# Variable Power

• posted

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. Thx!

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• posted

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...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |```
• posted

Rheostats are not out of the question for slot cars.

• posted

Whatsize Rheostat would I need lets say to make this variable from

5VDC to max 13.8VDC keeping the amps around 4-5?

Thx

• posted

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.

• posted

How old is your design? It cries out for power MOSFETs, etc.

• posted

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.

Thx

• posted

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

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.

• posted

to

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

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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.

• posted

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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.

Cheers

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

• posted

On Fri, 06 Jul 2007 21:20:08 +0000, Kevin R wrote: ...

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

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

Cheers! Rich

• posted

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 heatsink.

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