# RC circuit with Schmitt trigger - Derivation

1. Dec 20, 2013

### blueink

In lectures I covered how a Schmitt trigger can be used in an RC circuit to determine the capacitance of the capacitor (Where the Schmitt trigger switches at 0.3V+ and 0.6V+). The equation given was that C = 1.2 R \ T (time period)

Does anyone know how to derive this or where it comes from? My lecture notes do not explain it.

Thanks :)

Last edited: Dec 20, 2013
2. Dec 20, 2013

### Staff: Mentor

Please attach the schematic of a circuit appropriate to this discussion. I expect you will see that the charging path for the capacitor is a resistor connected to the output of the Schmit comparator.

3. Dec 20, 2013

### Jony130

Simply use RC equation witch describe the voltage across the cap in charging and in discarding phase and solve for period T. And from there you can solve for C also.

4. Dec 20, 2013

### skeptic2

I was using such a circuit to introduce a time delay. Since I was working at Motorola, I was using Motorola Schmidt triggers and Motorola claimed the specifications of all their ICs were 6 sigma qualified. Nevertheless the time delays of the circuit seemed to cluster around two different values. Testing the Schmidt triggers I discovered the switching voltages had two different values. I called the Semiconductor Division and asked how something like that could happen if the parts were all 6 sigma. I was told the switching voltages were not a 6 sigma spec. and not even tested. Lesson: Don't depend on Schmidt triggers for precise switching points.

5. Dec 20, 2013

### meBigGuy

I expect the motorolla data sheet did not specify min and or max switching voltage going positive and going negative. Generally you get one or the other, or typicals. So what does a 6 sigma typical mean? Or a 6 sigma max means it can be anything less than that.

For precise switching points you want comparators or opamps with a good reference voltage. (or even a 555)

6. Dec 23, 2013

### skeptic2

As I recall it did and that's why I was surprised when they told me it was not a 6 sigma spec. and that it wasn't even tested. I no longer recall exactly which Schmidt trigger I used and there are a lot of different ones available, but I checked a few datasheets and most do give the switching voltages.

I used a Schmidt trigger because I had an extra one on the board and the delay didn't need to be precise. Yes there are a lot of ways to introduce a time delay but I'm saying that a Schmidt trigger is not a good solution if you need a somewhat precise delay as in measuring the values of capacitors.

7. Dec 24, 2013

### meBigGuy

They, at best, give a range in terms of min/typ/max and the actual switching can be anywhere within that range. I doubt you can find a spec sheet that defines a precise switching point. You can't get that for a simple gate, much less a schmitt trigger.