Variable-Cap
A variable capacitor changes a circuit's capacitance for tuning or impedance matching. Mechanical tuning capacitors, small trimmers and electronically controlled capacitance devices use different mechanisms and controls.
A mechanical design changes plate overlap or spacing. A common rotary capacitor interleaves moving rotor plates with fixed stator plates; changing their overlap changes capacitance without letting them touch. Electronic alternatives include a reverse-biased varactor whose depletion region changes with bias, and switched capacitor arrays.
In plain terms
Like changing the length of a musical instrument's resonating space, adjusting capacitance shifts where a tuned circuit responds most strongly.
Why designers use it
- Adjusting an LC circuit's resonant frequency.
- Trimming a circuit during alignment or calibration.
- Changing reactance in an impedance-matching network.
Best for
- RF tuning
- Circuit alignment
- Impedance matching
Key specifications
- Capacitance range: Minimum, maximum and adjustment characteristic
- Control: Mechanical adjustment, bias voltage or digital interface
- RF operation: Loss, frequency, signal swing and working-voltage limits
When not to use it
- When the required stability, range, voltage or loss exceeds the selected tuning component's limits.
- As an automatic substitute for a fixed decoupling or bulk-energy-storage capacitor.
Common mistakes
- Forcing a mechanical adjustment beyond its stop or allowing plates to touch.
- Ignoring stray capacitance and the adjustment tool's effect during alignment.
- Applying a varactor bias or signal swing outside the specified limits.
- Assuming a tuning device's capacitance is linear with knob angle or control voltage.
- Using parent-category electrolytic polarity advice for a different tuning construction.
Where you will find it
- Rotating the tuning capacitor in an LC radio circuit changes the resonant frequency.
- A small trimmer can align a tuned circuit after assembly.
- An electronically controlled capacitor or varactor can change a tuning network without a moving shaft.
A short history
Variable capacitance can be achieved in more than one way. Alongside mechanically adjusted tuning capacitors, semiconductor-based products vary capacitance electrically for frequency matching. The tuning range, control method and operating voltage belong to the specific device, not to the name 'variable capacitor'.
Good to know
- Changing plate overlap changes capacitance without changing the dielectric material.
- A trimmer and a front-panel tuning control can share the same basic purpose while being designed for very different adjustment use.