I'm not a complete expert on this.

I'm not sure you can rectify 18vac to 18vdc.  My experience has only been with bridge rectifiers, and the diodes are loads, so you will get 14vdc through a bridge rectifier with an input of 18vac.

Rectifier

14 vdc is fine. I just don't now how to built it. I now i need a Bridge rectifier,electrolytic capacitor,a voltage regulator. I need to now the rating of these items,and how to assemble it. Any help is greatly appreciated.

You can buy a bridge rectifier at most electronic or electrical

supply stores.  I'm not sure where you live, or what stores would be in your area.  I think even Grainger may carry them.  I know you can get them from any Thermo-King or Carrier-Transicold transport refrigeration dealer, but theirs will be pricey.  I think most bridge rectifiers designed for industrial applications have a capoacity of about 5 amps.  By the way, you don't need to build one, they come as an assembly in a plastic box about 1 inch square.  Two corners on the square will have electrical connections one marked with a + the other with a -.

Bridge Rectifier

I'm no expert...

A Rectifier similar to this Mouser part should work for you.

Mouser #625-GBPC102-E4

Here is a page that should help with the circuit and calculating the size of the smoothing cap.

Power Supplies

Will you actually need a regulator in your case?

Dear Pollo, If I may ask,

Dear Pollo,

If I may ask, what device do you need 2Amps power for?

Bridge Rectifier

http://www.radioshack.com/search/index.jsp?kwCatId=&kw=bridge%20rectifier&origkw=bridge%20rectifier&sr=1
(use option 2 Model: 276-1146  

Capacitor

I hope you are intending to use this to _smooth_ the resulting DC voltage, and remove the "ripple" caused by rectifiying what is a Sine-wave AC signal.

Suggest
1000µF 50V 20% Axial-Lead Electrolytic Capacitor

2200µF 50V 20% Axial-Lead Electrolytic Capacitor but you _could_ use these Adjustable-Voltage Regulator LM317T |  Catalog #: 276-1778 Model: LM317T   with a pair of resistors to set the desired output voltage, Each of these is good for 1Amp with suitable heatsink. It _is_ possible to connect 2 such variable volt-reg units in _parallel_ such that the resulting output is 2Amp capable. However, I do _NOT_ reccomend this unless you are confident and capable working with what ammounts to welding-current level circuits! You _may_ be able to find the larger TO-3 cased LM317 unit, which can provide 2A from a single regulator. It will still need the pair of resistors to set the output voltage, and I'd reccomend a serious lump of metal as the heatsink!   Alternatively, avoid playing with components altogether, and just buy a suitable plugpack. http://export.farnell.com/xp-power/aed36us18/psu-external-36w/dp/9602968 http://export.farnell.com/xp-power/aem60us18/psu-desktop-medical-60w-18v/dp/1738305 or http://au.mouser.com/Power/Plug-In-AC-Adapters/_/N-wp53?P=1z0wd9nZ1z0wdd4   Proceed with caution, shorting or mis-wiring this level of current-capacity supplies can lead to some nice bright sparks being thrown... Happy Modelling, Aim to Improve, Prof Klyzlr

Recitifier

Thanks Prof Klyzir  for all your help.

   I have a Lionel CW-80 transformer which provides 80 watts, and 5 amps of alternating current. The transformer has two outputs.

1. The track output which is 0-18 vac variable voltage to the track.

2. The accessory output which is 0-18vac programmable voltage to the accessories / switches.

  This is what I like to do:

   On the accessory programmable output which is 0-18 vac at 5 amps. I want to add on to this output a bridge recitifier to change from ac to dc. Add a electrolytic capacitor to smooth out the dc,and if needed a voltage regulator to regulate the voltage to 12 volts. I am trying to achieve a clean smooth dc output voltage.

  I want to run Lionel and other accessories on this output.

1. So solenold operated accessories don't chatter, and hum.

2.So I can use dc relays to control accessories.

3. So sound accessories will not hum.

Thanks to everyone for all your help so far.

                             Pollocko.

Pollocko If you want the humming to stop

Just teach them the words.

Sorry I just couldn't help posting this.

Dan

Rectifiers

You have good info for the basic power supply. Fuse, transformer, bridge rectifier, filter capacitor. Just two diodes works also.

Put this regulator on the output of the filter capacitor. In the below link, look at the 1.5 volt regulator. Just put the appropriate resistor in place of the 47 ohm resistor or use a 1k to 5k potentiometer to vary the voltage.

http://home.cogeco.ca/~rpaisley4/PSupply.html

http://www.national.com/ds/LM/LM117.pdf

Clamp the metal tab on the regulator to a piece of aluminum for at least 1.5 amps capability. The LM317 is good for about 1.2 volts to 37 volts into it. It can regulate down to 1.2 volts.

http://tinyurl.com/3yvqz8r

For higher current, use the LM150 series, 3 amps or the LM138 series, 5 amps.

http://tinyurl.com/33sxvuj

http://tinyurl.com/294m2vc

The nice thing about these regulators is that they shut down if they get too hot.

In all cases, use a thin insulator between the case of the regulator or you will have voltage siting on the heat sink which is not a problem normally if the heat sink is isolated from being touched. The mica insulators usually come with the regulator and use heat sink grease from Radio Shack.

Once a year I remove the cooling fan from my PC CPU chip, wipe of the old grease and put on new grease and replace the cooling fan. I also blow the dust out of the heat sink with low pressure air.

Rich

Rectifiers

Radio Shack has heat sinks.

http://www.radioshack.com/search/index.jsp?kwCatId=&kw=heat%20sink&origkw=heat%20sink&sr=1

Hook up a small fan if there is a lot of heat. Many run off of 12 volts DC. Maybe find a fan in an old PC power supply or in the PC case. Improvise.

http://www.radioshack.com/search/index.jsp?kwCatId=&kw=cooling%20fan&origkw=cooling%20fan&sr=1

Rich

AC voltage

AC voltages are not measured from peak to peak. Instead they're measured at the average voltage point.  The PEAK volts in AC are about 1.4X higher.

Rectifier diodes have a foward bias voltage of roughly 0.7V so a bridge configuration will drop about 1.4V.

Your 18VAC transformer will have a peak output of around 25.2 VAC.  Less the diode drops brings it to 23.8V.

You can voltage regulator to drop the voltage further if desired. The 23.8 VDC (unfiltered) coming out of the rectifier) suffests a 35V rating on any filter capacitors in the circuit. Here's a rather crude diagram of the circuit (without a regulator).

Regards,

Charlie

I realize I am coming to this

I realize I am coming to this a little late but I have also been looking, off and on for some time now, for a DC power supply that I can build at home.  I came across this website which was very helpful for building a power supply, I hope this helps you out.

http://www.electronics-project-design.com/VariableDCPowerSupply.html

I have also found through various web searches a voltage regulator that can handle more current then the LM317T, the LM338T can handle up to 5 amps and will control the voltage from 1.2 - 32VDC.  Of course if you go with the LM338T, in the circuit I have linked, you may have to purchase some components that can handle the increased current abilities (primarily the transformer, rectifier & diodes) but the voltage and capacitance ratings of the components would not change.  The cheapest I found the LM338T is from a website called Avnet Inc. the LM338T costs about $1.08USD plus S/H (I think $9.00 for Fed Ex shipping - this is the only draw back to the website).

Here is the website http://www.avnet.com, once at the website type the P/N of the item you are looking for in the window labeled Avnet Electronics Marketing.  I don't know of any stores that carry it unless you have an electronics specialty store in your area.  I plan on building a couple of these eventually but right now my money has higher priority items in its sights.  Again I hope this helps you out.

Diode voltage drop

Isn't there going to be a voltage drop through the bridge rectifier?  I think you would need to supply slightly more than 18 volts AC to get 18 volts rectified DC output, but I'm no expert.

The link posted above would be a start for the circuit design, IMO.  Remember to use a heat sink to dissipate heat if using a voltage regulator.

Kevin is correct each diode

Kevin is correct each diode in the rectifier drops .7 volts, so in a bridge rectifier the diodes would drop a total of 2.8V give or take.  So to be safe the recommendation is a 24VAC supply transformer this way you will have plenty of voltage "to spare" for 18VAC.  The voltage regulator will do the rest for you and, also as Kevin stated, don't forget a heat sink for the regulator.