Price issue with 4400uf capacitor choice for AC-DC PS

[DaMaDo]

I'm trying to buy all the parts to build the AC-DC power supply in Randy Slone's Electricity and Electronics book and it calls for 2x 4400uf 50WVDC capacitors.

Are they really $40-$200 each?

Digikey's cheapest is $59 and mouser's and Newark's (stocked) are $44. Jameco, partsexpress, and mcmelectronics don't even have those.

Why can't I use the 4700uf 50V cap that costs $1.45?

It takes a week to get the parts so I was going to order them first and build it as I read through the sections, but this cap price is holding me back.

[alm]

4700 uF should work just as well, since the tolerance is likely something like -10%, +50%, it should be well within tolerance. This is with the assumption that it's just a smoothing capacitor. Timing capacitors might be more critical, but a 4400 uF precision capacitor would be very unlikely. If you'd care about the value, you could also put 2x 2200 uF in parallel.

My guess is that 10,000 uF would work as well. As long as the ripple is reduced to acceptable levels and the turn-on surge is limited to manageable levels, the exact value usually doesn't matter.

[amyk]

Are you sure it's 4400uF? That is not a standard size, only Cornell Dubilier makes them, which explains the absurd price.

If it's a filtering/reservoir cap then 4700uF (the next higher standard size) would be fine. No mention of ESR?

[DaMaDo]

Thanks guys, I wish I could tell you what they were for, but I guess I'll have to read it through to see if it's filtering or whatnot...then take a week off from this book while I wait for the parts. I have about 100 more pages to go (diodes then capacitors). I'll see if I can put some stuff off so I can just read it through today. It's odd that he chooses 2x 24v transformers instead of one 48v b/c they are hard to find but then lists these caps.

Here's a schematic in case that sheds any light on what they are for:

This is just the first section, the next section has the pcb with transistors etc

Schem made with Drawsch from http://www.dz863.com/

[alm]

C1/C2 are just for smoothing the ripple coming from BR1. BR1 produces rectified AC, C1/C2 convert it to something approaching DC. You're unlikely to notice any difference between 4400 uF and 4700 uF for C1/C2. You can also parallel other caps (with the correct voltage rating) until they add up to ~4400 uF, eg. 2x 2200 uF or 5x 1000 uF. Use whatever is the cheapest/in stock.

[Mechatrommer]

Are they really $40-$200 each?

with $200, you can get a very very very quality PSU (everything from regulation to protection proof). in there you'll highly probably will find big resevoir/filtering/smoothing capacitors + all the components you'll ever need to build one DIY PSU the schematics you are showing is the said resevoir/filtering/smoothing capacitors! the bigger Farad the better, 3300uF will do as replacement. dont trust the economist where you were refering to!

[DaMaDo]

Great! thanks, that's what I was hoping for.

I'll order them today.

[mmknee]

how are you doing with  your power supply, I am embarking on the same one! Thanks for already having an answer to this question!!!

[DaMaDo]

Well it's been in a stalled situation since I started spring semester. I'm hoping I'll have it done sometime this year... heh

At least before circuits class next spring.

Attached are pictures of the current state....I just need to solder the voltage regulator section onto a board and put it in the case and add the volt/amp meters.

I've already connected and tested everything and it all works. I still don't understand why I have both positive and negative voltage adjustment pots though. Every other PSU I can remember only has one knob for voltage, not two.

[Rerouter]

based on the layout, i would take a very solid guess that it is a split supply, with a positive, negative ang ground, e.g. positive and negative about a the ground, taking a guess from what i see, it is a rather high current one, with a pass transitor of some sort, (havent bothered to google part numbers) and with the 3 diodes, i'm also going to guess it uses and lm317/337 arrangement to offset its bias voltage and allow it to run down to 0V,

the way the second device is arranged, i would also take a guess that its a second pair of the 2 arranged in a current limiter

[onlooker]

The fuse is mounted too close to the (matal?) casing. It can be a safty concern.