Several years ago, I got some of these capacitors with 3 legs in a grab-bag of caps at Radio Shack.  I never did figure out what they are.  Now I'm trying to reverse this  sort of "rare" booster circuit and it has a couple of these 3-legged caps.   Can anyone tell me what they are/how they work?

Thanks,
JD

Probably like the dual caps used in old Fender tube amps for the cathode bypass caps.
They are two caps in one package and share a lead. The ones I have seen share the negative lead and have individual leads for the positive ends. They should be marked.

J.

Correct, they share the same lead. I've found some disk capacitors like that and the middle leg was the common lead for both. Seem pretty rare to find although I can see how it may be useful.

...there is a part that looks like a disc cap with three pins-it is called an EMI suppression filter.....it has a choke in the same package with a cap....they used to use them in one of those skylift equipment-cherry picker PWM controller boards I used to service....I think either Jameco or Hosfelt carries them....

A traveling encyclopedia-salesman pulls up to a farm in rural Texas, and on the way to the door notices a 3-legged pig in the yard eating. He rings the doorbell, the farmer answers and the salesman starts his pitch : "Sir --I just have to ask you about that 3-legged pig."
The farmer says "Son, that's no ordinary pig --last summer the house caught fire and that pig broke out of his pen, knocked the front door down, ran in squealing his head off to wake me and the wife. My little girl's room was burning too hot for me to get to her, but that pig crawled in, grabbed her by the leg and saved her life."
The salesman says "That's incredible --what happened to his leg?"
The farmer looks him in the eye, and says "Son,  a pig like that --you don't eat him all at once."

If it looks like a DIP tanalum, it's one cap, but with two positive leads so it *can't* be inserted backwards. Tantalum caps have been frequent enough sources of failed machinery and ... fires... that the demand created the backwards-less cap.

Some radial electrolytics for high frequency power supplies have three leads on one cap to lower the ESR losses.

The trick is to get out the old ohmmeter. If two of the three leads are connectecd together, it's a single cap with multiple leads for some reason, usually polarity insensitivity or ESR.

I have some dual .01 600v caps.   On them it says: .01X2 GMV Z5U
3 equal length leads(non Polarized)
I also have had these a while and they are definitly caps.  

Arn C.

If they look like a little box or a ceramic disc with bulges on the two leads that have continuity and the capacitance is fairly small, they're RFI filters and not caps. I've seen these at RadioSlack before in the cap assortments.

They contain a cap and you can use them as such, but each of the common leads has some inductance thrown in.

If you don't use them in computer circuits that interface to the outside world, good luck in passing FCC part 15 requirements. These days with CMOS you can get by without them by relying on parasitic capacitance to the ground plane (if you've got one), but back in the TTL/NMOS days.... forget it. They were very mandatory.  

Take care,
-Peter