RC time, using a Stop Watch, RC time is longer in time is the capacitor bad?

[danny davis]

Circuits that use RC for charging up a capacitor to cause a Delay in time

What value capacitors or resistors would it be to have a charging time of 1 second? , 5 seconds?, 10 seconds?

When using an RC network to do a charging time of 5 seconds but you get 20 seconds , what is wrong with the circuit? since the capacitors are old in the circuit can a capacitor that is in an RC network or a capacitor in an Op-amp intergrator circuit cause the charging time to get longer?

circuits at my work that are GOOD, work are 5 seconds in charging time for the lights to either turn off or on in 5 seconds after I push a button

I have to use a Stop watch to makes sure its 5 seconds the lights turn on or off after I push a button

If the capacitor is old , would it cause the time to get longer?

Because the bad boards at my work have different Turn on and off times that are not 5 seconds and they values on the RC networks are all the same values on the GOOD boards, What can be wrong with the RC timing?

BAD boards turn on and off the lights 30 seconds, 50 seconds , etc.

 

[Analog Ground]

Two comments: 1) Most ceramic capacitors have a large "voltage coefficient of capacitance" where the capacitance is much smaller as the voltage on the device increases. In an RC timing circuit, this can lead to longer than predicted charging time. This will be seen on all boards and right away. Your boards would have to have different types of capacitors. For example, ceramic vs tantalum. 2) If age is showing the problem, it is an increase in the leakage current of the capacitor. I am betting you have aluminum electrolytic type capacitors? I think these age the quickest.

From the times you mention which are seconds, I am thinking it is #2. In response to your question, the capacitor is bad.

 

[godfreyl]

When using an RC network to do a charging time of 5 seconds but you get 20 seconds , what is wrong with the circuit?

Could be resistance is too high or capacitance is too high.

 

[danny davis]

Why would leakage current Raise or increase the Time? or charging time?

 

[Tunelabguy]

Why would leakage current Raise or increase the Time? or charging time?

Leakage current means some of the charge that would have remained on the capacitor is "used up" as the leakage current flows. It is like trying to fill a bucket of water with small leak. The leak means that some of the water you pour in will not go to filling to bucket, so it will take you longer to fill the bucket.

By the way, the model of "charging up" a capacitor in the sense of "filling up" a bucket is a little inaccurate when an RC circuit is used to do the charging. The actual charging current gradually goes down as the voltage on the capacitor approaches the applied voltage. The capacitor is never "fully" charged in that model. It just gets closer and closer. Normally we set an arbitrary threshold at, say, 80% of the applied voltage to detect when the charging time is done. That does not mean the capacitor is done charging. It just gets harder and harder to detect small increases in the voltage across the capacitor as that voltage approaches the applied voltage. So we don't want to set a threshold too close to 100%.

 

[Audioguru]

Is "Danny Davis" the same as "Billy Mayo" the same as "Walters"? Billy Mayo today is also on another website forum.
Remember Walters?

 

[dick_freebird]

Consider also that in circuits where you're trying to
get a long RC with small C, your measurement stands
a good chance of corrupting the bare timing node -
10Mohm probe pr DMM input matters if your time
constant is made up of (say) 10Mohm and 1nF, the
timing node might never even make it over threshold
if the measurement is returned to GND same as the
cap - tau drops to 5s but Thevenin voltage becomes
VCC/2 and you end up having to go to many more
tau before you approach VCC/2 as threshold (if in
fact it's not even higher - in which case you'd never
switch until the probe is removed).

The same issues arise from things like the timing ramp
comparator input leakage, poor board cleaning / lack
of con-coat to keep surface leakage consistently low
against contamination & humidity, solder flux residue
and so on.

 

[danny davis]

Normally we set an arbitrary threshold at, say, 80% of the applied voltage to detect when the charging time is done. That does not mean the capacitor is done charging. It just gets harder and harder to detect small increases in the voltage across the capacitor as that voltage approaches the applied voltage. So we don't want to set a threshold too close to 100%.

How do you set the Threshold? do you mean on a Op-amp comparator to Switch On or Off at 80% of the charging voltage?

The same issues arise from things like the timing ramp comparator input leakage

What do you mean by the timing ramp comparator input leakage?

Can an Op-amp Intergator have input leakage that would cause the time to change to a longer time delay?

- - - Updated - - -

What I do at work is test the Timing of circuits

GOOD BOARDS:
1.) Timing of Output voltage to go from +10 volts to - 10 volts in 5 seconds with a switch is pressed
2.) When a Switch is pressed , measure the timing when a light turns on , should be 2 seconds

But I'm getting boards that are Longer timing
When the Capacitors are leaky in an RC network or In an Op amp Intergator circuit or the Op amp has input leakage, can this cause the Timing to get longer?

3.) There is Other Switches when TURN On takes 10 seconds to activate an Op-amp to switch it state, What is this timing RC networks called? they do this delay timing for 10 seconds so the input of the Op-amp comparators don't get false triggering's from noise or cause they want the Switch Turn ON state to stay on for 10 seconds before the Op-amp switches states or polarity on its output, What is this called when a designer puts RC delays charging like this?

4.) There is a Momentary Switch that Ground GND the Reset of an IC.
a.) Press Momentary switch GND Reset Once it will turn off the LIGHT
b.) Press Momentary switch GND Reset Again it will turn ON the LIGHT

What Kind of IC uses an Reset Pin that you can Ground GND that will turn On or off a light using the Reset Pin?

5.) Also what I do at work is I have to measure the TURN ON and OFF points of Lights by using a Pot. I turn the Pot very slowly until the light turns ON or OFF and have to measure the voltage and current of how much voltage and current is I guess going into the Op-amp Comparator circuit.
What is this called what I am doing? I don't know the electronic name for what I'm doing for this

The Pot is either hooked up to an Op amp Comparator or to TTL or CMOS Logic GATES

 

[dick_freebird]

I think at this point you should show a realistic schematic of
the circuit including test equipment applied, where.

Many cheap bipolar op amps & comparators can have uA range
input currents (more likely under extremes of environment, but
still). And that is easily worsened by handling (ESD) abuse even
if gross functionality is not lost - older vintage "30V" linears tend
to have dismal ESD ruggedness as far as input offset & leakage
drift.

 

[danny davis]

6.) Also what I see a lot of times on the bad boards is the company's CMM manual , component maintenance manual says the RANGES of the min and max voltages or the min and max Times in seconds are on a GOOD board.

I gets Voltages and Timings that are very close to the MAX values, can this be caused by leaky Capacitors go to the Max Voltage value or Very close to it? I guessing

Example#1
Test Point#1 , is 9.8 volts , time is 4.7 seconds ( BAD BOARD )
CMM manual will say voltage min is 7 volts , max is 10 volts
Timing Min is 4 seconds , Max is 5 seconds

 

[danny davis]

The Charging Pin is either
1.) The output of an Op-amp from the stage BEFORE
2.) Or the Charging pin is a Pull Up resistor connected to the Op amp from the stage Before

1.) Well some of the timing caps are in series with a charging resistor that goes to ground , this connects to the input of an Op Amp IC
2.) Others timing caps go to ground before the input pin of the IC op-amp
a.) The charging resistor is in series with a timing cap going to ground
b.) Or the charging resistor is a Pull Up resistor tied to the power supply , with the charging capacitor that goes to ground


RC timing networks , 3 types they use
1.) Resistor in series, capacitor is grounded
2.) Capacitor in series, Resistor is grounded
3.) Resistor is tied to VCC, capacitor is grounded, ( Not sure what this RC network is called tho) do you? The capacitor gets charged through the VCC through the resistor

There is an Op amp before the RC timing network and an Op amp After the RC timing network

Op amp#1 ----> RC timing network -----> Op amp#2

 

[danny davis]

The time range is min. 3.5 seconds and the max. is 5.0 seconds

if you get a time that is 2.5 seconds , what is wrong with the cap? high ESR

if you get a time that is 7.0 sseconds, what is wrong with the cap? high ESR?

 

[danny davis]

What RC network is this called? R58 is a pull resistor to +10, because R58 is what controls the time to the C13 capacitor, the 10 volts from R58 is what charges the cap.

Audioguru said there can not be a pull up resistor on the output of an op amp, and you can see R58 is a pull up resistor on the output of U9 pin#6

The problem I'm having is that U8 Pin#12 , threshold voltage is fixed internally , The time it takes C13 to change to the threshold is delaying the op amp U8 to switch it's output Pin#14 to late

To Find the threshold voltage is really hard because the C13 has to charge and my DVM meter will read random because it's switching decimal places from millivolts to volts and that transitions when the meter is switching is when the threshold is.

I'm trying to find out the threshold of U8 pin#12 , but there is that C13 charging cap on the input, It's hard to find out when at what voltage it is when the output switches from low to HIGH state

I tried using an oscilloscope
Channel#1 on U8 input pin#12
Channel#2 on U8 output pin#14

Channel#1 has a charging voltage expotential curve
Channel#2 has a Squarewaveform

My problem with that it needs to have two different Time sweeps in order to view them at the same time on the o scope

So how do I set up my O scope to do this test please?

 

[godfreyl]

you can see R58 is a pull up resistor on the output of U9 pin#6

Not exactly. There's a diode between the output of U9 and the RC so:
1) When the output of U9 is low, it discharges C13 through the diode.
2) When the output of U9 is high, the diode is switched off and R58 charges C13.

I'm trying to find out the threshold of U8 pin#12

U8 switches when the voltage on pin 12 is the same as the voltage on pin 13. Pin 13 is connected to ground, so the threshold is at zero volts.

 

[danny davis]

1) When the output of U9 is low, it discharges C13 through the diode.

So the C13 gets discharges through the output pin of U9 going inside U9 output to the ground of the IC pin?

U8 switches when the voltage on pin 12 is the same as the voltage on pin 13. Pin 13 is connected to ground, so the threshold is at zero volts.

The Voltage dividers of R58 and C13 starts at -11.23 volts
Other boards R58 and C13 start at -10.48 volts

Doesn't this starting point cause a different delay time to reach the threshold? what is this called ? i just call it a starting bias point , it's the node between the R58 and C13 were that meet together creates a voltage divider

 

[godfreyl]

So the C13 gets discharges through the output pin of U9 going inside U9 output to the ground of the IC pin?

Yes.

The Voltage dividers of R58 and C13 starts at -11.23 volts
Other boards R58 and C13 start at -10.48 volts

Doesn't this starting point cause a different delay time to reach the threshold?

Yes, it will make a small difference. I don't know what that's called.