# Need Help Building a Slot Car Motor Break-in Power Supply

#### Reaper802

Joined Apr 19, 2010
11
Several vendors sell these type of supplies but they all retail for well over $200.00. I'm positive I can build one cheaper. A good example follows: CAMEN Is proud to present the 8 AMP "MINI" POWER SUPPLY. This Power Supply sets an entirely new standard in miniaturization. A full 8 Amp supply (capable of running the highest powered motors in the sport) in a package that measures a mere 5" X 4" X 3". Other features include... Digital metering, circuit breaker safety system (no fuses to deal with), accessory plug (for powering tire truers, comm lathe etc.), and as an option, the inclusion of two ac plugs (eliminating the need for an extension cord to power your soldering iron etc.)$229 (Basic Machine)

#### Reaper802

Joined Apr 19, 2010
11
That supply at MPJA may just do the trick. I could feed the 12 volts into a voltage regulator to make it adjustable down to 1.5 volts or so. Anyone know where I can find the voltage/current monitoring IC and LCD? I could just add a simple voltage divider to power that part. Looks like I can get a 5x4x3" box for $10.00 or so. #### SgtWookie Joined Jul 17, 2007 22,221 That supply at MPJA may just do the trick. I could feed the 12 volts into a voltage regulator to make it adjustable down to 1.5 volts or so. Not 8A worth of current, unless you want to use several transistors and large heat sinks. If you want to avoid that, you will need to use a PWM circuit. Anyone know where I can find the voltage/current monitoring IC and LCD? I thought you wanted something cheap? I could just add a simple voltage divider to power that part. No, you would need a voltage regulator for parts like that. Looks like I can get a 5x4x3" box for$10.00 or so.
The supply will be fairly efficient, but it would still be a good idea to have some extra room in a case, and use a fan to exchange the warm inside air for cooler outside air.

#### Reaper802

Joined Apr 19, 2010
11
Pulse width modulation can't be used for this box. We use these supplies to break the brushes in on the arms com. Often this is done at 3 volts in water to prevent arcing on the com as the brushes cut in. The cars on the track do not use a PWM circuit, just the voltage adjusted by the controller. If I could use PWM my life would be easy, alas I can not.

In regards to keeping it cheap, my goal is to build a box that does the same things as the Camen but for under the $230 price tag. By the look of the parts so far I should be able to build the box for about$100.

Could you please explain why I would need a voltage regulator to supply the 5 volts I'm guessing the LCD voltage/current monitor will require vs. a simple voltage dividing circuit off one of the 12 volt outputs?

The supply is over 7" long so I will need a larger case and I did spec out an AC fan in my original post. Ether way it still seems like an elegant solution saving a lot of work with the transformer and the bridge rectifier.

Best Regards

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#### SgtWookie

Joined Jul 17, 2007
22,221
Well if PWM is out, then so is that $15 power supply. Linear regulators would have too much of a voltage drop to make that supply usable. You are then going to need a much larger enclosure, as you will need large and expensive heat sinks to keep the multiple output transistors cool, and fans to blow air over the heat sinks. You'll need a circuit like Rifaa proposed. Thread Starter #### Reaper802 Joined Apr 19, 2010 11 #### R!f@@ Joined Apr 2, 2009 9,751 Building PSU is not easy, ever wonder why they are so expensive in the first place, The regulator you showed are low dropout types, which means they cannot dissipate high power. If you try to get 5A at around 5V with an input of 20V, being this the voltage levels according to your desire, the regulator will burn out with in minutes. There is no such thing as high current cheap PSU #### retched Joined Dec 5, 2009 5,208 So how are companies like Camen and Koford building their power supplies in 3x4x5 form factors? Do you see the big fan on the side of the Camen power supply? Thats how they do it. When you want to use the same components for different currents or voltages, you MUST get rid of the heat somehow. If you start off with 10v 1A and you want 5v 1A, the difference must go away.. How does it go away? HEAT. The heat must be transfered into a heatsink from the part and then the heat has to be transfered from the heatsink to the air via a fan. If you pulsed it, you it could be off half the time (50% duty cycle) so you would generate much less heat. This is not a new science or art. It has been done for years and we've got a good handle on it. If you wanna do it, lets do it. It will be cheaper than buying off-the-shelf if you value your own time at$0 an hour.

But it can be done. Riffa did it.

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#### retched

Joined Dec 5, 2009
5,208
Any of them will do.

#### R!f@@

Joined Apr 2, 2009
9,751
hey retched...
OP got u ticked off didn't he.....

#### retched

Joined Dec 5, 2009
5,208
No. Did I sound ticked?

I was trying to get the "This is going to be simple" Part out of his head so we can get to work.

In the beginning I thought of a voltage regulator as a volume knob for power, But I didnt know or care what happened to the rest of the electrons.. Now I know they have to be dissipated as heat, and I was passing the info on.

I dont want to see someone put time and effort into buying parts they think will work because the datasheet says 5a somewhere on it, and end up with a smoking, stinking mess.

I have used those voltmeters and ammeters in my ATX power supply. They work well. and they are easy to impliment.

Rifaa, you already burnt up a bunch of stuff and now you have a design that has short-circut protection, inrush handling and thermal safeties. This would serve our OP well.

He may not realize that the other units have these things, not just a transformer and voltage/current regulators and a display.

Once we get that clear, he will have a nice power supply he can be proud of and be safe to use.

#### R!f@@

Joined Apr 2, 2009
9,751
no biggies.
OP does not realize why such prize tag comes. and he is comparing SMPS too, I think.

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#### Reaper802

Joined Apr 19, 2010
11
Perhaps I should have given a little more information about myself first. BS Electrical Engineer out of Penn State in 2003. My focus in school was DSP not power electronics or circuit design hence the reason I am looking for a little help. We don't really have any hardware guys at work so I figured I would give this place a shot.

The companies who are building these supplies are very small, like one guy small. For this reason I feel their markup is probably pretty high and the hobbyist in me feels this would be an educational and fun project.

Now with that out of the way back to the box. Lets start by redefining the requirements.

Input: 110 V AC
Output: 0 -> 12 V DC up to 8 amps
Voltage/current LCD display
Fuse or instantaneous breaker
Fan
3x4x5" form factor

I see the project having 3 main parts:

1 - A transformer to bring down the 110 to something more workable.
2 - A rectifying IC to turn that AC into a high voltage DC with all those amps.
3 - A voltage regulating IC that allows me to adjust my final output from 0 to 12 volts DC with all those amps behind it.

Thoughts?