"virtual" variable capacitor ???

[alvarohn]

Can I make a "virtual" variable capacitor with a capacitor in series with a potentiometer??? is that a normal practice in electronics?

The pot would make the cap smaller isn't?

And maybe adding a diode in parallel with the potentiometer I can make the cap look bigger? It would charge fast with the diode, and discharge slow with the resistor?

Sorry to ask this, but the normal electronics sites are full of sceptics.

best,

Alvaro

[dR-Green]

Can I make a "virtual" variable capacitor with a capacitor in series with a potentiometer??? is that a normal practice in electronics?

The pot would make the cap smaller isn't?

And maybe adding a diode in parallel with the potentiometer I can make the cap look bigger? It would charge fast with the diode, and discharge slow with the resistor?

No.

Sorry to ask this, but the normal electronics sites are full of sceptics.

That's because you're wanting to make a chocolate teapot. When you understand the concept and attributes of chocolate and tea then you will know the answer.

[alvarohn]

I didn't mean to really turn a cap into a variable cap.

It was for a comparator cap dump circuit, in which there is a little capacitor that changes "on time".
I have just tried and it works, I can change the "on time" turning a pot in series with the little cap.
Now I can set the "cut off" voltage of the dump, because I think it depends on the impedance of the charging battery. For example; with the same “on time” charging 1 battery it dumps until 17 volts, and adding another battery in parallel, the dump is until 14v.
best,

Alvaro

[dR-Green]

There is also such a thing as a trimming capacitor which is a variable capacitor but with very small values.

[vidbid]

Variable Caps

Can I make a "virtual" variable capacitor with a capacitor in series with a potentiometer??? is that a normal practice in electronics?

The pot would make the cap smaller isn't?

And maybe adding a diode in parallel with the potentiometer I can make the cap look bigger? It would charge fast with the diode, and discharge slow with the resistor?

Sorry to ask this, but the normal electronics sites are full of sceptics.

best,

Alvaro

I haven't seen it thus far, but here's what I'm aware of:

1. 
2. 
3. 

Video for #3.

Regards,

VIDBID

[mikrovolt]

When a circuit could benefit from a sine wave rather than a square wave and frequency needs to be accurate a crystal is used as a stable frequency.
This can be coupled to a variable tank circuit which uses a tuning diode.
In this way a variable capacitor is sort of virtual. This method can bend a crystal frequency quite a bit and does not inherit the imperfections of the crystal's sine but stimulates the variable tank and has an excellent sine output.

Some other names for these diodes are tuning varicap diodes, varactor diode, variable capacitance diode, variable reactance diode or tuning diode all basically a type of diode whose capacitance varies as a function of the voltage applied across its terminals.

Because resistance is inherently linked to voltage by Ohms law. A precise DC voltage adjustment circuit can incorporate a potentiometer.
The variable resistor uses its three ports to form a voltage divider outputting pure DC. The tank circuit with a varactor diode or a mechanical
variable capacitor creates a low level sine wave. The amplification stage or active element removes very little power from exciter.

[alvarohn]

When a circuit could benefit from a sine wave rather than a square wave and frequency needs to be accurate a crystal is used as a stable frequency.
This can be coupled to a variable tank circuit which uses a tuning diode.
In this way a variable capacitor is sort of virtual. This method can bend a crystal frequency quite a bit and does not inherit the imperfections of the crystal's sine but stimulates the variable tank and has an excellent sine output.

Some other names for these diodes are tuning varicap diodes, varactor diode, variable capacitance diode, variable reactance diode or tuning diode all basically a type of diode whose capacitance varies as a function of the voltage applied across its terminals.

Because resistance is inherently linked to voltage by Ohms law. A precise DC voltage adjustment circuit can incorporate a potentiometer.
The variable resistor uses its three ports to form a voltage divider outputting pure DC. The tank circuit with a varactor diode or a mechanical
variable capacitor creates a low level sine wave. The amplification stage or active element removes very little power from exciter.

Tnx for the info I will read something about that.

In my circuit, the pot in series with the "on time cap" works ok.

best,

Alvaro

[mikrovolt]

I will share one of my circuits where the frequency is adjustable from 1.76 Mhz to 4 Mhz Possibly someone will find it ok as a simulation platform for design.
I think running it on ltspice would be helpful.


Colpitt 160 and 80 meter signal generator.JPG