Making a power supply

[Craneguy]

Ok so I’m brand new and my knowledge of electronics is basic at best, I’m just a dumb crane op who gets paid to play with expensive toys. I had an old pyramid power supply that went belly up but the transformer, rectifier, and case is still good. The pyramid claimed to be good for 18 amps constant/20 surge. Im getting around 25 volts dc out of the recitifier so I purchased a DROK adjustable buck converter rated @15A/750W. I can fit it into the old pyramid case beautifully. I looked at some info on astron supplies and noticed they generally use 2,000 uf per load amp. So if I’m understanding all of this right I **think** I should probably use a 30,000-40,000 uf cap for a filter cap directly after my full wave bridge rectifier, which will then feed my buck converter, which will then goto my binding post and then to my load, hopefully giving me roughly 3-24 volt variable power supply capable of up to 15 amps. My main question is does this sound ok to do or am I building an electronic Molotov cocktail? If it is ok, am I in the right ballpark for the filter cap? Thanks in advance for any input/advice.

[RES]

1x 30000uF/40V (min. 40000uF/40V for 20A, not too large -> rectifier bridge (diodes) must be big enough to handle the inrush current at switch on (low ESR cap(s), short large charge/load on bridge diodes), or 4 x 7500uF/40V in parallel. (15A). Verify the rules of thumb for a full wave rectifier bridge power supply to be sure.

[Eldi4]

I will not go into 1x30000uF choice, because the dimension would be huge (although your 20A transformer would be huge too), and if it's goes wrong you will have to buy a new one, but if you use 4x7500uF, you'll get lower ESR,and if one goes wrong you are only replacing one of them not all, and although the overall volume will pretty much same,  the placement of 4x7500uF is easier and better to see.

as for inrush current you can use NTC to reduce it, four of em, 2 on the ac output, and 2 on the rectified dc output.

everything will goes okay as long as you connect everything correctly,linear power supply is a pretty easy build.

[Craneguy]

Thank you for the advice. After some more homework and learning about esr’s I ordered 4x7500 uf caps. As for the NTC thermistors tho I have quite a bit more homework to do tho because I didn’t realize I needed 4. I thought they were only needed on the AC side of the rectifier   
Thanks again for the help.

[soldar]

... I didn’t realize I needed 4.

You don't. You only need one.

[Craneguy]

That’s what I was told before as well. 1 on the AC side of the recitifier. Thanks for the input. Gonna read up on them tonight.