Fixed DC

[Bennekers]

I have a power supply with 16V fixed DC. I always wondered why that was, since the most needed for accessories is usually 12V DC.
Is there a simple way to transform from 16V DC to 12V DC. Are there transformers available at a reasonable price or is it cheaper to buy a 12V DC power supply?

[rrinker]

The 16V fixed DC output on MRC packs was designed to power their handheld cabs that you could connect.
It's not too difficult to drop the voltage - a few diodes connected in series will do the job. .6V per diode, so use about 7 of them. You need the rectifier-type diodes, not the little glass signal diodes, and they should be rated for at least whatever current draw you plan.
What do you plan to power with the 12V?

--Randy

[Bennekers]

Thanks Randy,

I have among others several train detection units that needs 12V DC. I could also use the DC output from another power supply, but that would tie up that power supply.
Are those diodes available at Radio Shack?

[SteelMonsters]

Yes Radio shack has some.

To price them out here is a good page.
http://www.jameco.com/webapp/wcs/stores/servlet/CategoryDisplay?langId=-1&storeId=10001&catalogId=10001&categoryId=11286&pagenum=1

Just remember that they will get toasty if you continuously draw there full amperage. Also , they disipate some excess heat theough the leads It's not too easy to damage the diode since it's pretty rugged, but burnt fingertips is easy..
.

[rrinker]

Also keep in mind if you are using the variable output to run your trains, loading down the fixed DC output is going to reduce the power available to run the trains. Depending on how much current is required, it might be better to just get a different 12v power supply. There are also seveal web sites that show you how to modify an old computer power supply to use for such purposes.
If you go the diode route, get at least the 3 amp 100v ones. 1N5401. 10 for 75 cents or so. That should keep them from getting too toasty, as I doubt the power pack can supply more than 1 amp continuously.

--Randy

[jkeaton]

Depending on how much amperage you need for your train detection units, you could also buy a 12V IC (Integrated Circuit) regulator such as a 7812 - good for 1.5 amps. These are dirt simple to use - they have just three terminals, one + power in, one regulated + power out, one connection to ground. If overloaded or overheated they shut themselves down until they cool off before they get damaged. Plus, the regulated power out is exactly 12 V, even if your supply voltage varies (which it will, as the load put on its other output by your train varies).

BUT - they do throw heat - for example, at a steady one amp load, given your 16V input and a 12V output, the 7812 has to dissipate 4V times 1A or 4W of heat - the same as a nightlight bulb (which is hot enought to burn your fingers!). So, you MUST mount the 7812 on a sheet of metal - preferably thick aluminum or at least thick steel - so the waste heat gets carried away. Usually the 7812 comes with a metal tab with a hole in in it. Drill a matching hole in your new sheet of metal heatsink, and bolt the two together. For best performance (if you really are drawing that one amp all the time) buy a little heat conductive grease at the electronics shop and smear it between the metal tab on the 7812 and the heatsink.

It's not that you can't use a string of diodes to reduce the voltage - but for track detection equipment (or anything else that needs a steady voltage), the IC regulator solution is better - and not expensive.

Jim

[rrinker]

Yes but don't forget to put a filter capacitor on that fixed DC output - it might even be only half-wave DC, depending on the power pack. Even if it is full-wave, the filter capacitor is a good bet, the regulators don't like noisy power sources. Adding a filter capacitor to 16VDC will raise the actual output slightly - I'd STILL stick a couple of diodes int here to drop it a little and spare the regulator. My very first computer was a single board kit that I built, and they took this same step - the cheapest transformer they could get was 8VAC, after rectifying and filtering it they had north of 10VDC, and with the regular running at near capacity current-wise , it would run too hot and shut down (nice thing about three terminal 7800-series regulators, self protecting!) so they included 4 or 5 diodes on the circuit board to drop the voltage prior to the regularor - it worked! Still works, 20+ years later.

--Randy

[Bennekers]

Thanks, I like the regulator approach if I can find the right one. Are those usually available at any electronics store? I know something about electronics, but not too much 'hands on'.
I am not too worried about the power supply, I think it will deliver enough power for variable and fixed DC. I am not going to hook up accessories on it.

Thanks for the input.

[Bennekers]

If I am not mistaken the power supply can deliver 5A at 12V.

[rrinker]

If it's a 5A supply, you will need multiple regulators, the common 12V regulator, a 7812, is only good for anywhere from .5 to 1.5 amps depending on the variation, and that's only if mounted on a heat sink. With 5 amps of source current you can build 5-10 of these circuits and provide that many different 12V outputs to divide the load, if needed.

--Randy

[jkeaton]

You should be able to find the 7812 - or a functional equivalent - at nearly any electronics store. Even Radio Shack still carries them (at least here in Canada) and they don't carry a lot of parts any more.

And I should have thought to mention filter capacitors on the output - duh! Sorry about that. rrinker's suggestion of a few power diodes on the input side if you find it running hot is a good one - just make sure that they, too, are rated to handle the current and have someplace to vent their heat (which at 0.7V times 1A is 0.7W, not as much as the regulator but still not inconsiderable).

Jim

[Bennekers]

The detection units should not draw more then 1.5A alltogether (3 ISS units). As Randy said I still need power to run the trains.
I am planning on using some diodes and a filter capacitor, what rating? (mF)?