i am trying to connect a trimmer capacitor on breadboard. but the trimmer has three pins. I know that two of the pins are connected together. does that mean I can choose one of the two connected pins as one plate of the capacitor and left the other pin as floating?
the trimmer looks like this:

 

Say you number the pins so that the nearest (on the picture) is pin 2, the rightmost pin is number 1 and the most further is 3.

The capasitance between 2 (middle pin) and 3 will increase when you screw it clockwise. I think the capasitance between 1 and 3 will be constant.

 

Yes to your question.

The trimmer has a continous rotation, watch the trimmer vanes as you adjust the capacitance
when using a 'non metallic' trimming tool.

Old plastic knitting needles filed down to a blade, make a cheap tool.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

It has three pins for better mechanical stability. Since it is adjustable it could twist lose on a PCB having just two pins.

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Proper Planning Prevents Piss Poor Performance

 

I followed the links from the properties of the picture you posted and found these datasheets:
http://www.rapidonline.com/netalogue/specs/12-0100e.pdf
http://www.rapidonline.com/netalogue/specs/12-0110.pdf
They clearly show the pin connections for two different size trimmers.

__________________
Ron

 

I haven't tried inserting a trimmer on to a breadboard. I usually use extra wires with the legs to easily insert into the breadboard.

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Likes to have some rare,nostalgic transistors for my breakfast...

 

A breadboad has more capacitance between all its connections than the trimmer capacitor.
The trimmer capacitor is used in high frequency RF circuits where a breadbard doesn't work and should not be used. Use a pcb.

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Uncle $crooge