RC timing measurements using a DVM frequency mode doesn't work why?

[betwixt]

You test our patience to the limits!

Try this: walk from one end of your room to the other and time how long it takes to do it. From that can you calculate what time of day it is?

An RC network does NOT have a frequency, it does not produce any repetitive signal which you can measure. If you don't believe us, connect your 1K and 0.01uF capacitor is series across the input of your frequency counter and see what it tells you. Try then in parallel and see if it makes any difference.

What it produces is DELAY in voltage change because the resistor limits the flow of current into and out of the capacitor. To measure the delay you have to look at the signal entering the RC network and the signal leaving it and see how much time difference there is between them. A frequency counter is useless for that, you need an oscilloscope, ideally one with a dual trace. You look at both signals and count the time divisions on the X-axis to see how much time difference there is between the input and output waveforms.

I will say this: if you are going to the trouble of measuring time constants like that, it would be quicker to remove the resistor and capacitor from circuit and test them conventionally and it would almost certainly be more cost effective to simply replace them altogether.

Brian.

 

[danny davis]

An RC network does NOT have a frequency, it does not produce any repetitive signal which you can measure. If you don't believe us, connect your 1K and 0.01uF capacitor is series across the input of your frequency counter and see what it tells you. Try then in parallel and see if it makes any difference.

It measure nothing , it's zero , on the frequency counter

What it produces is DELAY in voltage change because the resistor limits the flow of current into and out of the capacitor. To measure the delay you have to look at the signal entering the RC network and the signal leaving it and see how much time difference there is between them. A frequency counter is useless for that, you need an oscilloscope, ideally one with a dual trace. You look at both signals and count the time divisions on the X-axis to see how much time difference there is between the input and output waveforms.

Yes I know how to do this , I do this all the time at work to measure the RC time

I will say this: if you are going to the trouble of measuring time constants like that, it would be quicker to remove the resistor and capacitor from circuit and test them conventionally and it would almost certainly be more cost effective to simply replace them altogether.

Yes this is what I do at work

To measure the delay you have to look at the signal entering the RC network and the signal leaving it and see how much time difference there is between them.

Time Delay = Frequency

Time can be converted into frequency
Frequency can be converted into TIME

A frequency counter should work , I just need the right Test signal to inject on the input of the RC network

 

[betwixt]

NO NO NO!

Time delay has nothing whatsoever to do with frequency. If you were catching a bus to work and it arrived 5 minutes late it wouldn't mean time has accelerated or slowed down, it just means it took longer to reach you.

Frequency is the number of times a repetitive action occurs within a set time period. It has to be defined by what that time period is and how long it takes for the next occurence.

Try this experiment:
Connect a battery (any voltage) through a switch to a resistor, a capacitor then back to the battery so they are all in series. Connect your oscilloscope (or frequency counter if you like) across the capacitor and see what's there when the switch is open. Short out the capacitor just be sure there is no charge in it. Now close the switch and see what happens. Hopefully you see the voltage rise to meet the battery voltage, the time it takes to reach it depends on the resistor and capacitor value. Now without opening the switch, use a clock/stopwatch and see how long it takes for the voltage to go down and rise up again. The reciprocal of the time in seconds will be what you call frequency. The rest of us think you will be waiting for an awfully long time.

Brian.

 

[danny davis]

use a clock/stopwatch and see how long it takes for the voltage to go down and rise up again. The reciprocal of the time in seconds will be what you call frequency.

Yes I do this at work is measure the Time with a stopwatch

I convert the time into frequency

But you're saying it's a different kind of time than a time period or a repeated waveform related to a time period

 

[betwixt]

Did you try the experiment? - I think it will show the difference between frequency and time constant.

Brian.

 

[Audioguru]

An RC network does not make a frequency.
They can be parts in an oscillator that makes a frequency. They can be a highpass or a lowpass filter. They can make a delay time. Which is it?

Since they are simply a resistor and a capacitor then why don't you measure the resistance and capacitance?

EDIT: I did not see your post until now since it was on a different page.
Danny, Billy, Walters or whatever he calls himself today did the very long test WRONG. He did not wait long enough for the oscillation of the RC to begin.:roll:

 

[betwixt]

It's gone quiet, maybe he's still waiting... :wink:

Brian.

 

[Audioguru]

It's gone quiet, maybe he's still waiting... :wink:

Brian.

He is waiting for "HELL to freeze over".
Did you see the Arizona University answer to the exam question? OMG! http://www.quickmeme.com/p/3vr3nn

 

[betwixt]

Thank you. Best laugh I've had in ages!

Brian.

 

[danny davis]

Since they are simply a resistor and a capacitor then why don't you measure the resistance and capacitance?

Yes this is what I do , to measure the tolerance and drift , cap leakage , cap ESR

An RC network does not make a frequency.

Since it doesn't make a frequency, what kind of TIME or DELAY is this?

An RC network TIME DELAY is not the same kind of TIME you find when you have a DC alternating waveforms, like pulse or squarewaveform, you can measure the time delay between pulses and the time of the period of the waveform

Mostly TIME Delay you can convert into frequency , am I right? this kind of RC time delay you guys are saying is that I can not convert into frequency

I Thought any TIME delay you could convert into frequency , but this type of TIME delay from an RC network is different

An RC network does not make a frequency.

How can I trick the RC network into having a frequency? using a Testing Signal to inject into the VIA of the RC network to measure the TIME DELAY

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An RC network does not make a frequency.

Frequency= The SPEED OR RATE of alternating polarity swing , Cycling back and forth between polarity or a voltage

The Charging and Discharging Rate or time can be converted into a frequency?

The RC network itself is not a frequency or a time delay

You have to inject a Test signal to make the RC network into a frequency? and a time delay?

 

[Audioguru]

Since it doesn't make a frequency, what kind of TIME or DELAY is this?

Use a 100k series resistor feeding a 10uF capacitor to ground. Apply DC to the free end of the resistor and measure the DC voltage across the capacitor (to ground).
The voltage will slowly rise then when it reaches 63.2% of the input voltage its time should be exactly 1 second.

An RC network TIME DELAY is not the same kind of TIME you find when you have a DC alternating waveforms, like pulse or squarewaveform, you can measure the time delay between pulses and the time of the period of the waveform

No.
Continuing pulses or a squarewave signal are created by the output of an oscillator, not by an RC. But the RC are the frequency-setting PARTS of the oscillator.
An oscillator does not have a delay. Instead it has an oscillation. The RC in the oscillator has delay because it takes time for the capacitor to charge and discharge.

Mostly TIME Delay you can convert into frequency, am I right?

NO!

How can I trick the RC network into having a frequency?

Connect it as the frequency-setting RC in an oscillator circuit.

Can I use a Test Signal to inject into the VIA of the RC network to measure the TIME DELAY

What is the "VIA" of an RC network? A VIA is a path on a pcb from one side to the other side.
A delay circuit is a series resistor feeding a capacitor to ground. It takes time for the capacitor to charge and to discharge. You can measure how long it takes for the capacitor to charge or to discharge.

Frequency= The SPEED OR RATE of alternating polarity swing , Cycling back and forth between polarity or a voltage

Correct.

The Charging and Discharging Rate or time can be converted into a frequency?

Yes, in an oscillator circuit.

The RC network itself is not a frequency or a time delay

The RC network can produce a frequency when it is the frequency-setting parts of an oscillator circuit.
The RC network can produce a time delay.

You have to inject a Test signal to make the RC network into a frequency?

NO!
You do not inject a signal into an oscillator. The oscillator produces its own signal.

 

[betwixt]

From the Oxford English Dictionary:

Mathematics & Physics The number of times a specified periodic phenomenon occurs within a specified interval, as:
a. The number of repetitions of a complete sequence of values of a periodic function per unit variation of an independent variable.
b. The number of complete cycles of a periodic process occurring per unit time.
c. The number of repetitions per unit time of a complete waveform, as of an electric current.

Build the circuit in post #23 and tell us the results.

Brian.

 

[danny davis]

The voltage will slowly rise then when it reaches 63.2% of the input voltage its time should be exactly 1 second.

1 seconds time = 1hz frequency

Mostly TIME Delay you can convert into frequency, am I right?
NO!

Why No? When is a TIME delay different than a Time than you can convert to frequency? I guess I don't know the difference between these two TIME's

So a Time delay you can't convert to frequency? but a time period you can convert to frequency?

I guess what you guys are saying is if you have 2 waveforms and the time delay is 1 millisecond part from eachother , 1 millisecond = 1khz, there is no frequency at 1Khz between the 2 waveforms that have a time delay apart from each other. It's just a 1 millisecond time delay

When an RC network is "periodic" it's considered an oscillator?

When an RC network is periodic , meaning it's charging and discharging Periodic , than you can measure the frequency only if it's periodic?

Most RC networks are not periodic ? only in an oscillator or timer circuit like a 555 timer?

Since an RC network is not periodic, what is it called when a circuit is has a TIME delay but isn't periodic?

How can I trick the RC network into having a frequency?
Connect it as the frequency-setting RC in an oscillator circuit.

The Charging and Discharging Rate or time can be converted into a frequency?
Yes, in an oscillator circuit.

So If I connect an Oscillator circuit ACROSS in parallel to an RC network , I can then measure the frequency of the RC network from the Oscillators circuits output? So I would have my fluke meter frequency counter on the output pin of the oscillator and the oscillator will be across in parallel to the output of the RC network in circuit

What I just did was convert an RC network that is not periodic into a periodic frequency so i can measure the frequency of the RC network that is in circuit

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Also to note that when I turn my fluke 87 meter to frequency counter, I just hold my meter probes in the AIR and it will measure 60hz
So this is a way to test that the probes are un-shielded and you can measure what the probes are picking up in the AIR using the frequency counter, this is way it won't work with measuring logic levels

 

[Audioguru]

1 seconds time = 1hz frequency

You are completely WRONG again.
A capacitor that charges to 63.2% of the DC voltage has no frequency because it is not oscillating. If it charges in one second then an oscillator circuit causes it to discharge in one second and charge and discharge over and over then its frequency is 0.5Hz, not 1Hz.

Why No? When is a TIME delay different than a Time than you can convert to frequency? I guess I don't know the difference between these two TIME's

Maybe you also have trouble understanding English.

So a Time delay you can't convert to frequency? but a time period you can convert to frequency?

Get rid of the word, "convert".
An RC has a charging time and a discharging time.
An oscillator has a frequency. The frequency of many oscillators is set by an RC network.

I guess what you guys are saying is if you have 2 waveforms and the time delay is 1 millisecond part from eachother , 1 millisecond = 1khz, there is no frequency at 1Khz between the 2 waveforms that have a time delay apart from each other. It's just a 1 millisecond time delay

How can you have a delay between two waveforms? If you have a single waveform made with 1 millisecond repeating pulses that occur every 1 millisecond then one cycle takes 2 milliseconds and the frequency is 500Hz.

When an RC network is "periodic" it's considered an oscillator?

An RC network is not periodic and is not an oscillator. An oscillator is a circuit with a transistor or IC. An RC network sets its frequency.

When an RC network is periodic , meaning it's charging and discharging Periodic , than you can measure the frequency only if it's periodic?

Yes, the waveforms in an oscillator repeat over and over so they are Periodic.

Most RC networks are not periodic ? only in an oscillator or timer circuit like a 555 timer?

A Wien bridge oscillator, a classic Cmos oscillator, a phase shift oscillator, a Bubba oscillator, a 555 oscillator and many more oscillators have their periodic frequency set by an RC network.

Since an RC network is not periodic, what is it called when a circuit is has a TIME delay but isn't periodic?

It is called a time delay circuit. A monostable (can be made with a 555 timer IC) can be a time delay circuit.

So If I connect an Oscillator circuit ACROSS in parallel to an RC network , I can then measure the frequency of the RC network from the Oscillators circuits output? So I would have my fluke meter frequency counter on the output pin of the oscillator and the oscillator will be across in parallel to the output of the RC network in circuit

You must look at an oscillator circuit to see how the RC network is connected. Many RC oscillators produce a frequency that is not accurate.

What I just did was convert an RC network that is not periodic into a periodic frequency so i can measure the frequency of the RC network that is in circuit

That makes no sense. Instead maybe you connected an RC network properly in an oscillator circuit and measured the frequency of the oscillator.
Why? Then you are measuring the frequency of an oscillator that has nothing to do with the timer or phase shifter or filter in the circuit you are trying to fix.
You need an accurate TIMER or PHASE SHIFTER or FILTER, not an oscillator.

 

[danny davis]

A capacitor that charges to 63.2% of the DC voltage has no frequency because it is not oscillating.

Ok I get it

Since an RC network is not oscillating, what is it called when an RC network is not oscillating? or not connected to an oscillator circuit

Since the RC network is Non Periodic , has no frequency, but has a charging and discharging time delay. It would be called?

What I'm asking is because some RC networks are for Non-periodic circuits and other RC networks are for Periodic circuits

So what it is called when a RC network is in a non -periodic circuit? because the charge time and discharge time is not periodic , it should be called what? besides non- periodic , what would it be called?

 

[betwixt]

RC networks are NOT periodic, never have been and never will be. They are simply time delay circuits. You CAN use them to make an oscillator but not without additional components, in themselves they do not oscillate at all.

They have a TIME CONSTANT, as explained several times before, it is a measure of how long a change in input voltage takes to appear at the output. Time constant is normally measured in seconds or fractions thereof.

You will often see circuits where the signal entering the network is alternating and hence the output is also alternating. The output is still just a delayed version of what went in.

The signal feeding an RC network MAY be periodic, it MAY have a measurable frequency but the term 'periodic' applies only to the signal applied to the network, not the network itself. Even if a fixed, stable, squeaky clean square wave is fed fed to an RC network, it will only delay it.

Audioguru has already explained how the time constant is calculated in post #34.

Brian.