Variable trimmer capacitors

[M84AB1]

Hello guys,

as the topic suggests there is something that I am not quite sure about regarding these trimmer capacitors:

http://www.jaycar.com.au/productView.asp?ID=RV5722

I am not able to find any data sheets regarding these and their leg/pin layout. How do these behave? Like variable resistors? How do I wire them up as capacitors? Which two of the three pins/legs do I use...etc

Anyways, if somebody could shed some light onto this, that would be greatly appreciated.

thanks in advance

[Zero999]

It behaves like a variable capacitor 8.5pF to 100pF depending on the position of the screw.

[Murphy]

Never seen one of those with 3 legs but there are still probably only 2 connections. Would help to have a meter or a datasheet. First thing I would do to figure that out without a cap meter is check continuity to see if 2 of the pins are connected together internally.

[DJPhil]

Here's a picture, taken from this thread.

As they mention in the thread, remember to use something non-metallic to adjust it, and try to keep your hands away as well.

[M84AB1]

Thanks a lot for your input guys. I greatly appreciate it. However, I have played with this trimmer for a bit and this is the results I got.

When i connect an AC signal source to one of the two common pins, i get the same signal on the other common pin (for obvious reasons). However I get nothing on the third pin. If two of the three pins are common (connected together) then shouldn't the output occur on the third pin. Does not seem to be the case for a coupling capacitor application.

As a decoupling capacitor however, the third pin connects to ground whilst one of the common two can be used as the second terminal.

I don't know but it seems a bit odd. I want to use these trimmers for input and output networks of a Class C RF power amplifier.

cheers

[DJPhil]

I hopped on to Digikey and had a look to see if they had a datasheet for a variable cap matching the look of the one you have. I found this one, and the datasheet is here. It covers a whole range of caps, you'll have to scroll down to the page marked "10mm TOP/BOTTOM & SIDE ADJUST" on page 7 to see the 9-100pf part. As we suspected, the two opposing pins are the same, and both attach to the 'rotor' (the bit that spins), and the third pin is the 'stator' (the bit that doesn't).

It seems strange that it would behave differently in different ends of your circuit. I am by no means familiar with RF design, but it might be related to the Q of the capacitor. The 9-100pf cap from the digikey catalog uses a polycarbonate dielectric, and these seem to all have a Q of about 200 @ 1MHz. To put that in perspective, the Q of most smaller PTFE variable caps seems to fall around 1500 @ 1MHz, with some of the small SMT caps (see "7mm SURFACE MOUNT") sitting at 1000 @ 10MHz. It appears that the smaller package sizes and smaller ranges have a higher Q, and this makes sense given the small amounts of capacitance involved.

If you are delicately balancing impedance with and adjustable LC filter then you may want to use a smaller adjustable capacitor. This is all greek to me really, but I found a half decent reference for Q in radio circuits here.

Can someone with more RF experience back me up on this?

[M84AB1]

Thank you very much DjPhil, your input has been very helpful. I would have replied earlier but I was not at home and did not get a chance to further test the trimmer. As it turns out, it does reproduce the input signal (20MHz) on the third pin when fed into any one of the two common side pins but only on certain positions of the rotor. The reason why it did not work before was due to a faulty wire on my bread board. In any case, thanks to you guys I and other people interested in variable trimmers did learn about their pin configurations.

Cheers guys!