Is a DC Transformer needed for some LED's?

Why not post a link to the item(s) in question - "steady stream of DC" is insufficient information.

Your LED accessories have internal electronics to convert the AC from a CW to DC as needed by the LED devices.  Since the welder apparently needs DC, if you want to run it from CW AC, you need to perform this AC-to-DC conversion.  With some soldering and component (diodes, capacitors, etc.) you can do this for a couple dollars.  Or you can buy an AC-to-DC voltage converter module for, say, $5 with relatively simple 2-wire in, 2-wire out hookup.

Or you can use a DC-output adapter (a.k.a. wall-wart) like an unused cellphone charger.

More specifics on your options can follow after getting the DC requirements of your welder device.

In your Lionel stuff with LEDs, AC is being rectified to DC before it hits the led lamp.

 Like Stan says knowing more he can produce a more "ideal supply".

  But more than likely, if its a well built unit, it just needs a bridge rectifier, or a few diodes to run off the transformer.

 Capping the max voltage might be important too. A small voltage regulator might be needed too, and that's where design, while still basic, needs "more" added, based on the light unit.

 Ask the supplier for a spec sheet, at least min.-max. voltages, etc . If they don't have one, they are likely "middle men" & not close with the mfg, so no telling what you'll have. Might be just a lamp, or it might be on a switching board.

 $5-$20 is my guess. Once done, it can still run on dc power well too. 

i bought some cool and inexpensive led police cars and mail truck from guy at led depot on ebay 2 were ac or dc so hooked up to terminal board but other was just dc so used a old phone power cord converter i had like stan reccomended

Just curious: Why isn't anyone recommending picking up an old used DC transformer used to run things like HO trains, etc.?  I had always thought that's what I'd do to run some of the LEDs planned for our layout.

I know almost nothing about this topic, so all replies will be helpful for me.

Thanks.

Steven J. Serenska

LEDs all use DC.   IF there is AC coming into a device with LEDs, it needs to be converted.

Most LEDs I have worked with use 4.5 volts DC and very very little current.    You need protect them for a current overload.  

The simplest way to power an LED from an AC transformer in the 12 volt range is to wire a bridge rectifier across the output poles to convert to DC, and then install a 1000-1500 ohm resistor (maybe 1/2 watt) in series with the LED.   

IF you still have a Radio Shack open inthe area, a bridge rectifier will run a couple of bucks and a package of resistors will run less than 2 bucks.  

You can play with the resistors to adjust brightness.   the bridge can be a small one with a 25-50 amp capacity.

Bob

If your not familiar with Evans Design do a search and view their options on LEDs. You will find it very informative.

They offer a welder and camp fire arrangement kits that you can choose the power options. I use the AC kits because I just wanted everything powered on my lay out with AC train type transformers.

Larry   Have fun

That's not an awful idea, but not always ideal....

  I like to play with cheap boards that normally operate at 2-3volts. One accidental bump to high voltage could burn out an unregulated circuit easy.

 That's why wall warts tend to work out so well for some folks. They have a constant supply. Normally new ones are pretty clean in delivery too. I've seen a few "noisy" supplies, ac leaks in older small DC variable "power packs" too.

25-50 amp?

6amp would likely be extreme overkill for just a flashing led strobe.

 A small, old school transitor sized 3amp bridge rectifier is likely still way more than enough.

Smaller, cheaper than that, a few diodes placed properly, is the same thing as a bridge rectifier.

http://en.wikipedia.org/wiki/Diode_bridge

 What Stan wants to do is smooth out the "ripples" that still remain, with proper capacitance.

  I.e. a capacitor acts like a tiny quick release battery, to even out the power ripples when AC reverses towards 0 volts/negative volts, on each AC cycle(60 hertz). Polarity change, and voltages are constants in an active "pure DC". 

To answer the question,you don't need a DC transformer. all you need is a diode or bridge rectifier and a series resistor. Here is a single headlamp made for AC input. A flashing LED might need a filter capacitor. 

LINK

Dale H

A word of caution regarding using HO DC supplies:

A typical small HO supply uses a series rheostat (variable resistor) to drop the output voltage.  That means that the actual output voltage depends upon how much current is being drawn.  For a mimimal load, the first step when turning up the knob may produce almost the full rated output voltage!

I use a diode and resistor for display of turnout position using an LED as shown. This LED is powered by the AC voltage from the RailKing turnout itself. No other circuit is needed. The two pushbuttons shown operate the turnout.