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RC circuit
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Nikos A.:
Hi everyone,

I try to design an RC delay circuit but it doesn't work as it is expected.

First of all, I want to choose the largest possible resistor value in order to minimize the capacitor's size. I noticed that when I am using big resistors e.g 1Mohm then capacitor's output voltage never reaches Vdd (in my case Vss is 3.3V). For example, using a 1.2Mohm resistor and 6.8uF cap the output voltage after around 10s is only 1.5V and stops increasing. If a use a small resistor e.g 6.8k then capacitor's voltage reaches 3.3V in around 225ms

I cannot understand this behavior...

Also, I want to achieve fast discharging. So, I placed a diode in parallel with the resistor but it didn't work.. The discharging response remains the same with and without the diode..

This is the circuit



Any advice?

Thanks in advance
JackJones:
What is connected to the output? If you're drawing current from it it won't reach Vdd.

For example, imagine if you have a 1 Meg resistor connected from output to ground. That would form a voltage divider with R1 limiting the max output voltage.
Nikos A.:
At the moment I have not connect a load on the output, I am justing probing it.
JackJones:
It could be that the cap itself is leaky. If it's an electrolytic cap it might have a fairly high leakage current. You could try with another type, like ceramic of foil.

Or the probe itself might be dragging it down, but I would expect it to have ~10 Meg input impedance, usually. For multimeter anyway, oscilloscope with a 1x probe would be 1 Meg which would drag it down to about half Vdd.
Nikos A.:

--- Quote from: JackJones on May 15, 2020, 10:27:14 am ---For multimeter anyway, oscilloscope with a 1x probe would be 1 Meg which would drag it down to about half Vdd.

--- End quote ---

You were right!! After measuring the voltage with my Fluke multimeter the capacitor was charging to Vdd voltage (3.3V). So, oscilloscope's probe creates a voltage divider that affects when using high-value resistors... Is there any trick to overcome this issue?

Do you have any comments concerning the diode? I was expecting a fast discharge rate but it is remaining the same with and without the diode.

Nick
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