THanks for photo of schematic. If you don't mind <20% efficiency, it might be ok.
Transformer is 18Vac 4A = 72Watt max.
This full wave bridge in theory (with the right parts) can deliver 25.2(41% over AC) with no load but the 100Ω series gives low ripple but also huge loss.. eg 10V drop at 0.1A, so if you want 1V drop, consider what load you need.
for 72W @ 24V @3Adc 24dc/18Vac=1.33
using my chart 1.33 @ 72W >> equals 6Ω (not 100)
Using Steven's photo you want the capacitor to hold up the voltage between pulses say 5~10% so if using 50Hz the pulses are 100Hz and instead of a time constant of 100mS, which would give you ~60% ripple you choose say 600ms so the load cap from T=RC, C=0.6sec/6Ω = 0.01Farad or 10,000 uF @30V or higher.
Now there happens to be 10 thousand capacitors that fit that description from $3 to $50 but many can't handle the ripple current. So using www.Digikey.com 's auto-filter we enter 10000 uF then select 30 or 35V then sort by Amp rating and for the cheapest that can handle 4A easily with margin. Great! that narrows it down to 11 parts (in stock) ranging from $3~$11. So I pick the most popular brand in reliable caps, Panasonic. Ripple Current 4.42A ESR (Equivalent Series Resistance) 50 mOhm, 10,000uF @ 25V 22mm x 45mm high.
Now there is a huge inrush current, so we need to protect the diodes and the cap.
So we look for Inrush Current Limiters (ICL) from DK site again. and look for 3A continuous rating. Now I see only stock one of these and its only rated for 2A , so you need two. so buy 5.@$2.02 each.for spares.
The specs for the surge resistor are R @ 25°C =120 Ohm, R @ Current= 1.18 Ohm.
Great! So it will have the soft filtering at low loads and thus low surge and very low ripple and drop to 1.18Ω at 2A or around 2Ω @ 1.5A with two in parallel thats 1Ω so the the 6Ω series R , that we calculated before, can drop to 5Ω. or so.
Not too bad for a 78W 24V supply.. Now the 12V supply is going to get hot with 1A so then we consider.. maybe we should have split the supply to +/-12V so we dont lose so much in the 3-terminal regulator. But we need a current spec from your 1st.
So for $6 to $15 incl spares plus shipping. the can ship same day on VISA.
That's how I would design around your transformer. But without knowing your application. especially the 100Ω pot ( poor regulation ) that's all I am going to do.
Oh yah the bridge, with the ICL resistor, you only need a 4A 50V bridge.
with these specs $0.91 instock. Without, you need a bigger one.
Generally there are many Design Rules for safety of components and safety of public for consumer quality power supplies. Then for commercial and industrial a few more specifications. So for simplicity, when a consumer needs an unregulated DC bridged power supplied , they simply buy a wall adapter with the appropriate rating. These DC voltage ratings will be accurate when loaded with the rated current and then rise as the load is reduced to the peak voltage levels.
Since you wish to learn about how to specify components, there are only a few simple tradeoffs with choice you need to specify.
1) Budget cost of parts,
2) tolerance for ripple voltage,
3) expected load current and voltage
4) temperature rise of bridge.
5) The voltage drop on the bridge at expected load current
- bridge power loss (=load current* voltage drop)
- bridge temperature rise = Pwr * deg 'C/watt Junction to Ambient rise
- ambient rise. If you enclose the project you need to consider having ventilation or consider the temperature rise inside the enclosure and the max temp. rating of the capacitors.
6) If you choose large uF with low ESR capacitors, you can reduce ripple significantly
7) If you add a series R after the bridge you can reduce the peak current and ripple voltage
8) If you add a series inductor after the bridge you can reduce ripple even more
9) If you need a precision regulator an adjustable LM317 or variation will work
10) If you want short circuit output protection (SCP) , a PTC can be used as a series resistor
11) if you need over-voltage protection (OVP) from flashover on transformer unless rated for 6kV (max passthru on power meter)
The bigger the bridge the lower the voltage drop and temperature rise. Typically 1V drop @1A = 1 Watt but if not heat sunk can get very hot, so choose lower drop Schottky bridge. But check the voltage Drop at your load current and temp rise coefficient.
Don't worry too much about voltage rating unless you live in Africa, India or some part of the world with wild power fluctuations. 50% margin should be adequate.
Here are some additional detail on items 7 not given in the other answers.
Courtesy of Dr Keith Billings "Switched Mode Power Supply Handbook" excellent book.
There are more curves for Ripple vs Cap value and R value.
There is a different curve for a half bridge if you choose that.
Can you confirm output rating in Vac @Iac of transformer? It is not the same as DC.
What accuracy do you need for Vdc output Ripple Vp-p? i.e. regulated or not?
"My input voltage is 110VAC, with a transformer converting it to