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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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