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I'm looking at various sources of variable capacitors, and many of the sellers quote single capacitance values for the capacitors. Is this typical? If so, is there some rule of thumb from which I can assume the range of capacitance?

Example: http://www.amazon.com/Through-Trimmer-Variable-Capacitors-Yellow/dp/B00G9BIT5W/

Many examples there and on ebay.

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Could you add some examples? I just checked digikey and all of their variable capacitors have ranges on them. –  horta Jun 4 at 14:36
    
Can you give an example? –  Andy aka Jun 4 at 14:36
    
I added an example. –  Fadecomic Jun 4 at 14:40
    
The smallest range I've seen is down to around 30% of the max value. My confidence would be high that the min value would be less than 13, perhaps as low as half that. –  MrWonderful Jun 5 at 0:02

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up vote 5 down vote accepted

A quick search through digikey shows many variable caps that are some value up-to 40pF. I would suspect something similar here in your example. They're most likely some value up-to the value they list. I would assume that the value they give is the maximum value that it can reach.

Here's some of the examples I looked at here.

I didn't see any capacitor ranges that started at 40pF and go up. I think the attribute of a variable capacitor that most circuit designers would be interested in is the maximum value. Then they would just assume that they can trim out to something less than that to tune to.

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Ah okay, that was my assumption as well. But I thought there might have been some convention I wasn't aware of. –  Fadecomic Jun 4 at 16:11

First, the description from the seller is not very good. But I'll try to ferret out a good guess for what the missing parameter is.

The 40pF is the maximum capacitance. There is a fairly wide range of constructions and minimum capacitances, but if we confine ourselves to a search for 6mm ceramic variable capacitors, and look for similar construction, the Sprague-Goodman GKG series is very close.

enter image description here

Note that the color codes match as well (yellow for 40pF).

It's only a guess that the minimum is 6.5pF, I would not be surprised if it was a bit higher, but if you design for 10pF~40pF you should be okay. The tempco should be specified as well- the Sprague ones have a -750ppm/°C coefficient.

It is a bit silly that the parametric search parameters at both Digikey and Mouser sort by the minimum capacitance. The maximum and a range or max, then min would make a lot more sense to an Engineer.

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