While working on my circuit, I found that I have some basic questions about simple capacitors:
1) Is there any minimum operating voltage for Tantalum and Electrolytic capacitors? I think that the formation of an electrolytic layer in such a capacitor could perhaps require some minimum operating voltage.
2) Until which frequencies does an electrolytic capacitor work well? When should I consider tantalum capacitors?
Depends on who makes them. I have elecs in old radios that are around 50 years old and when last measured still had their 10uF or whatever and little leakage. Some are from companies that stopped making caps when I was in diapers. I usually check these electrolytics when I repair or restore a vintage radio.
But then again there were certain PC motherboard caps that had a life span of a few weeks, I heard.
Tant explosions: I remember when they stuffed a proto board wrong, all the tants reversed. Then it was connected to a 5V/100A supply. Some said it was like popcorn, others likened it to a machine gun. We had to replace some acoustic ceiling tiles later.
A trick used to reform the aluminum capacitors on Tektronix tube scopes was to slowly raise the input line voltage with a variac. The idea is to have a capacitor voltage increased slowly, to minimize internal leakage currents and possible damage.
With the old valve stuff that needs reforming, the rectifier valve starts to conduct over a very small v range, so you have to wind the variac up very slowly indeed around that area to get that method to work.
I've reformed 1930s 'lytic caps that way, though not with 100% success, but earlier kit tends to use paper caps housed in wax, and I've not found any way to deal with those. Can they be oven cooked at just above
100C safely? If so it might be poss to dry them out that way.
Sure, but you have to furnish enough energy to make the bang, and you generally have to exceed the cap's specs. An MnO2 tantalum provides its own explosive chemical energy, and they detonate when operated well within their ratings. The new polymer tants are better.
But electrolytics can explode "nicer". Not by excess voltage but when I exceeded the current rating for too long one of them (a really big one) sent its aluminum can skywards. Straight up like a Saturn V except that the ceiling was in the way. It also made a nice roaring sound. The paper then flocked down like snow in a fairy tale.
That day I learned that there is indeed such a thing as a maximum current rating for caps.
Depends on who made the cap. I have some that are fine after 40-50 years on the shelf. True new-old-stock.
The death sentence for an old cap in, say, a radio or a TV is when replacing an old selenium rectifier with silicon diodes. Unless there is a power resistor in series it can blow the cap right out of its moorings.
Sometimes, to preserve the antique look, I have scraped out a cap and mounted a modern electrlytic in the can. That way it still looks like the old can. The most pretty can was a Ducati cap. I guess that this motorcycle company must have made caps for a while after WW2 until the cycle biz picked up again. Maybe to tide them over the economic slump.
Firstly, a little terminology: Tantalums are electrolytics too, just made with something more expensive (or it's a more expensive process), what are commonly called electrolytics are technically "aluminum electronlyics." So tantalum is short for tantalum electrolytic, and electrolytic is short for aluminum electronlytic. This shorthand also gets by having to used the alternative international spelling/pronunciation of aluminum/aluminium.
I'm don't know about tantalums, but I've heard suggestions that electrolytics be operated at at least half their max rated operating voltage, else they're more likely to lose capacitance over the long term (several years). It's also suggested that anything that has electrolytics in it be powered up for at least a few minutes about every six months or so, so the electolytics get voltage and don't deform. Have you looked at capacitor manufacturers' websites? Surely they would have something on this.
I'm sure it's related - the initial forming of the layer in manufacturing is done by applying a voltage.
You have to get info on the ESR and inductance of whatever particular value of capacitor (and for the manufacturer and type) you use, to decide if it's acceptable at the frequency you use. Many of the better "suitable for switching power supply use" electrolytics work fine into the hundreds of kHz. The Digikey catalog has some short blurbs about each type of capacitor and what applications it might be best for, but again, the maker's websites ought to have the best info.
1 - Tantalums tend to have a higher capacitance for their size.
2 - Aluminum Electrolytics performance tends to degrade as they age. This may be an issue if you are using them for de-coupling.
3 - Tantalum supposedly works better at high frequencies (you will need to compare datasheets and look the ESR and ESL)