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

R!f@@

Joined Apr 2, 2009
9,918
Are u sure you can put in LCD DMM with a 8A variable in to such a small case
on second thought u can if you put the heat sink outside at the back.
I do have such a small one but the heat sink is outside.
12VAC 8A Tx can be smaller than those measuements
 

R!f@@

Joined Apr 2, 2009
9,918
OK. Here is the deal.
Find a transformer of appropriate size that will satisfy you.
but you need these specs. 110VAC to 15VAC at 8A continues. so to say a 120VA transformer.
Laminated core or a toroid. any thing will do
 

SgtWookie

Joined Jul 17, 2007
22,230
Well, if you really want a small form factor, you'll need to go with a switching supply.

It's possible to make them inexpensively in very large quantities, but one-offs or small runs can be rather pricey.

Your computer power supply is an example of a switching supply. It's really quite efficient for the small form factor.

Linear regulators output nice, smooth DC. However, as has already been mentioned, you pay the price with the dissipation of power as heat. In the worst case scenario, where your load is at high current with low voltage across it, the regulator is dissipating most of the power.

If you would like to do some reading on switching power supplies, here is a good resource for you:
http://smps.us/
In particular:
http://www.smps.us/smpsdesign.html

This is a rather basic introduction to linear regulators:
http://www.national.com/appinfo/power/files/f4.pdf
This is a somewhat basic introduction to switching regulators:
http://www.national.com/appinfo/power/files/f5.pdf
However, if you haven't looked at the fundamentals for a while, it's worth the time to read through.

Linear regulators are fairly simple compared to switchers. However, there is no way that they can match the efficiency of a switching regulator.
 

Thread Starter

Reaper802

Joined Apr 19, 2010
11
Good question about the 8 amps. I know mid end motors will draw up to 4.5 amps when breaking in. These supplies are also used to power tire truers and com lathes. I will have to check and see how many amps they want. If I could get away with 6.5 amps it would be a huge savings in the build.
 
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SgtWookie

Joined Jul 17, 2007
22,230
I am uncertain as to that particular supply's action if the 6.5A rating is exceeded. Some switching supplies shut down immediately, some will fold back to a low current level until the load is removed.

For that price, you could run two in parallel to double the current, and still be so far ahead of the money curve it's almost silly. You'd want to ensure that the supplies' output voltages were adjusted to very close to identical so that they won't "fight" each other. A 0.1 Ohm power resistor on the output of each would ensure good load sharing, and make the adjustment less critical.
 

Thread Starter

Reaper802

Joined Apr 19, 2010
11
All the feedback I am getting from the guys with tire truers and com lathes is that they never pull more than 5 amps. Motors seem to never pull more than 5.5 amps. So at this point it seems the cheep power supply is going to work.

Now how do I get 24 volts DC at 6.5 amps into a voltage regulator? Would I use 7 or 8 voltage regulators rated at 1 amp each in parallel?
 

SgtWookie

Joined Jul 17, 2007
22,230
Go here:
http://www.national.com/mpf/LM/LM117.html#Overview
and download the datasheet for the LM117/LM317.
(they're very similar, the LM117 is rated for a wider temp range).

It's a good idea to read through that datasheet several times just to get an idea of what that regulator will do. They are really quite remarkable for such an old design.

Then look at the top of page 17, at the schematic entitled "High Current Adjustable Regulator". You won't be using the exact same transistors, and the final schematic will differ a bit, but the example could be a decent starting place.

Instead of the LM195 transistors, you can use 2N3055 transistors in a TO3 case.
 
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