How a trimmer can be trimmed..
|Sometimes one has
made a tuning circuit and the frequency range is not exactly as one
Here are some tips on how to solve the matter. It is so simple that you could have thought of it yourself.
What is a capacitor?
Next, we need to remember what exactly makes a capacitor a capacitor:
In case of a varcap, plates are positioned parallel, so together they provide more area, e.g. more electrons can be stored.
There are a few more properties (temperature, moisture, material,
dielectric, a.s.o.), but these are not of interest for this article.
|Cutting down the Cap|
|In the situation one has a varcap of 150 pF and one needs a varcap of 100 pF, the solution is to remove some of the vanes. This is how:|
| - Measure
the capacitance of the varcap:
let us assume the meter indicates 148 pF.
- Count the number of
We can also count the total of vanes: 40.
We want a varcap of 100 pF.
Take off 11 vanes and take a measure. Than decide if you want to go on removing that extra vane. You can't put them back on!
|Tip: Take a
good look which vanes form a pair!
|Increasing the Cap|
|Let's imagine you have a 100 pF varcap but need a 150 pF varcap. Remedy is simple, again: Go to the Paraset Warehouse and buy a new one.|
|Trimming the Cap|
value slightly down:
Increase the distance between the last two vanes. On tuning capacitors in commercial broadcast receivers one can observe that the last vane is cut in parts to facilitate easy trimming.
value slightly up:
Looking at the photo's to the right, you can see two varcaps I deliberately ruined for this tutorial. The upper one has aluminium vanes, the one at the bottom has silver plated brass vanes.
|Varcap Vane Value Calculator|
|I made a little calculator in Excel. You can download it here, if you like.|