RC circuit - Page 1
Forum section: electronics / projects

author: Nikos A.
author_rank: Regular Contributor
timestamp: May 15, 2020, 09:03:56 am
content: 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 225msI 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 circuitAny advice?Thanks in advance

author: JackJones
author_rank: Regular Contributor
timestamp: May 15, 2020, 09:38:42 am
content: 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.

author: Nikos A.
author_rank: Regular Contributor
timestamp: May 15, 2020, 10:12:05 am
content: At the moment I have not connect a load on the output, I am justing probing it.

author: JackJones
author_rank: Regular Contributor
timestamp: May 15, 2020, 10:27:14 am
content: 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.

author: Nikos A.
author_rank: Regular Contributor
timestamp: May 15, 2020, 01:18:19 pm
content: Quote from: JackJones on May 15, 2020, 10:27:14 amYou 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

author: FenTiger
author_rank: Regular Contributor
timestamp: May 15, 2020, 01:51:27 pm
content: What are you doing to make it discharge?If you just disconnect Vdd then the diode won't conduct - there'll be nowhere for the current to go afterwards.If you disconnect Vdd and then short it to ground, you'll forward bias the diode and it should work as you expect.

author: Nikos A.
author_rank: Regular Contributor
timestamp: May 15, 2020, 04:10:57 pm
content: Quote from: FenTiger on May 15, 2020, 01:51:27 pmIt works now, thanks!!And looking at the internal impedance of the oscilloscope I can now understand why the measurement was affected.My 1.2Mohm resistor in series with the internal impedance of the oscilloscopes (1Mohm) creates a voltage divider which output is exactly 1.5V (this is exactly what I was measuring with the oscilloscope)Is there any way to overcome this issue?

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 15, 2020, 04:20:05 pm
content: Some active probes have (sometimes much) higher input impedance. They are pretty expensive though, so I guess really not worth it just to capture a capacitor slowly charging and discharging.One simple approach you could use would be to build your own, simple, active probe based on some op-amp as a unity-gain buffer. A simple circuit won't give you exceptional performance, but should be more than adequate for this one task.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 15, 2020, 04:50:17 pm
content: Quote from: Nikos A. on May 15, 2020, 04:10:57 pmI have to tell you at least two things:1) Use oscilloscope with 1/10 divider always (10 MOhm impedance and low input capacitance), except a few situations where you can't use divider.2) Different multimeters may have different input impedance on voltage mode (1 Mohm, 10 MOhn, and more). So you may need multimeter with at least 10 Mohm (or 100 MOhm) input impedance for such high impedance measurements. But there is no problem to use any multimeter - you just have to remember to take it's impedance into account.

author: Nikos A.
author_rank: Regular Contributor
timestamp: May 15, 2020, 04:51:08 pm
content: Quote from: SiliconWizard on May 15, 2020, 04:20:05 pmQuote from: Vovk_Z on May 15, 2020, 04:50:17 pmThank you guys!!

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 16, 2020, 02:30:25 am
content: Quote from: Nikos A. on May 15, 2020, 09:03:56 amIn addition to the other comments...avoid drawing four-way connections. In your drawing, you should have, at least, drawn a dot at the four-way. This circuit is so simple, there is no confusion, but a very bad habit may develop. Just don't do it. Break the common connection and offset the wires.

author: Nikos A.
author_rank: Regular Contributor
timestamp: May 16, 2020, 06:35:57 am
content: Quote from: Wimberleytech on May 16, 2020, 02:30:25 amHi, Wimberleytech yes you're right. Actually, I always place dots at junctions but in that case, I just draw the circuit in a simple editor (that do no add junction automatically) and I was a bit lazy

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 16, 2020, 10:51:26 am
content: Quote from: Nikos A. on May 16, 2020, 06:35:57 amLooks like you are drawing in visio...my favorite vector graphic tool!