Tantalum Capacitors?

[Ken256]

It looks like most people on the forums have trouble with aluminum electrolytics, but I have an old oscilloscope with a shorted tantalum in the power supply. At least I think its a tantalum. It is cylindrical shaped, axial leaded, with one end rounded to indicate polarity. 4.7uF, 50V, Kemet. The cap is shorted and burned through.

The power supply is a conventional linear design. Transformer, rectifier, filter caps and linear regulators. The shorted capacitor was on the input side of a LM337 regulator and took out a bridge rectifier, but not the fuse.

I was wondering if anyone has experiance replacing tantalums that failed in this way? Should I replace it with a tantalum? I've read that they can fail from current surges. I've had other equipment with tantalum filter caps that failed in the same way (bang! smoke...). Should I look at replacing the tantalums with aluminum electrolytics or some other type? Also, I counted 12 tantalum capacitors on this power supply board. I guess the good ones could fail at anytime. Should I recap the whole power supply? But recapping could be expensive, a 10V, 220 uF, axial tantalum is listed at $12.28 on digikey.

Thanks for any help.

[Radio Fox]

Re: Tantalum Capacitors?

From my own experience, I tend to avoid tantalums wherever possible. They appear to be far superior to electrolytics in many ways (low ESR, infinite shelf life, self-healing etc.), but like yourself, I have seen far too many explode or go short-circuit in the past to have any faith in them.

To be fair, they may be more reliable nowadays, but unless they are used to save space, or are in some critical circuit in which an electrolytic isn't suitable, then I would always replace a tantalum with an electrolytic.

One thing that I have never understood about tantalums, is that some spec sheets state that they should always be used at only 50% (or sometimes less) of their marked voltage. so if you do replace your tantalum, I would make sure it is rated to at least double the voltage it will see in the circuit.

[kc8adu]

Re: Tantalum Capacitors?

tantalums are not forgiving of excessive inrush,spikes,reverse polarity.
newer ones are much better but i still use the 50% of rated voltage where i can.and i often use sm tants in place of sm lytics.esp in stuff that runs warm.example is the standard c5900da.
every one of these rigs i see need all the lytics replaced.
ones that were sent in to standard when under warrenty and had a bunch replaced need them again.of over 100 of these rigs i have rebuilt i have never had a repeat failure.

[stj]

Re: Tantalum Capacitors?

i have seen more tant corpses than most, they were used on space invaders boards.
(i fix a shitload of those)

it's reverse polarity spikes that will make them blowtorch.

if you lose one, think yourself *very* lucky if the board is o.k. - i'v seen a lot burn a hole clean through the fiberglass pcb.

i think the modern ones are more protected against internally shorting out.

[Ken256]

Re: Tantalum Capacitors?

Thanks for the replies. I am leaning towards a tantalum replacement. It looks like I would need 10 times the capacitance to get the same ESR, if I replaced them with Al electrolytics. So I would need something like 47uF or 68uF, but would still be cheaper. The Al caps would be bigger though, and wouldn't quite fit the holes/pads. I was thinking of a full recap because it was such a pain to remove the power supply board.

Its interesting what kc8adu wrote about voltage derating, spikes and inrush current. The one that shorted was rated at 50V and was filtering the input of a 3 terminal regulator at 36 V (after the big electrolytics). But this is a 30 year old scope, so maybe a blown capacitor is expected and a replacement cap will last some time.

[stj]

Re: Tantalum Capacitors?

put a .1uf polyester in parallel - it may protect it from high frequency noise.