One thing to keep in mind - if you use a single diode as @Consolidated Leo posted above, you can still use you outside rail as ground. If you use a rectifier bridge or some other device (refer to a second post by @Consolidated Leo), please take careful note that the rectifier bridge or other device generates its own ground that you must use, and can not use the outside rail.
As an old rule of thumb I was taught many moons ago, for a full wave rectifier, you use 1000uf per 1amp being supplied. For half wave, I would double. For 18VAC type layouts, a full wave rectifier would yield about 25VDC, so I suggest the cap should have a working voltage rating of 50VDC or higher. Note that these thousand uf caps are polarized, the case these days normally has a segmented bar/arrow with a minus sign in it pointing to the negative terminal of the cap.
BTW - I know this if TMI, but - to get an idea of what VDC a full wave rectifier will create from VAC, you multiply the VAC by 1.4 (officially, use the square root of 2, but 1.4 is close enough for government work). This is due to the fact that AC is spec'd not as a peak to peak value, but as an energy equivilant to DC. So for example, you house has 120VAC, but the peaks are + and - 168v, and a full wave rectifier would yield about 168VDC.
I found an online calculator to determine the value of a series resistor to limit the current through a LED. This calculator is for a single resistor in series with a single resistor. You simple enter the DC supply voltage, the forward diode drop (2 to 5 volts - I normally use the 2 value), and the LED max forward current, which is normally 20ma. My thanks to @gunrunnerjohn for catching my decimal point mistake, which I have corrected above.
https://www.digikey.com/en/res...-led-series-resistor
