12v power supply

Thread Starter

Heinz57

Joined Dec 16, 2009
24
Hi,

I am planning to build a 12v power supply. It's for a model railway. It will have probably 2 transformers in it (so I can have 3 or 4 outputs).

First of all transformers, can anyone help me track down a 240vac - 12vdc open case transformer? I've done a google search but Its giving me ac - ac transformers.

Outputs, for the outputs I plan to use probably XLR connections (obviously with power cable and not signal cable) I assume this is ok?

Any help is appreciated,

Heinz
 

SgtWookie

Joined Jul 17, 2007
22,201
One of the easiest ways is to simply convert an ATX form factor computer power supply.

Cheap, effective, and can be done quite quickly

Google "ATX Bench Supply" for lots of ideas.
 
One of the easiest ways is to simply convert an ATX form factor computer power supply.

Cheap, effective, and can be done quite quickly

Google "ATX Bench Supply" for lots of ideas.

I am involved in R/C airplanes and this is what I use for my battery charging power supplies....

pretty easy to do.

Not sure if you need to be a member to see the below link, but here is a nice guide by a good friend of mine:

http://www.flyinggiants.com/forums/showthread.php?t=12159&highlight=frankenpower
 

Thread Starter

Heinz57

Joined Dec 16, 2009
24
How much current do you need?

All transformers are AC devices. To get a DC output, you must rectify, filter, and regulate the output - http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
So I'm best to use a recifier. I've found some transformers on Rapid online which look suitable for the project. Any other components I might need?

I'm currently working on some rough schematics of the project.

Cheers
 

SgtWookie

Joined Jul 17, 2007
22,201
If you are determined to build your own power supply, consider downloading National Semiconductor's datasheet for the LM117/LM317 voltage regulator. There are a number of ideas in the Applications section for high current output regulators, both linear and switching.

However, you will be very hard-pressed to match the economy, reliability and efficiency and ease of simply converting an ATX form-factor PC supply.
 

Thread Starter

Heinz57

Joined Dec 16, 2009
24
The main reason I dont want to do the conversion is I've got a method of building my own which is pretty simple to follow. I can't understand the conversion.

I just need a bit of help working out some values of components now. I have a schematic below of a circuit of LEDs, this will be build at the same time, and powerd by the power supply unit, housed in a separte box and connected via the leads mentioned in the first post. I need a little help in what value of Bridge Rectifier to use if the voltage is 12v and what resistors to use, in the locations on the schematic. If the voltage is 12v. I see the current is 10 and 20mA respectivly in the LEDs but is this the current I put in ohms law? I'm not sure.

Cheers,

Heinz

EDIT: Please excuse the dodgy drawing with the components, I did my best using MS paint
 

steinar96

Joined Apr 18, 2009
239
I would recommend a series regulator and a filter capacitor after the bridge but it might not matter though. As the schematic is right now the diodes will briefly not conduct when the AC wave falls down to zero between peaks (fixed with a filter capacitor and even better also with series regulator for more voltage stability). So you might experience flickering or reduced light output. Though at 100hz i'm not sure you'll notice the flickering. Can someone elaborate on that ?
 

Thread Starter

Heinz57

Joined Dec 16, 2009
24
Hmmm, is there something I could put in to stop the problem?

Also, please excuse that a couple of the 'diode icons' on the bridge rectifier are the wrong way around. I didn't notice until I posted it. Top right and bottom left are the wrong way around. My bad
 

SgtWookie

Joined Jul 17, 2007
22,201
You're trying to run LEDs in parallel on a single resistor (the yellow ones)
This is bad practice. If one of the LEDs opens up, the others will have to take the remaining current - and will rapidly burn up.

You can get away with running several in series, if you use a current limiting resistor with a controlled voltage source, or an active current limit circuit.

You're trying to make things too simple, and it won't work well.

You need a large capacitor after the rectifier bridge, and a voltage regulator to ensure constant voltage to the LED strings.
After you have a rectified, filtered, and regulated voltage, then you can figure out how many LEDs you can have in series or parallel.
 

Thread Starter

Heinz57

Joined Dec 16, 2009
24
Thanks for the info, I've beard it in mind and re-done the schematic


Should that work ok? Any advice on values?
 

steinar96

Joined Apr 18, 2009
239
The capacitor is in the wrong place. As it's positioned now it will actually block DC current :rolleyes:.
The capacitor is connected between the 12V line and ground. Between the diode bridge and linear regulator.
 

Thread Starter

Heinz57

Joined Dec 16, 2009
24
The capacitor is in the wrong place. As it's positioned now it will actually block DC current :rolleyes:.
The capacitor is connected between the 12V line and ground. Between the diode bridge and linear regulator.
So that means I just need to swap the capacitor and regulator around?
 

SgtWookie

Joined Jul 17, 2007
22,201
More like this:



P1 is the wall plug. Note that it's shown as conventional in the USA. Here, the short blade is the "hot" or "live" blade, the long blade is neutral.

The fuse is required for safety. The power on/off switch S1 is optional.

C1 is the filter capacitor; 3300uF is about right for up to a 1A load.
C2 and C3 should be located as close to the regulator as possible.
A 7812 is a fixed 12v positive regulator, good for up to 1A. With light loading, a heat sink may not be necessary. If you're planning on running more than a dozen LEDs, use a heat sink.

Only two LEDs with their switches and resistors are shown; one white, one yellow.
 

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Thread Starter

Heinz57

Joined Dec 16, 2009
24
Thanks for the schematic, much appreciated! I've interperited it with all the LEDs (Shown below). I'm assuming what I've done with the '+' and '-' on the Bridge is correct? If it's wrong let me know and I'll alter it. The transformer is not shown on this diagram, this LED circuit will be (as mentioned) located in a separte box and connected via a cable.

This is the schematic for the power supply. I havn't used symbles but it's labled accordingly. The wires are connected in various places with terminal blocks (as shown) the power input comes from IEC (I've got it down as 5A but if you think 1A is better, that's what I'll put in. The IEC input also has a switch on it.

As you can see I've earthed it. The method I have says something about including 'cutouts' but it isn't a nessessity. I take it they arn't realy worth while seeing as I've earthed it.

Cheers
 

SgtWookie

Joined Jul 17, 2007
22,201
I'm afraid your supply wouldn't have worked like that.

You had the lower left diode in the bridge shorted.

Like this, instead:



I have no idea what your actual power requirements are.

If you just have these few LEDs, you won't need much power.

You fuse the primary side to protect yourself and the transformer from overload, and from internal shorts.

If you so wish, you can fuse the secondary side of the transformer as well.
 

Attachments

Thread Starter

Heinz57

Joined Dec 16, 2009
24
Thanks very much. What value bridge should I use? Would it be 12V?

You mentioned something about heatsinks. One idea I had was installing some sort of fan in the power supply casing, simaler to that on a computer. Would this work?

The basic power requirements are just to light the LEDs. I'm using a 12v supply because we use it in the railway hobby to power all sorts of accessories (or at least we do this side of the pond)
 

SgtWookie

Joined Jul 17, 2007
22,201
Thanks very much. What value bridge should I use? Would it be 12V?
I'd use a bridge rectifier rated for at least 50v, at least 1A.

Note that the voltage at the output of the bridge will be around 17v at low load. Your capacitors should be rated for at least twice that much voltage, or 35v.

You mentioned something about heatsinks. One idea I had was installing some sort of fan in the power supply casing, similar to that on a computer. Would this work?
Sure, if you can find a 12v DC fan. Many of the newer fans are brushless DC motors that need a driver circuit to to work.

The basic power requirements are just to light the LEDs. I'm using a 12v supply because we use it in the railway hobby to power all sorts of accessories (or at least we do this side of the pond)
Yes, railroader hobbyists (and many others) do so as well.
 
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