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Simple DC Power Supply Schematic Needed

Posted: Wed Jun 18, 2008 7:39 am
by Craig
Hi Guys,

My Dad asked me to build a DC power supply for his garden rail road. He supplied me with a transformer that he bought a Hammond 185G24 transformer. The datasheet on it can be found at It can output 24v @7.3 amps when the secondaries are wired in series, and 12v @ 14.6 amps when wired in parallel.

He wants it to power two trains, and he already has two controllers for it that can each handle 24 volts and 10 amps, although I'm sure that each train won't use anywhere near 10 amps, but I would like the design to handle it.

The DC output does not need to be very clean, just something very basic will do. I have searched around, but can't find any decent designs that can handle over 1-2 amps.

Any suggestions will be greatly appreciated!



Posted: Wed Jun 18, 2008 8:12 am
by gerty
I don't know a thing about train power supplies, does it need to be regulated?. If not you can just put a bridge rectifier on the output, you can get them up to 35 amps for about $5.

Posted: Wed Jun 18, 2008 8:22 am
by Craig
Excuse my ignorance, but I don't know the difference between a regulated and non-regulated power supply.

Posted: Wed Jun 18, 2008 8:52 am
by sghioto

The specs on the other two controllers would be a big help. What is the max and min voltage output from the controller when the train is operating?

Steve G.

Posted: Wed Jun 18, 2008 9:09 am
by gerty
Regulated simply means the stated voltage out will remain constant under load. A unregulated supply ,such as the transformer and bridge rectifier I mentioned earlier, will have an output voltage that'll vary according to the load. It'll usually have a higher than stated output voltage without a load.
For example a supply rated at 12v and 1 amp will probably put out 14 volts unloaded and 12 with a 1 amp load. A regulated supply will put out 12 volts with or without a load. As stated earlier I know nothing about trains, but since they are just motors I wouldn't think it would need regulated power.

Posted: Wed Jun 18, 2008 9:27 am
by Craig
I agree with you gerty. It's a very basic setup, and the train is just going in a circle.

Sghioto, here are the details of the controllers:
Input: 16 V DC
Train Output (DC): 0-21 V
Train Current: 20VA
Accessory Output (DC): 15 V
Accessory Current: 7 VA
Total Output: 22 VA, 5 A MAX

For more info, see ... -5401.html.

Posted: Wed Jun 18, 2008 10:07 am
by dacflyer
i am pretty sure this is just controlling the motor on the train.
me, i'd use a variac to control the transformer and then just use a bridge rectifier. that transformer you got sounds like wayyy over kill.

how long is the track ? around the garden or around the house :P

the power pack you have now is 16 volts, but your transformer is 24 volts..becareful, you do not wanna smoke the motors in the engines,
and typically the accories on them power packs is ac to run the street lights and other lights for buildings etc.

Posted: Wed Jun 18, 2008 10:10 am
by dyarker
I'd suggest setting up the transformer for 12V. Full wave bridge rectified.

Your latest set of specs say controller input is 16V. 12V times 1.414 (the RMS to peak conversion factor) is 16.968V, minus about 1.5V for forward drop of rectifier would be just about right.

That 10A max doesn't make sense with the train current spec of "20VA".
20VA at 21V looks like just under 1A to me. A mistake on the spec sheet maybe?


Posted: Wed Jun 18, 2008 10:28 am
by sghioto

I would connect the xformer windings for 12 volt operation. Use a 25 amp bridge rectifier and two 4700uf/25 volt capacitors .This will provide a 16 volt dc output for the controllers

Steve G.

Posted: Wed Jun 18, 2008 10:47 am
by Craig
Thanks for the info. How do I setup a bridge rectifier? Is this a single component piece that I can go buy, or do I have to make one myself? What parts would you recommend?

I posted this before I saw the previous post. Again, thanks for the info, I will go to my local electronics store to see if I can find a 25 amp bridge rectifier, and pick up a couple of capactiors as well.

I will let you guys know how I made out.

Posted: Wed Jun 18, 2008 10:58 am
by sghioto
You can get a discrete bridge rectifier and capacitors from Mouser. The parts are in stock and no minimum order is required.
Steve G. ... LWlg%3d%3d ... d6xw%3d%3d

Posted: Wed Jun 18, 2008 10:59 am
by gerty
A bridge is a one piece device with 4 terminals ,2 marked with ~ for ac input and 1 each + and - positive and negative out.

Posted: Wed Jun 18, 2008 11:20 am
by haklesup
You state a DC voltage but then spec power in VA which is typically only used for AC power. So now I have doubts that you want a DC variable supply (which by most designs would be regulated)

Most of the circuits suggested are not variable. The simplest solution is the variac into a 12V transformer plus rectifier adf caps. if it turns out to be AC, the variac into a 12V transformer would be good and safe.

If you need variable DC at 12V, 20W, you will only need 600mA so a 1A variable DC supply should be sufficient

These specs are crazy, For example

Input: 16 V DC
You would need a DC-DC converter in there if the primary were DC lower voltage than the output

Train Output (DC): 0-21 V
OK, I'll take your word for operating Voltage looks reasonable

Train Current: 20VA
VA is power not current and is an AC not a DC term. at 1V, 20VA is 20A but at 21V, 20VA is only

Accessory Output (DC): 15 V
You wnat this thing to have two seperate outputs, just use a separate supply for accessories.

Accessory Current: 7 VA
Again VA is not current. at 15VDC, 7W gets you 460mA unless this actually referrs to the power used by an unspecified 120VAC mains then the DC output current could be much higher and is inpredictable from the data.

Total Output: 22 VA, 5 A MAX
This is only possible if the voltage does not exceed 4.4V. So you can't have high current and high voltage at the same time. This also implies that the supply is unregulated.

Posted: Wed Jun 18, 2008 1:16 pm
by Craig
The specs that I posted were copied from the website of the product that I want to power. I don't need to build something to those specs, I need to build something that will power a device with those specs.

I just picked up a GBPC2502 25 amp bridge rectifier. I will try to put it together tonight or tomorrow.

Posted: Wed Jun 18, 2008 2:56 pm
by CeaSaR
The question that should be asked is:

What are the specs on the trains? That will give a better idea of what you
really need. Also, what type of trains they are and maybe a link to them
would help.

Garden trains sound like they are probably G guage or larger. The motors
in them could be either AC or DC, depending on manufacturer, hence the
need for train specs / links.