Building a 12V power supply: Putting Solid Caps in Series?

[TimLaw]

Building a 12V power supply: Putting Solid Caps in Parallel?

Hello,

This is my first post on Electro Tech and it is kind of weird because I don't have the ability to help any of you, but hopefully in the future I will be able to provide assistance. But for now, I am a plain old newbie.

So, I am in the planning stages of building a 12V power supply. I will be basing the design off something like this: **broken link removed**

Something I'd like to try to do with this particular project is use all solid organic polymer caps instead of a huge electrolytic capacitor. I know well made electrolytic caps aren't bad, but this is something I'd like to try.

Thing is, there is no one polymer cap that can fit the bill just like a electrolytic cap can. What I was thinking was that I could put about 15 35V 150uF caps in series.

Now would this work?

What would I have to take into consideration if I was to go this route?

Anything else I should know?


This would be the solid polymer cap I would use. ~$49 for 15 of them. A bit pricey but it may be worth it. Or not: **broken link removed**

Whether or not I make the supply with all solid caps is still up in the air, but I'd like to know if it a) is even possible b) what problems could it cause.

Thank you very much. I appreciate it.

-Tim

 

[Nigel Goodwin]

For astart you would need to put them in parallel, NOT in series.

I've no idea if they would be any good or not, but I don't see how they would offer any improvement over an electrolytic?.

 

[crutschow]

The likely lower ESR of polymer caps might be helpful in a switching supply but I don't see any particular advantage in using them for a standard power supply. I don't see any problems with using them either, other than taking up a lot more volume, but I suggest you save you money and just use electrolytics.

 

[TimLaw]

Oh right...Parallel....Stupid me. I swear the book I am reading is more confusing than helpful.

I suppose its a dumb idea economically because it costs about 20x more than electrolytic.

Possibly helpful if you needed a really low profile supply, as these things are super short in height in comparison to the huge electrolytics you might seen in PC power supplies or anything that might require quite a bit of power. Spread across a moderate sized board, you could probably get some decent power.

It was just an interest. Since I am not too experienced, I think I'll stay electrolytic for the time being right now...but..

If you read the recommended parts list or the schematic, it calls for a 10uF cap. Suppose I wanted a solid polymer cap there. There would't be any problems with using a solid cap there and an electrolytic cap for the big one. I work on PCs part-time. I see tons of boards that use a combination of different types of caps, chokes, regs. But the engineers behind those things are quite a bit more experienced than I am.

Can any one refer me to a article/guide that might help me choose parts in any situation? I don't want to have to be so bothersome. Or I could just keep on reading this nightmare of a book called the Electronics Self Teaching Guide...My head!!

Thanks to all. Much appreciated.

 

[TimLaw]

Okay, I am going to complicate matters a bit here...

I actually have an application that I could use this supply for. I have a couple spare hard drives that I was just thinking I would make them into externals. Now I could just put them inside, but that wouldn't be fun, now would it?

There's a problem, though. Hard drives mostly require 12V power for running the motor, but they also need 5V for the read/write heads.

Now, not sure the logistics of this, but if I built a 12V supply on top of a 5V supply and then made some magic, could I get 12V and 5V? Sata power cables wiring are yellow, black, red, black. Each black I assume is a ground? And then the yellow might be 12V and the red might be 5V....

Is it possible?

Thanks.

Thanks.

 

[Brevor]

Sure that should work.
Tip: Use big heat sinks on the TIP31's and forget those polymer caps! Use the money you save to put a blue LED on it.

 

[MrAl]

Hello there,


Solid's have an advantage over Electrolytic's in that they dont dry out so supposedly they should last longer in an application. Reliability should be a bit better.
Thing is though, the MTBF (Mean Time Before Failure) for a part relates to the 'run' time, not the shelf life, so if the cap is not operating technically the part is not aging as much. Many power supplies are only turned on for use once in a while. This means that for something like a power supply, if it is not running 24 hours a day 7 days a week then you might never see the advantage of solids over electro's so you may be just wasting the money, literally.

I've seen Electrolytics age and become a hunk of junk on the circuit board but that was with something that runs 16 hours a day almost every day of the week. A 1000uf cap with ESR around 0.1 ohm becomes a 100uf cap with ESR around 2 ohms, ripple voltage goes way up and triggers the overvoltage detection circuit and turns the power supply off. Once the cap (or cap's) are replaced the unit works fine again.

If you are going to use this with hard drives then you may want to think about it a bit more. If they have to run for long hours then the solid's are going to be worth it in the long run which may prevent a hard drive crash at some point. For short run times though i dont think it would be worth it. It's mainly all about the run time length, and of course what might get damaged if a failure occurs.

 

[TimLaw]

Thanks to both of you. I read a bunch more on power supplies and how they are constructed. I actually get how they work now. So, I was wondering what kind of filter I should use. I broke open my old cell phone charger and saw they used a TC filter. But I am not sure.

Thanks. I'll be back later.

 

[crutschow]

What's a TC filter? Do you mean RC?

 

[TimLaw]

Oh shoot I meant LC filter. LC as in it uses a choke/inductor of a sort. RC as in it uses a resistor.

I was wondering what type of filter I should be using.

 

[Brevor]

You don't really need a filter, your cell phone charger is a switchmode type of power supply. These operate at high frequencys and can cause interference, thats why it needs a filter. Your power supply operates at 60 Hz (a very low frequency) so a filter isnt necessary.

 

[TimLaw]

Okay that makes sense. Thanks.