# Variable Power

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

1. ### Kevin RGuest

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!

2. ### Jim ThompsonGuest

http://analog-innovations.com/SED/SlotCarController.pdf

...Jim Thompson

3. ### CharlesGuest

Rheostats are not out of the question for slot cars.

4. ### Kevin RGuest

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

Thx

5. ### Meat PlowGuest

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.

7. ### Kevin RGuest

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

8. ### CharlesGuest

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.

9. ### Ross HerbertGuest

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
http://www.auslot.com/forums/index.php?s=5c47c7c500a1ede3b0c33ea40dfd31af&showtopic=458
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 RGuest

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

12. ### neon

1,325
0
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 GriseGuest

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

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

Cheers!
Rich

14. ### Ross HerbertGuest

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.