Testing 3 lead tantalum capacitors?

[W4PJB]

Hopefully this is the right forum for this question.

I'm working on some old HP equipment, and there are a number of 3 lead tantalum caps in the power supply. I have a bench LCR meter. What is the best way to test these? Either both caps I've tested are bad (the readings jump around wildly), or I'm doing something wrong. I've tried testing off of the outer legs, and the inner leg to either outer leg. 120kHz / 1V. These caps are either unobtanium, or expensive, or both. So I want to make sure I get this right!

[Sensorcat]

Looks similar to this:

https://content.kemet.com/datasheets/KEM_T2046_T396_T398.pdf

... which would mean you could replace them with an ordinary tantalum, electrically equivalent.

[ArdWar]

It should be outer negative (both common) and inner positive. Check the PCB trace whether they simply connect both negative into the same net or if they use it as 3T capacitor. Most of the time you may substitute it with common 2 lead tantalums.

[W4PJB]

Thank you for your replies. It's hard to find information on these rare 3 legged beasts. Most internet searches for capacitors with 3 legs come back to HVAC posts. Having some context makes the information that is available much more logical. I was probably way overthinking their purpose, when it seems they exist simply to simplify installation and reduce assembly errors.

Those Kemet 396's are what I've been looking at but are available in very limited sizes. 1uF, 10uF, 22uF, and 47uF. And the 47uF versions are a whopping $44 each! I also need a 100uF, which is 52 week lead time, and 800pc minimum order. So clearly that isn't going to work.

Good to know I can replace these with standard 2 leg version if I use caution when installing. It also explains why one leg was nearly impossible to properly de-solder, even with my Hakko desoldering gun. If one leg is just there for support and to prevent reverse installation, it probably does not have a via (edit: solid via) through the board, just tracings top and bottom.

[SteveThackery]

If one leg is just there for support and to prevent reverse installation, it probably does not have a via through the board, just tracings top and bottom.

I thought the idea of a 3-leg capacitor is that you can install it either way round. In which case the two outer holes will be connected on the PCB.

[squadchannel]

The 3-terminal tantalum (by KEMET) capacitor is unique in that it can be mounted on the board without concern for polarity.
Therefore, as ArdWar says, the outer 2 pins are negative and the center 1 pin is positive.
It is also used in the IBM 5170. I replaced it with a regular 2-terminal tantalum.

[W4PJB]

Sorry, I should have been more specific. No "solid" via. Connected, yes, but only with a circuit tracing on top and bottom of the board. So when unsoldering (especially being a thick board) it's difficult to heat up the solder on both sides sufficiently to remove it with a vacuum gun.

[SteveThackery]

Sorry, I should have been more specific. No "solid" via. Connected, yes, but only with a circuit tracing on top and bottom of the board. So when unsoldering (especially being a thick board) it's difficult to heat up the solder on both sides sufficiently to remove it with a vacuum gun.

So there you go: the outer legs are connected together in the capacitor and should be on the PCB, thus allowing you to insert it either way round.  I agree that through-holes can be tricky to solder-suck.