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How does a 3 terminal capacitor work?

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zape

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I have a non-working digital alarm clock and the output driving the buzz goes through a three-terminal electrolytic capacitor. This is something new for me. Any idea of how it works?

The buzz has two wires, one connected to the supply and the other one to the capacitor middle terminal.

The other capacitor pins are connected one two the supply and the other to the collector of a npn transistor. So I guess when the transistor is saturated this latest pin will be connected to sthg near to ground and the buzz shall be fed with the supply.

I have measured the capacitance between the pin connected to the supply and the one in the middle and I read something like 1 microfarad.

Measuring also the impedance between the extreme terminals it seems to be open. Is it right?
 

chemelec

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zape said:
I have a non-working digital alarm clock and the output driving the buzz goes through a three-terminal electrolytic capacitor. This is something new for me. Any idea of how it works?

The buzz has two wires, one connected to the supply and the other one to the capacitor middle terminal.

The other capacitor pins are connected one two the supply and the other to the collector of a npn transistor. So I guess when the transistor is saturated this latest pin will be connected to sthg near to ground and the buzz shall be fed with the supply.

I have measured the capacitance between the pin connected to the supply and the one in the middle and I read something like 1 microfarad.

Measuring also the impedance between the extreme terminals it seems to be open. Is it right?

I can't say for sure what that capacitor is, But usually Electrolytic capacitors with three or more terminals are just Multiple capacitors in one package.

One terminal is a common Negative.
The other two terminals are each of the two Positives.

These types of capacitors were very common in old Tube (Valve) amplifies and television sets.
 

Verba

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are you sure that is a capacitor ?
Could you put a picture of it ?
 

btbass

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Three terminal caps are a mini filter network.
You have an inductor, then the cap to ground, then another inductor. You should have very low resitance between the in and out terminals, the inductors, and a cap between in or out terminals to the center terminal.
 

vandelay

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When I was building tube amplifiers a few years back, I frequently bumped into some 3-terminal electrolytic caps, simply two capacitors with common ground as described by chemelec above. They were usually high voltage parts (300V+) and in the range 47uF to 220uF. I can even remember some paper in oil capacitors that had mixed values in the same package, like 1uF+2uF+3uF...
 

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