Decoupling with polarized (tantalum) 0.1uF

Would you use polarized 0.1uF tantalum capacitors for decoupling (instead of the usual ceramic 0.1uF)?

No. The ceramic will be cheaper and will have lower series resistance, which is important for decoupling.

Using a tantalum capacitors for decoupling power is a bad idea. The most common failure mode for tantalums is a short and so can cause a fire when they go.

Thanks.
Can’t that failure (with fire) happen in other situation than decoupling 5V-powered devices? What is the advantage of using tantalum capacitors? When do you use them (low voltage filters)?

I was planning to use tantalum caps(.1 uf and 2.2 uf) on my L4931 power supply because they’re physically smaller than the equivalent electrolytics - bad idea?

What is the advantage of using tantalum capacitors?

as bill says:-

because they’re physically smaller than the equivalent electrolytics

Can’t that failure (with fire) happen in other situation than decoupling 5V-powered devices?

Yes it can but in other places in the circuit a capacitor going short has other things to limit the current, used as a decoupler it does not.

I have worked in places where there was an absolute ban on using tantalums in decoupling and in others where they were total unaware of the problem. If you have a choice use something else but in a benign environment (cool <35C) and used at 25% less than the voltage rating then the risk is not too great.

What is the advantage of using tantalum capacitors? When do you use them (low voltage filters)?

The tantalum capacitors are used where you need a large capacitance in a small space at
a fairly low cost. I think that this advantage has gradually eroded over the last ten
years. It is not difficult to get low cost small ceramic capacitors up to 10uF.

You need to careful in designs that specifically call out a tantalum or electrolytic
capacitors. Some times these designs rely on the series resistance of the capacitor for stability. A ceramic capacitor could be used in these applications if you add a small
series resistor.

(* jcl *)

Some of us have to be careful about the inclusion of tantalum because of this:

:frowning:

(I’ve seen tants connected incorrectly- they burn with a very intense white light!)

You can use a monolithic though right ? They are the only sorts of >= 1µF Ceramics I can get readily… other than polarized tants…

They are not as good due to having higher inductance and so they don’t take out as much of the hight frequencies but a large capacitor (polarised or not) in parallel with a small ceramic is the way to go.

Thanks Mike… I’d read that in a tutorial somewhere (might even have been yours! :slight_smile: ), and was using 0.1µf ceramics and Low-ESR Electro’s of +1µF or greater (22µF, 10µF or 4.7µF depending… ) but I needed some 1µF’s ceramics for my RPM Tach circuit and best I can find locally is some Mono’s… There seems to be a big gap between 0.1µF ceramics and anything higher at least locally. If I want a 1µF+, then I need a Mono. Want an electro or tant, no probs… got them coming out my ears… :-[ At least the 0.1µF’s are fine though…

I still really need to learn the ‘rules of thumb’ for capacitor selection for decoupling… I figured 0.1µF was pretty standard, but some datasheets seem to use all manner of odd rated ceramics…

but some datasheets seem to use all manner of odd rated ceramics.

At one place I worked the small decoupling caps were always 22nF.

When I asked why they said that value was the cheapest by £0.0005. It’s not much but when there are 100 in a box and you are making 1 million boxes it all adds up.