# Confusion about variable capacitors/diodes

In a lot of circuits (especially with the 555 timer), I see capacitors used to set the duration/frequency of something.

Now mostly the value of a capacitor can be calculated, however, what if I want it to be variable, and controllable from a microcontroller.

I know there exists varcaps/var. diodes etc, but if I look at the data sheets they only mention they are for radio/TV tuning. For resistors there exist variable resistors ICs, and I would expect something similar for variable capacitor ICs.

Do I miss/oversee something here?

(update) Example:

See link Example circuit

Using for C: 100 nF Using for R1 and R2: MCP42010 (100 kOhm version, see Datasheet

Slight problem is that the wiper resistance is 125 ohm, resulting in a minimum frequency (using the calculations in the circuit link) when using 100 kohm is 48 Hz, and maximum frequency (when using 125 ohm) is 48 Hz.

• If you plan to use an MCU, why do you not integrate the timer function in it's program, saving yourself the hassle with the 555? – Bart Jun 13 '17 at 9:43
• @Bart ... maybe that's best indeed, for me it's just a 'learning' question ... I can imagine there are many examples, not only the 555 timer. Like some high frequency IC where involving an MCU will be too performance costly to use as 'timer'. – Michel Keijzers Jun 13 '17 at 9:45

Do I miss/oversee something here?

Varicap diodes vary the capacitance by varying the DC voltage on reverse biased PN junction. PN junctions don't have a massive amount of capacitance hence you only get several tens of pF for something like a BB171 made by NXP: - The data sheet refers to it as a VHF device but I use it at frequencies from 1 MHz to 15 MHz.

For resistors there exist variable resistors ICs, and I would expect something similar for variable capacitor ICs

There is one digitally controlled capacitor that i have come across but, like the varicap diode, this was for applications that required a capacitance in the tens of pF range. I guess there is no massive reason why bigger value digitally controlled capacitance chips couldn't be available other than market demands.

I mean, you can make a low-speed to medium-speed oscillator by varying the resistance and if you wanted a high-speed LC oscillator most varicap diodes are in the right capacitance range to suit the higher frequencies.

• What's wrong with using a variable resistor - nobody would consider using a variable capacitor at this low frequency. – Andy aka Jun 13 '17 at 10:08
• If you want a 20Hz frequency, you start with a larger cap like 100uF, then you get practical resistor values. – Neil_UK Jun 13 '17 at 10:20
• @MichelKeijzers it seems to me that you want something that doesn't exist within the constraints of using a 555 timer. If you want an oscillator that can smoothly run from 20 Hz to 4800 Hz this requires specialist design over and above what a simple 555 can permit. It doesn't work with a programmable variable resistor and it doesn't work with a variable capacitor. – Andy aka Jun 13 '17 at 10:37
• You'll never get a digipot with a temperature stable range of 800:1. I use digipots in circuits that process signals and I won't go higher than 16:1. Using a real pot will be somewhat similar but if you are prepared for temperature induced frequency instability then that's OK. – Andy aka Jun 13 '17 at 13:32
• Look at the temperature coefficient graph for potentiometer mode on page 7 (fig 2-3). At 16:1 ratio the TempCo is about 20 ppm/degC. At 32:1 it's about 40 ppm/degC and getting close to be off the scale if you went to 64:1. In other words, you cannot rely on a digipot for having a big range (256:1 for the one you listed) whilst producing frequency stability. It's the same with varicaps - they have a temperature coefficient that isn't bad but aint that great either. Those who know, know that stable variable oscillators are gold-dust. – Andy aka Jun 13 '17 at 13:36

For a variable-frequency oscillator using resistor*capacitor time constants, it might seem natural to change the oscillator frequency by either changing the resistance and/or the capacitance. Variable resistors have limitations as you have noted. Voltage-variable capacitors have a limited capacitance range that is non-linear...mostly useful for high-frequency oscillators using a resonating inductor.
Other approaches allow frequency to be varied over a wide range. Some versions have impressive linearity. These circuits use a fixed capacitor that doesn't change value. What changes the frequency is a voltage or current that is variable. Many simple function generators use this technique. A 555-type oscillator does not - it has fixed threshold voltages (at Vcc/3 and at Vcc*2/3).
It is possible to modify the standard 555 relaxation-type oscillator by charging its fixed capacitor with a current source rather than a resistor. A variable current-source results in a variable frequency: Output frequency varies between 180 Hz to 10kHz. While a variable resistor is used to provide a control voltage, any variable input voltage source performs the same function.

• Thanks for the answer! Looks like it's much more complicated than I thought (but learnt something) – Michel Keijzers Jun 14 '17 at 12:24