Converting AC Output to DC or Is a Rectifier What I Think it is?

[jonathan]

Specifically, here is what I want to do:

I wish to take advantage of some of the unused 16V AC outputs on my power packs for LED lighting on my layout.

I would like to convert the AC output to DC and run a bus to tap into for scenery lighting.

Running a few searches, I believe I need a 'rectifier' to accomplish this task.  I found some generalities, but not a specific size device (or math formula) to make it happen.

A search of Radio Shack revealed no less than 160 choices of various rectifiers and diodes... All very inexpensive, but no option gave me the specifics for voltage conversion and the like.

So my question is:  Which rectifier and/or diode do I need? 

I can noodle through the LEDs and resistors.  Trying new electrical stuff again.  I'm gonna burn the house down one of these days. 

Regards,

Jonathan

[mhampton]

A diode is a very simple rectifier.  If you simply put a diode after your AC source, you would have "half-wave" DC.  What you probably want to find is a full-wave bridge rectifier (essentially composed of 4 diodes) with a reverse voltage rating of at least 22.5 volts and a current rating that is at least what you can get from your AC source.  Hook your source to the AC legs on the  bridge - usually marked with a sine wave.  The DC legs should be marked "+" and "-".  The DC you get won't be smooth, but that shouldn't matter for lighting applications.  When you do your noodling for the LEDs and resistors (and if you don't check the output with a voltmeter), use that 22.5 volt value rather than the 16 volts of your source.

[poliss]

Brian Lambert has information and diagrams for bridge rectifiers and powering LEDs on the Electrical Bits section of his website.
http://www.brian-lambert.co.uk/Electrical-2.htm

[jonathan]

Thanks, gents.

That was a big help!

Great tip on the increased DC voltage.  I wasn't expecting that one.

Thanks again!

Regards,

Jonathan

[richg]

For some years I have stripped components out old electronics. I normally use a 1000/35v ufd cap for filtering. I usually prefer a filtered DC, though that is not always necessary.  I make sure to put a resistor as a minimum load on the capacitor in case I switch off whatever is connected to the cap.
I have found a lot of full wave bridge rectifiers, also.
I also have used the LM317 voltage regulator. With the pot, you can vary the lights on a layout or any other device. Say you have a buss for aux power on your layout, stick a VR where you need a different voltage.
Some run a 24 volt buss and stick a regulator where they need a lower voltage. The buss voltage has to be about three volts higher than what you need.
There are many more possibilities you will find as you learn more about this.

http://www.zen22142.zen.co.uk/Circuits/Power/lm317.htm

Rich

[jonathan]

I also like to salvage components. however, I'm not that savvy to recognize various pieces parts. Fortunately, those parts can be had for cheap at the local Radio Shack.   

Regards,

Jonathan

[Joe Satnik]

Dear All,

Analog voltage regulators are nifty, but have their limits. 

If you drop significant voltage, e.g.,

24 volts unregulated down to 4 volts regulated = 20 Volt drop,

you must multiply that by the current draw, e.g. 1 amp, 

to get the dissipated power in the regulator, in this case 20V x 1A = 20 Watts.

20 Watts is huge, and will burn up a regulator if it is not made for that much power,

or is not mounted properly to a big enough heat sink.

Switching regulators can handle big voltage drops much better,

but have more complicated circuitry surrounding them to make them work.

Hope this helps.

Sincerely,

Joe Satnik