Low value caps in PSU - why electrolytic? why not film?

Family Hoof

Well-known member
Joined
Dec 17, 2004
Messages
406
Location
Brooklyn, New York, U.S.A.
I've noticed in dozens of power supply schematics that unless it's a 100nF, non-polar and electrostatic caps (i.e. film) are never specified. With something like a 10uF Vreg bypass cap, for example, a polarized electrolytic or tantalum is always suggested. What are some of the electrical reasons (not just cost, size, practicality) for this? What possible benefits might I obtain by using film caps, or even high quality non-polar electros, in these types of PS applications?
 

Gus

Well-known member
Joined
Jun 3, 2004
Messages
4,778
Location
NJ
I like solens in guitar amps. I like to use films when ever I can. I like films for the cathode bypass caps in guitar amps. FWIW 1uf PET will often have a ESR of about 1.5 ohms the better 10uf electros seem to be about 1.5 ohms ESR. Hint.

If you use film(PP) in a tube amp power supply add some resistance after the diodes and before the first cap. The ESR is so low that it can cause surge currents that might hurt the diodes and also cause the power transformer to have a mechanical hum. Toroids seem to be more affected by the use of low ESR caps. I believe it is from the mismatch in the diodes in how they conduct.
 

dramadisease

Well-known member
Joined
Jun 17, 2004
Messages
110
Location
portland - or
i used solen fast caps in my tube psu i just built.
chokes solens and oil caps.

too many hifi guys in my day to day travels not to try it out!
 

Svart

Well-known member
Joined
Jun 4, 2004
Messages
5,134
Location
Atlanta GA USA
Price. I know you wanted a *real* reason, but try to find a 10uf+ film(polyester) cap and look at the price tag. you can usually find 10 electrolytics for the price of one large film cap. Films can be seen as superior in a lot of ways but it all goes back to price. Polypro are even more expensive, polycarbs are rare and teflons are rare and expensive. Polystyrene are rarely made in large values. the size is the next item used for determination. I have a handful of 10uf polyester caps. they are about 2.75 inches long and around 5/8 inch in diameter.
 

Bjorn Zetterlund

Well-known member
Joined
Jun 3, 2004
Messages
123
Location
London, UK
The soundcraft K1 series powersupplies use 1uF electros close to the regulators...I'm sure that is for no other reason than cost. Regulator manufacturers will most likely suggest the layout and components that will make their part work best; manufacturers need to build things economically and will cut costs/corners where they can.

Bjorn
 

jensenmann

Well-known member
Joined
Oct 10, 2004
Messages
2,598
Location
Karlsruhe, Germany
Some regulators are sensitive with respect to ringing caused by the LCR-activities of the bypass-cap and the regulator output. Usually the ´lytics have a worse ESR than Filmcaps which means they have a worse Q and thus tend to ring less.
And of corse price is probably the more important argument for manufacturers.
Jens
 

Family Hoof

Well-known member
Joined
Dec 17, 2004
Messages
406
Location
Brooklyn, New York, U.S.A.
Thank you, Jens! That's the kind of answer I was looking for. It's already blantently obvious that cost is the primary factor.

I've also been wondering if maybe the polarization of electrolytic caps is desirable for some reason but can't figure out how or why. Am I out of line here?

Don't think I'm going to start using film caps in power supplies but I thought it was worth discussing. Thanks, everyone. :thumb:
 

Bjorn Zetterlund

Well-known member
Joined
Jun 3, 2004
Messages
123
Location
London, UK
All electrolytics are meant to be polarized, apart from the non-polarized ones, naturally! Using them in a way where they don't get a proper voltage across them is just a bit unkind...and not what they are really meant for. Quite a few people here have phantom on permanently to keep the polarization voltage on the input elcaps...theoretically something to do with making the dielectric perform as intended (not something I know very much about). Of course you can get away with it, and so many manufacturers do. How many desks use elcaps for signal coupling and 'design in' an offset? I can only think of the 9098 neve/amek discussed at length at the 'old' home, and an old midas monitor desk (probably 20 years+) that I was given to strip for parts at work; a fine example of industrial engineering...and a real bitch to lift!

Bjorn
 

jensenmann

Well-known member
Joined
Oct 10, 2004
Messages
2,598
Location
Karlsruhe, Germany
Björn, you´re talking about a Pro4? Two days ago I had the pleasure....


But talking about caps in a PSU there´s always polarization voltage present and distortion isn´t an issue. So nonpolar ´lytics in this application is a waste of money.
Jens
 

Bjorn Zetterlund

Well-known member
Joined
Jun 3, 2004
Messages
123
Location
London, UK
Hi Jens,

It didn't even have a name as far as I know. All trf input and output, no fader and all rotary knobs, 9 VU meters, 24in 8 out. Mostly 741's and discrete circuitry.

Bjorn
 

jensenmann

Well-known member
Joined
Oct 10, 2004
Messages
2,598
Location
Karlsruhe, Germany
Yes, that´s a Midas Pro4 Monitordesk. I know some companies in my area who still have them in use. They did some of the biggest german festivals in the 70´s with that board. But carrying this b*tch is no fun at all.
:sam:
Jens
 

Latest posts

Top