How to select R and C?

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gary36

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I have a particular design where timing has to be set from 1s to 90s. I have two options, R=1M(pot) and C=100uf or R=10M(pot) and C=10uf
Which one of the above would be better choice with respect to timing accuracy and long term stability?
 

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Look up the leakage current in the data sheet for the capacitors. Multiply by the corresponding pot resistance.
That is the error voltage you get from the leakage current. The 1MOhm + 100uF option is probably best.
Also consider a higher frequency oscillator with a divider chain for such long delay times.
 

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Which one of the above would be better choice with respect to timing accuracy and long term stability?
Click to expand...

1) What are your goals for accuracy and stability ? Range of T and V effects your environmentals have to
deal with ?

2) What device are you using with these passives to generate delay/timing ?


Regards, Dana.
 

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Accuracy and long term stability can be expected with film capacitors. This limits achievable capacitance values.
There are also timers with analog time programming and frequency divider, e.g. TI TPL5xxx series.
 

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10M Ω implies tiny Amperes. Very often neighboring devices (example, IC's, op amps) have 1MΩ input impedance. As well as common DMM's. Your RC network is likely to be 'loaded' by such devices, throwing off expected volt levels. The tiny Ampere level may be insufficient to drive another device.

In real life you can discover a range of your 10M potentiometer where you can dial the resistance that provides the needed RC time constant. However I think the 1M potentiometer is a better choice. Does your project run on batteries? Then you might prefer to look for ways to minimize power usage. It depends on what trade-offs you decide to make.
 

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Hi,

when talking about accuracy and long term stability I would not go to an analog RC solution at all, especially with periods of 1s and longer.
Any 8 pin microcontroller with XTAL would be by a factor of 1000 more accurate, easily...

Klaus
 

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