I have had so little time to do train stuff that I have to relearn everything each time I get back to it... so I apologize if I repeat anything here... and any cross-post...

My original plan was to rectify 16V AC for powering accessories and building lights at each point of usage. i.e. run a bus of 16VAC and tap that as needed. My initial LED light circuits were based on that principle...
<http://layout.mixmox.com/1/LED_module>

Then when I did the lighting for my signal box,
<http://layout.mixmox.com/1/signal_box>
I found I burnt out some LEDs because I had done my calculations on the RMS voltage and not the peak voltage so I had to rethink my circuits. In doing that I again considered running a properly regulated clean DC bus wire under the layout for my lighting.

Then I was given a box of old (1960's) power supplies, transformers and magnetic voltage regulators, including a huge 15 Amp 24V power supply (360W) and a very fancy 15V 22W regulated power supply. Suggestions regarding the fancy power supply was to use it 'in the lab' for testing etc.

Now I want to run this logic past the electronic experts here to ensure that my thinking makes sense and not too many sparks...

The variable voltage power supply will only produce 1.5 Amps and each LED will use 20 to 30 mA so that would only drive about 60 LED lights, so I conclude that 1.5A will not be adequate.

After spending some hours looking at the circuit and huge capacitors on the monster power supply and figuring out what the magnetic voltage regulators are all about I retuned to my old idea of using an old PC power supply...

The label says that it produces the following outputs:

3.3V 24A
5V 35A
12V 12A

I believe that these voltages are smoothed and regulated suggesting an ideal layout DC power source. The unit is also in a nice compact case, is fused and has its own cooling fan. If it ever dies, I can find another one and plug it in.

Does anyone see a problem using a PC power supply?

Assuming that is a GO, my next decision is: "which voltage should I use... 3.3V, 5V or 12V..."

White LEDs have a forward voltage of about 4V - the ones I have, have a Max forward Voltage of 3.3V suggesting that the 3.3V supply would be ideal. If however later LEDs need more, I won't have any headroom... I am not sure on what the minimum forward voltage is for these LEDs - possibly 3V.
Also, I suspect that fluctuation and voltage drops may result in less than 3.3V resulting in some LEDs not lighting up.
24Amps would drive about 1000 LEDs. Plenty.

If I use the 5V supply I would not be able to drive two LEDs at a time but a series resistor of 56 Ohms to 85 Ohms would run a single LED at 30 to 20mA and each unit would use about 0.1W allowing about 1750 such lights, also plenty, but the disadvantage is I cannot place two LEDs in series as the forward voltage is not available.

If I use the 12V supply, a single LED would need a 290 to 435 Ohms resistor and at 30mA that starts exceeding a 1/4W resistor rating and would call for 1/2W rated resistors - probably not a big deal. One LED plus resistor would use about .36W and I could drive a total of about 400 such lights. Significantly fewer lights as more energy is going into resistors... and I have no idea how many lights I will need.
With 12V I could power up to three lights at a time which could be advantageous.

So can anyone suggest any further advantages or disadvantages for the 3 available voltages?

I am currently driving my k83 type devices using a separate 16VAC supply (not digital power) - I suspect I should not steal any of the 12V DC for such purposes though.


Should I consider any measures to prevent accidental 16V Ac or any other spikes into a low voltage line that is driving items that do no tolerate over-voltages? If so how? I guess I could place a diode on the DC line to prevent half of any AC from going the wrong way... is there a smart way to limit the voltage very fast so that a single accident does not destroy all installed LEDs in one flash of HO scale lighting?

Dale (sometimes in the dark)

Very interesting topic - thanks Dale.
Tony has a very good point regarding OTHER uses for the power supply - perhaps you may want to add some street lights or other non-LED consumers at a later stage? In which case why not go for a higher voltage and just put resistors in where needed?

Also, bear in mind that not all LEDs have the same Vf; I've used some white LEDs with a Vf of 2.2V, in which case you could run 2 LEDs of the 5V supply.

They are getting cheaper all the time, earlier this year I bought some at 50c each (US) including shipping. I like the fact that they give off almost no heat and their life is very long - allowing me to embed them in buildings without having to ensure that I can get back to them to replace them...

Gi Dale,

Here my ideas to your lighting problem:

Using the 5 V output of a PC power supply is certainly a good idea. There are two points I would consider:

- You should protect your light circuit against an occasional short circuit. The power supply can provide 20 A or more. This might fry your electric wires in case of a short circuit. I would subdivide the light circuit into 4 or 5 separate branches, each protected with a 3 - 4 A fuse. No risk any more to damage something.

- M* transformers have a reliable insulation between the primary mains and the secondary 16 V circuit. I don't know how this is for PC power supplies. Probably there are good and bad ones. Perhaps it is wise to have an eye on the safety labels. Have a strange feeling in my stomach since I once had a glance on the wire jungle inside a PC power supply casing.[:0]

Hans Martin

-

Yes thanks..
I have drawn a diagram how I think this should be done. It can be seen at

<http://layout.mixmox.com/1/zener_voltage_protection>

Did i get it right ? Any suggestions?

Yes.


Doesn't matter really.

An advantage with using 12 V is that you can use the same power feed for other MRR accessories, which often also are 12 V DC.

An advantage with 5 V is that you can incorporate electronic circuits which usually use 5 V.