Tantalum Capacitors

Use 2.5 times the applied voltage or more.

Or use an aluminum electrolytic 2 - 3 times capacitance.

I don't think there's a reliability issue here. C109 (located on the Power / MIDI board schematic, is the initial output filter capacitor in a 3.3V switching power supply. There are other 100 uf caps surrounding it, but it seems to be the only tantalum cap, which suggests that the designer needed some specific characteristic of the cap to make things work. Methinks a change of capacitor type might not be such a great idea.

What would make the cap explode? My guess(tm) is that someone plugged in a wrong voltage power supply, which blew up FET (F1) which then applied overvoltage to the tantalum. It much have been quite a large jolt as tantalums using produce a low resistance "short" which then gets hot and starts smoking toxic fumes. I would look around for other parts in the power supply section that may be been destroyed, such as the other caps along the 3.3V rail.

I notice now that there seems to be some very light heat discoloration around the common mode choke L10-L11-L12.

I don't see anything immediately wrong with F1 and it passes the "smell test" so I dunno.

Dry-slug tantalums across power rails are bad news. High dV/dT literally ignites them; MnO2 is the oxidizer and tantalum is the fuel.

Derate them 3:1 on voltage, or use something else. Polymer aluminums are good, and some come in a tantalum-like surface mount package.

Cheers

Is there anything I can use temporarily in its place, that I might have in stock, to try and get that switcher up and running while I wait for the appropriate part?

What a horrible schematic! A mosfet is "F1" and a fuse is "FU1" ! Connector names are all over the place. And worse.

Replace C109 with most any 100 uF cap.

What's with the 21 resistors between AGND and DGND? That's crazy.

That's what I've read everywhere. Yet, I spent 10 years shipping marine radios that were literally crammed with dipped and molded tantalum caps on power supply rails with never a problem. The only ones I've ever seen go up in smoke were reverse polarized (which produced an impressive red glowing piece of slag and plenty of white smog). Mostly, these caps were 25V caps on the 12V (nominal) power supply lines and 16V caps on the 8 and 10V regulated lines. There were also a bunch used in audio circuits.

However, we never used tantalums on the output of a switcher, where I would expect problems. I guess using a tantalum in this 3.3V switcher would qualify. However, at the time (1970's) the literature declared that high voltage spikes were the culprit, not voltage slew rate. Since these often appear together, I can see where there might be some confusion.

The original cap is a 100uF 10v tantalum which is already 3:1 derated in a 3.3 VDC power supply. However, that doesn't included voltage spikes from the nearby inductor.

Judging by the age of the Korg, I would guess(tm) axial leads not SMT.

Nope, the power supply is almost all SMT, including the tantalum. The only through hole parts is the switcher inductor and the other large capacitors, which are a mix of organic polymer and regular electrolytic.

This thing cost an arm and a leg when it was new 15 years ago. Inside the very large case there actually isn't very much - there's a board which holds all the front panel controls, and everything else including the power supply and output jacks are on a single mainboard measuring maybe 8"x10".

Bad guess(tm). Sorry. I couldn't tell from the "manual" because none of the caps are listed in the parts list.

This should work for SMT on Digikey: The eBay link includes both through-hole and SMT.

The tantalum thing is very erratic. Some batches blow up, some are fine.

I know for sure that tantalums sometimes blow up at below their rated voltages, with no overshoot spikes. It's dV/dT, namely peak current, that can ignite tiny particles of tantalum, which then burn in the solid MnO2 electrolyte.

Could be. Or maybe there was a lot of dV/dT. Or maybe some other failure mechanism.

Tantalums are just right for some things, but have to be used carefully.

Tantalums have such a low HF resistance that it is sometimes recommended to put a current limiting resistor in series if there is a serious ripple voltage around. I've witnessed a lot of IGBT's being blown up because those resistors were failing. The voltage feeding the tantalums had such a large HF ripple due to the switching of the IGBT's that it blew the tantalums out of the control board, after which the IGBT's also went to pieces.

joe

That's why in those cases a series resistor might be recommended in order to limit the current spikes.

joe

They're all marked "NU", which seems to mean Not Used. Other parts of that sheet have "NU" resistors in similar nonsensical places, like across the fuse or across the power switch. I suspect there are places for all of these resistors on the board, but they don't populate them at the factory except for test.

Matt Roberds

Add series resistance to a tantalum cap and you have just created an electrolytic replacement.

Sorry, I forgot to mention to put the resistance in between the power line and the tantalum.

joe

Al polys can make voltage regulators oscillate. Tantalums have a nice middle-of-the-road ESR that makes 7800s happy.

The ignition problem is quite real--see

Cheers

Phil Hobbs

My favorite cheap "MDO" regulator, the LM1117, loves a 10 uF tantalum on its output.