What type of capacitor is most stable for use in 555 timer circuit?

Question

I am building a 555 timer circuit with a frequency of 15, 30 and 60 Hertz. I will most likely use a 1µF capacitor but I was wondering what kind of capacitor would be best for keeping a stable value even when the temperature changes (goes down most likely)?

I will be putting potentiometers so that if the values do change they can be calibrated again, but that will only be done before we deploy the systems. Once deployed, we cannot calibrate again until retrieved days later).

Basically, I am asking what kind (ceramic, electrolytic, etc.) of capacitor will keep its capacitance constant at a value of 1µF and with varying temperatures.

Answer 1

As DKNGuyen said, if you want a stable capacitor, use C0G/NP0 ceramic.

However, if your actual goal is to have a stable frequency rather than discuss capacitors, then your original idea of using a 555 timer is not the way to go, as this chip will have worse drift than the C0G capacitor.

A much better option would be to use a quartz oscillator, for example a 2.4576 MHz oscillator which will cost about €1 in 50ppm/°C stability, then divide by 4096 using a 74HC4040 ripple counter. You could use a 74HC4060 too, with a crystal instead of an oscillator. Also the quartz oscillator is pre-calibrated and you don't need to adjust it.

EDIT I misplaced a decimal point...

I mean, a 1.2288 MHz oscillator followed by 74HC4040 to divide by 4096, resulting in 300Hz. Then a 74HC390 or similar which can divide by 5 then 2 then 2 which gives 60Hz, 30Hz, 15Hz.

Or a 192kHz oscillator then the same two 74HC chips, but this oscillator is only available on digikey in tiny MEMS flip chip so maybe not the best option. Anyway you get the idea, pick a frequency and a convenient division ratio with 74HC chips...

The BOM cost for both solutions will be less than a precision capacitor and a potentiometer, and that doesn't count the salary of the intern who gets to tweak the frequency just right...

If you need better stability than what a cheap XO will provide, you can use a TCXO. That will cost a bit more (a few €) and it will be available in less convenient frequencies. So you can use, for example, a 12.288MHz TCXO and divide by 60000, you'll get a few ppm stability over temperature.

Note a cheap microcontroller makes a nice programmable divider if you need one.

If you already have a microcontroller in your project, why not use that?

Answer 2

Regardless of the caps you use, a 555 is a 0.1-1%-ish stability device over a modest temperature range.

Typical (not maximum) drift of the bipolar NE555 is specified at +/-150ppm/°C, so to get 0.1% typical drift with perfect external components you only need to change the die temperature by 7°C (either from ambient changes or from the die heating).

You can slap 10 C0G 0.1uF 1206 caps in parallel and get 1uF with +/-30ppm/°C tempco and use some nice expensive 10ppm or better resistors but you're just putting lipstick on a pig.

Answer 3

Neither. What you want (or rather, what is practical and most cost effective) is a C0G/NP0 ceramic capacitor. Not any ceramic capacitor, but a C0G/NP0 dielectric. Good luck finding that in a size as large 1uF. Even 100nF is quite rare, and expensive.

Other stable alternatives such as mica, glass, or poly max out at even lower capacitances and are much more expensive.

Also, temperature isn't the only thing that affects capacitance. The DC-bias the capacitor experiences does too.

Answer 4

use a 30.72kHz crystal and a CD4060, digikey is sold out and says it's obsolete, but some other places still have this crystal

Q8 will give 60Hz, Q9 30Hz, and Q10 15Hz