Author Topic: RC circuit  (Read 1204 times)

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Offline Nikos A.Topic starter

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RC circuit
« on: May 15, 2020, 09:03:56 am »
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
 

Offline JackJones

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Re: RC circuit
« Reply #1 on: May 15, 2020, 09:38:42 am »
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.
 

Offline Nikos A.Topic starter

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Re: RC circuit
« Reply #2 on: May 15, 2020, 10:12:05 am »
At the moment I have not connect a load on the output, I am justing probing it.
 

Offline JackJones

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Re: RC circuit
« Reply #3 on: May 15, 2020, 10:27:14 am »
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.
« Last Edit: May 15, 2020, 10:34:55 am by JackJones »
 

Offline Nikos A.Topic starter

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Re: RC circuit
« Reply #4 on: May 15, 2020, 01:18:19 pm »
For multimeter anyway, oscilloscope with a 1x probe would be 1 Meg which would drag it down to about half Vdd.

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
 

Offline FenTiger

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Re: RC circuit
« Reply #5 on: May 15, 2020, 01:51:27 pm »
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.
 

Offline Nikos A.Topic starter

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Re: RC circuit
« Reply #6 on: May 15, 2020, 04:10:57 pm »
If you disconnect Vdd and then short it to ground, you'll forward bias the diode and it should work as you expect.

It works now, thanks!!  |O

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?
 

Online SiliconWizard

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Re: RC circuit
« Reply #7 on: May 15, 2020, 04:20:05 pm »
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.
 

Online Vovk_Z

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Re: RC circuit
« Reply #8 on: May 15, 2020, 04:50:17 pm »
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?
I 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.
 

Offline Nikos A.Topic starter

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Re: RC circuit
« Reply #9 on: May 15, 2020, 04:51:08 pm »
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.

I 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.

Thank you guys!!
 

Offline Wimberleytech

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Re: RC circuit
« Reply #10 on: May 16, 2020, 02:30:25 am »
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
In 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.
 

Offline Nikos A.Topic starter

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Re: RC circuit
« Reply #11 on: May 16, 2020, 06:35:57 am »
In 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.

Hi, 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 :)
 
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Offline Wimberleytech

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Re: RC circuit
« Reply #12 on: May 16, 2020, 10:51:26 am »
In 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.

Hi, 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 :)

Looks like you are drawing in visio...my favorite vector graphic tool!
 


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