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

[Billy Mayo]

We have an event and some period of time

What is considered the Event?

Since T = R * C the RC time is 4.2 seconds for C13 to show a 63% change in a positive direction. Keeping the numbers round and simple the difference between -12 and 10 is 22 volts and 63% of 22 volts is 13.86 volts. Therefore we should cross zero in about 4 seconds. Just about 1 R*C time constant.

Thanks for this info, i need this to help me out

R * C , you did R58 and C13? I thought the RC network was C13 & R59? so what is R59? is it a timing resistor or something else?

 

[Billy Mayo]

Another circuit board at work they use two op amp intergrator circuits in series one after another

The First Op amp intergrator is the RC time delay to reach the input threshold of the #2 second Op amp intergrator

They are in series one after another , 2 stages of intergators

What does having 2 stages of intergators do? a stepper slope?

1.) To measure the input threshold voltage crossing point of the 2nd Op amp intergrator, how would you do this please?

2.) To measure the "TIME to reach" the input threshold of the 2nd op amp intergrator, how would you do this please?

 

[MrAl]

Hello,

If they were true integrators then the first one would produce a ramp and the second one would produce an exponential ramp, but it sounds more like they are just used for timing in your circuit. So two would produce a longer delay and probably a non inverted signal also. Of course if the first one is a delay and the second is a true integrator, then the circuit produces a delayed ramp. We'd have to see the circuit to know for sure what it is doing.

 

[Billy Mayo]

Of course if the first one is a delay and the second is a true integrator, then the circuit produces a delayed ramp.

Thanks Mr. Al you just nailed it , now i got the name for it delayed ramp

How would you guys test for these things:
1.) To measure the input threshold voltage crossing point of the 2nd Op amp intergrator, how would you do this please?

2.) To measure the "TIME to reach" the input threshold of the 2nd op amp intergrator, how would you do this please?

 

[Reloadron]

R * C , you did R58 and C13? I thought the RC network was C13 & R59? so what is R59? is it a timing resistor or something else?

The R8C network is R58 and C13, R59 is the opamp input resistor for the next stage. Thus I calculated the R*C time using R58 and C13.

Keep something in mind through all of this. When using R*C circuits like this for timing applications the rate of charge for the cap or discharge is not at all linear. Sort of obvious because as we have covered for the cap to fully charge to the applied voltage takes about 5 R*C time constants but yet the cap charges to 63% in one R*C time constant.

How would you guys test for these things:
1.) To measure the input threshold voltage crossing point of the 2nd Op amp intergrator, how would you do this please?

2.) To measure the "TIME to reach" the input threshold of the 2nd op amp intergrator, how would you do this please?

There are a few ways to go about doing this. My first choice would likely be a DSO (Digital Storage Oscilloscope) because some of the RC time constants you are looking at are pretty slow. Another option I would consider is the use of a counter with two input channels that allows for time interval measurement. Looking at one of your images we know that the output of U9 is going to swing high at pin 6. When that happens capacitor C13 which was being held at about -12 Volts will begin to charge in a positive direction. We know when C13 passes through 0 volts that U8 pin 14 will go positive. So, U9 output is a START event and U8 output is a STOP event. The time interval is the measured time between the START and STOP.

Ron

 

[Billy Mayo]

The time interval is the measured time between the START and STOP.

If I didn't know the Threshold parameter of U8, how would u use the oscilloscope to find the TIME TO REACH to the threshold point?

Start and stop time to reach to the threshold point? how can this be done please?

 

[large_ghostman]

would be easy to make a counter with a pic and capture and compare, LCD and away you go!

 

[Billy Mayo]

Ron, how can you do this with an oscilloscope? how can you do this with a counter?

 

[large_ghostman]

on the counter you select if you want to start count on rising or falling edge on one input, then do the same for the other input! simples, when first is triggered it start to count and when other is triggered it stops, then on the display is number's, these number are the time taken, normaly you can select the time setting for S ms us etc.

 

[Reloadron]

If I didn't know the Threshold parameter of U8, how would u use the oscilloscope to find the TIME TO REACH to the threshold point?

Start and stop time to reach to the threshold point? how can this be done please?

Before all else remember you are looking at long RC time constants. In your circuit the time constant for R58 and C13 is about 4.2 seconds and from previous oscilloscope discussions we know you need a DSO (Digital Storage Oscilloscope) as an old analog scope will only show a dot crawling across the screen. Keep that in mind!

If I didn't know the Threshold parameter of U8, how would u use the oscilloscope to find the TIME TO REACH to the threshold point?

I would set the scope up for two channel operation Using channel 1 I would connect to the junction of R58 and C13. I might use an external trigger and trigger off the output of U9 pin 6. I would set my triggering for Source: External, Level: Midrange, Slope: +, Coupling: DC. I would set my sweep time for maybe around 2 seconds / Division. Channel 1 vertical to the junction of your R58 and C13 RC network and Channel 2 to the output of U8 pin 14. Another option using a 4 channel scope would be to trigger internally off channel 1, channel 2 the RC junction and Channel 3 the output of U8.



Look at the attached image in detail as it is just about what you have in your circuit. My U1 simulates your U9 and my U2 simulates your U8. I use a +/- 13 volt supply as you have (V1 and V2). My V3 is your 10 volt reference supply and finally my V4 simulates the voltage into your U9. The image shows the waveforms as they would look on a scope. Looking at the second trace down (red trace) labeled V[C1Rate] we see the capacitor start charging when the output of U1 swings positive. With the exception of me using a signal I label V4 to simulate a changing voltage input what you see is what you have.

@ my little ghostman buddy:

would be easy to make a counter with a pic and capture and compare, LCD and away you go!

Yes I agree but there is a downside to doing this. While a PIC is a marvelous fun device they do have limitations. Sometimes creating a specific piece of test equipment is a good idea but for test like this I prefer the use of GPTE (General Purpose Test Equipment). For circuits like this I like the GPTE route netter than making something specialized.

NOTE: My attached image is large so when viewing it make sure you expand it (enlarge it) so the traces don't look lousy.

Ron

 

[large_ghostman]

I was being a bit tongue in cheek sorry
What he needs is something i always have on my bench

It would do everything he needs in one accurate instrument! everything except show a trace that is. but its very accurate, does frequency and counting as well as all normal DVM measurements, Has loads of functions and 5 1/2 digit accuracy

 

[Reloadron]

Ron, how can you do this with an oscilloscope? how can you do this with a counter?

I mentioned using a counter earlier in a time interval mode. However, we can revisit it. Look at my cartoon again. For your drawing we know that when a voltage exceeds .267 Volt (267 mV) into u9s non inverting input that the output of U9 will swing from -13 to 13 volts. That event starts things going and that level is set by the divider of R60 and R61. That transition starts things going with the RC network of R58 and C13 through diode CR19. So C13 starts charging heading for 10 volts. C13 begins charging from about a -12 V where it was sitting. When C13 passes through 0 volts the output of U8 will swing from around -12 to 12 volts. So the output of U9 has a positive going pulse and the output of U8 has a positive going pulse. If I call the first pulse U9 "A" and the second pulse U8 "B" I am measuring a time interval A to B. That is how it would be done using a counter. I labeled the START and STOP pulses on the cartoon I provided in my drawing image.

Ron

 

[Reloadron]

@ Little Ghostman

It would do everything he needs in one accurate instrument! everything except show a trace that is. but its very accurate, does frequency and counting as well as all normal DVM measurements, Has loads of functions and 5 1/2 digit accuracy

Now would be yanking my chain? How can it have 5 1/2 digit accuracy? Me thinks it has 5 1/2 digit resolution and in time interval mode it has ranges of 1mS, 10 mS, 100mS, 1S and 10S with an accuracy of +/- 0.005% Rdg +/- 1 Count +/- Trigger Error. Trigger Error is covered in the manual. Me no thinkie that 5 1/2 digits is an accuracy specification but rather a measure of the instruments Resolution. So what you think there LG?

Ron

 

[Reloadron]

Uh Oh, all the attached images just seem to have went away? Noticing this Template Errors: PAGE_CONTAINER at the top of pages all morning also? Maybe there is a connection but my image and the LG images are gone from our post. Poof and they were gone.

Are you guys seeing the images or is this just a me thing? Maybe time to see if ElectroMaster has posted anything about this.

Ron

 

[large_ghostman]

Ermmmm, WELLL I think I should try and proof read a little bit more as for the time setting's, i have never used the counter on this, so i dont know. It sit's on the bench and mainly gets used for Quick measurements, mainly because it has O scope like probes, when i do timing stuff and i need accuracy, i normally use something else (depending what's plugged in and ready to go ), order of preference is O scope then this

or if bench space is tight I use

 

[large_ghostman]

i can see my new pic?

 

[large_ghostman]

I dont normally use the bench DVM in the pic, I have a Racal one I much prefer

 

[Reloadron]

Thinking I need to look at my settings in FireFox as things are fine (for images) in MSIE.

LG I haven't seen a Racal Dana counter in decades.

Eventually I need to get my test bench setup again. Since I retired I have been moving things. Between my shooting hobby and my electronics stuff is all over the place. What I really need when spring gets here is a large dumpster and clean some stuff out.

 

[large_ghostman]

I love the Racal stuff! We had that one tested against a Agligent one that was in calibration, and its spot on! same with the Racal bench meter , 6 1/2 digits and spot on. That Tektronix meter would do 99% of what billy need's, plus has the shielded scope type probes! even has temperature reading.
Dad like's new stuff i like the old stuff! One exception is my tenma DMM, ace bit of kit! and has usb connection

 

[Billy Mayo]

Another option using a 4 channel scope would be to trigger internally off channel 1, channel 2 the RC junction and Channel 3 the output of U8

.

What would be on channel#1 to trigger off of? the output pin of U9?

Thanks

The way I did it was
Channel#1 was the Input pin of U8
Channel#2 was the Output pin of U8
External Trigger was the output pin of U9

I used a Digital Storage Scope

Using the external trigger on the output pin of U9, does what? it helps sync what?