I thought that was not
    the best way to reduce things.

  8. Charles

    Charles Guest

    You could try a series fixed resistor of a couple of ohms to test the

    Assuming one track draws 2 amps, a 2 ohm resistor would drop 4 volts (V = I
    x R). If your experiment seems to yield good results, 10 ohm rheostats
    might work just fine. The motor current is not going to be constant, by the
    way. Again some experimentation is in order.

    I have not worked on slot cars for many years, but when I did the supply was
    full-wave dc with no filtering. This suggests changing to half-wave as a
    possible solution.
  9. Ross Herbert

    Ross Herbert Guest

    I am wondering why you would need to make the voltage to each track
    adjustable between 5 and 13.8V. The simple hand held rheostat slot car
    controllers usually found on slot car tracks do just this, as I am
    sure you would be aware. Your question implies that you have something
    else in mind.

    To start with, using a simple variable resistor is not an ideal way to
    control slot cars. Due to friction and inertia losses in the car and
    resistive losses in the rheostat itself, the voltage required to get
    the car moving from stop is considerably greater than that required to
    keep it running at any desired speed. This means that controlling the
    car's speed is going to be quite jerky when starting. Also, depending
    upon the regulation ability of the dc power supply, if one car comes
    off the track, the remaining car may get a sudden increase in voltage
    which will cause it to surge.

    A very simple method to do what you want without going to a fully
    fledged PWM controller is outlined in this forum
    See the SCR controller posts by Rene on Aug 12 2004. While his
    description is not technically correct, the essentials are there and
    the article is easy to follow if you are prepared to give it a go.

    PWM speed control is the way to go really.
  10. Kevin R

    Kevin R Guest

    The sole purpose behind this is to have each lane individually
    powered. Since I have a 3 yr old that is just itching to drive these
    slot cars, I need to slow them down so he can not thrash a $50 car.
    Dropping the voltage to say 5-7VDC should do the trick. Also 7VDC is
    great for breaking in new motors. I've done some research and it
    looks like a LM350 will do the trick. It's rated at 3amps, but I
    can't tell what max input amps are. So what I would like to do is
    use two LM350's each controlling a lane but hooked up to a single
    13.8VDC 10amp power supply. Has anyone played with the 350's that
    know if the 10amps from the power supply is going to cause issues, or
    will the 350 be able to handle it? I know I need more then the
    LM350's as I'll need a few resisters and pots and other misc things to
    build the circuit. I was planning on using the schematic that is
    shown with the LM350, but maybe use a tad bigger capacitor.

  11. Fred Bloggs

    Fred Bloggs Guest

    What's wrong with the PWM circuit Analog Innovations posted???
  12. neon


    Oct 21, 2006
    Lokk up the LM317 regulator and inplement it a a variable current source the device will give you what you want a stedy current source of but it is limited to 1 amp so after that you need some current boost why regulate the voltage when motors needs current to run. you will need some significant heat sinks even for yur idea of the lm350 which also can be a current source if designed for it. LM317 simpler thought.
    Last edited: Jul 7, 2007
  13. Rich Grise

    Rich Grise Guest

    Fifty Bucks for a slot car????!!!?? =:-O

    You're spending way too much money on kid toys.

  14. Ross Herbert

    Ross Herbert Guest

    The "input amps" will be marginally greater than the "output amps",
    probably in the order of milliamps. The output current is purely
    dependent on the load requirements so if the car doesn't take more
    than say 1 - 2 amps max then the LM350 will be ok.
    As long as the 13.8V supply has a large output filter capacitor (eg.
    4,700uF - 10,000uF) then you won't need a huge capacitor at the input
    of the LM350 - say 470 - 1000uF or thereabouts would do. Since the car
    motor isn't fussy how clean the dc supply is you only need to satisfy
    the instantaneous current demand for a 100mS or so. I'd still put
    something like 47uF across the output of the 350 though. You will need
    to mount the 350 on heatsink particularly when using the TO220 package
    - also in case the output gets shorted (track shorted). The 350 will
    go into over-current and over-temp protection mode but it will still
    dissipate a fair bit of power in this condition. The TO3 (K) package
    is far more efficient at dissipating heat than the TO220 (T) package
    but for your purposes the TO220 would be easier to mount on a
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