If you're going to replace a tantalum w/ an electrolytic . .

[JW]

How do you know which side to put the negative lytic lead on?
Or, I don't know, are tantalums bipolar or not or sometimes? Hmmm. . .

Just wondering. I've heard of people doing it, and there's some values that i'm not able to get cheaply in film caps. So I thought I'd give it a try with electrolytics.

Is it just measuring w/ a VOM? And the side w/ the posotive lead that gives me the higher voltage is the positive side? That is, if tantalums are polarized.

 

[soundguy]

tantalums are polarized like an electrolytic, look for the marking on the cap.

dave

 

[bcarso]

Tantalums are definitely polarized. Unfortunately the polarity indication can be misleading unless there is a "+". Sometimes there is a band or stripe, but it is almost always indicating positive. Note that most aluminum electrolytics have a stripe indicating negative! So you need to be very careful.

In circuit a 'lytic should always have a positive or zero voltage on the plus end relative to the other end.

 

[gyraf]

and there's some values that i'm not able to get cheaply in film caps. So I thought I'd give it a try with electrolytics.

Film caps?

I think you're confusing the cap types..

Jakob E.

 

[JW]

See,

that's another thing that confuses me. If they're polarized then why are people replacing them with films?

Or is that just my imagination? What do you guys usually replace tantalums with? Besides other tantalums, that is.

 

[gyraf]

Replace tantalums with tantalums. After all, you'll have to trust that the designer knew what he was doing.

Only execptions are:

1- in powersupplies. replace with electrolytic paralleled with a small (~10n) ceramic cap.

2- Stuff that is badly designed around 1976/77, where some engineers thought that tantalums were the new and fancy answer to all their needs..

Jakob E.

 

[Gus]

Go to the meta there should be a link for the Marsh Jung paper.

Replacing caps with other cap types is a very deep question. One could write for pages on this.

I use different caps type like a spice added to food.

The Art Of Electronics has some good info.

Tants are low ESR compared to Al electros so just changing them can make a difference depending on what the use is. Is it a coupling cap or a power supply bypass for an OPAMP. If it is a Power Supply bypass I would leave it a tant.

For Audio DA and ESR seem to have a bigger effect than some other specs.

 

[mike_relay]

[quote author="gyraf"]1- in powersupplies. replace with electrolytic paralleled with a small (~10n) ceramic cap.[/quote]

Hey Jakob, (or anybody else who might know)
what does the ceramic cap do in this case?
-mike

 

[JW]

They're coupling caps in the Yamaha PM1000. But they're all over, so they're in other non coupling places in the circuit too. The Pm1000 kind of has a reputation of being pretty well designed, but I don't know, it might fit the 70's tantalum fad bill?
I'm in the process of ordering a bunch of stuff, so I will be ordering some replacement tants too from a couple brands, but for some reason I had it in my head that I could replace them w/ films too. So I was going to try that and compare, but if that's an absolute no-no, then I'll spend my money trying more brands of tantalums.
I could just leave the old tants in of course, but I'd like to try some replacements to see what I get. [/img]

 

[Gus]

put PM1000 etc into the search funtion above.

 

[JW]

Hey Gus,

Yeah, I've read most of the PM1000 threads in the archives.
The one that talks the most about the tants ends inconclusive as to whether (tubejay's) problem with replacing them w/ films was related to the fact that it was a film cap or to the fact that it was too big of a film cap.

Quite a few folks just recommended replacing w/ a 1 uf film instead of just using a tant in place of a tant.

 

[bcarso]

[quote author="mike_relay"][quote author="gyraf"]1- in powersupplies. replace with electrolytic paralleled with a small (~10n) ceramic cap.[/quote]

Hey Jakob, (or anybody else who might know)
what does the ceramic cap do in this case?
-mike[/quote]

The principle is that the ceramic has lower impedance at high frequencies, where the series inductance of the 'lytic is getting significant. So the combination should have low impedance over a wider frequency range and thus filter better.

Some people use three capacitors in parallel to extend the range further.

This doesn't always work as well as expected btw, since sometimes you get unwanted resonances with one capacitance and another's inductance.