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

Discussion in 'General Electronics Chat' started by Billy Mayo, Nov 26, 2013.

  1. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Pretty much what I mentioned when I said:
    You can look at the junction of R58 & C13 or you can look at pin 12 of U9, same thing. The 10K input resistor matters not.

    Using a 2 channel scope I would, as mentioned, use external triggering so I could observe the slope of the capacitor charging and U8 changing output states. Using a 4 channel scope I would do it as I mentioned and just run my trigger signal into channel 1 and use channels 2&3 to see what I have an interest in. Either way the event we want to trigger off of that starts things going is when U9 pin 6 goes high. All of that should be clear in the image I posted.

    The output of U9 is where things begin happening. Thus I make it the event that triggers what I want to see. The drawing I attached shows everything including the time intervals. Did you look at the image I attached and do you understand it?

    Ron
     
  2. Billy Mayo

    Billy Mayo Member

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    I did look at it

    I just confused about the triggering part

    If you don't use any triggering, what will happen? it's the same thing it seems

    What is the triggering doing? it's syncing what? or doing what?

    I tried it with triggering and without triggering and I got the same results from input and output of U8

    So I don't understand how the triggering works or how does it help with the measurement, what is it doing?

    Yes I know it's triggering off of U9 and that's the start event
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    No. You are completely wrong.
    When the output of U9 goes high then the diode is reverse-biased and is turned OFF for most of the charging of the capacitor.

    No. See answer above.


    No again.
    The 741S opamp does not have a ground pin. Its output goes high to about +11.5V and it goes low to about -11.5V.
    When the 741S output is low then the capacitor is quickly charged by the opamp through the diode to about -10.9V at the capacitor.
    When the 741S output is high at about +11.5V then the diode is reverse-biased and does not conduct allowing the 560k of R58 to slowly charge the capacitor to about +12.1V.


    No again.
    U8 does not have its power supply shown so we guess it is +13V and -13V.
    Its positive and negative threshold voltages are the voltage on its pin 13 (hint: ground).

    The threshold voltage is simple to see on a 'scope that has two channels.
    One channel is looking at the voltage on the capacitor and the other channel is looking at the output of the opamp acting like a comparator.
    The threshold voltage is where the traces cross each other.

    But why are you wasting time looking for the threshold voltage? It is set by the voltage on pin 13 of U8.


    I call it a peak detector. The capacitor charges quickly with one polarity and charges slowly with the other polarity.
     
  4. dave

    Dave New Member

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  5. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Audio Guru I has a question for you. In your fine reply you mention:

    My question is on the Bold and Italicized text where you mention the cap charge. This is the cartoon:

    Comparator 1.png

    They use the 10 Volt VR supply for the RC network so wouldn't the cap start charging towards 10 volts rather than the 12.1 volts you mention? Not that it is a big deal but they seem to have gone through placing their divider circuits and RC circuits at the regulated 10 volt reference supply. Really matters not as U8 does its thing long, long before the cap reaches 10 or 12.1 volts anyway. U8 is going to change states as that sucker comes through 0 volts (GND).

    Ron
     
  6. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Billy, the only thing the trigger does is to initiate a sweep. Triggering triggers a sweep. That is all it does. Now if you don't use any trigger one of two things can happen. If the scope is set for Auto Trigger then the scope will trigger using an internal generated signal and you will see a trace, if the scope is set for Normal triggering then without a trigger nothing will happen as in no sweep. That is it! The only damn thing a trigger does is initiate the sweep. Once the sweep is initiated there is a trigger holdoff circuit that prevents the scope from triggering again till the sweep is completed.

    Sync is another story and if you want sync then the trigger signal needs to be the same frequency as the signal we want to view, or a multiple thereof. You can have trigger initiate a sweep but not have sync. (a stable display).

    Something initiated the sweep? Was the Mode set for Auto? No trigger no sweep unless the scope triggering is set to Auto.

    Ron
     
  7. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Billy start here


    It's called SCOPES for DOPES, I couldn't find a video under the search term i actually used, but after tweak the search phrase this came up. It's brilliant, and assumes no prior electrical knowledge, mainly based on analogue scopes but all the basics are there. After that one i have a few that area bit more technical.
     
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  8. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Yeah, we really need to get you up to par on scopes. That weakness will haunt you because the oscilloscope is such a popular and important piece of test, measurement and diagnostic equipment. Once you have a good handle on scopes and scope triggering it will allow many counters to come easy as most counters use the same or similar triggering concepts.

    Ron
     
  9. Billy Mayo

    Billy Mayo Member

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    Well I'm measuring long delay times 4 seconds up to 17 seconds , so I have to use a Storage scope , So the Ext. trigger doesn't START an event for or Start the sweep for the storage scope section

    I have to push my finger on the start button at the same time as a START Event I want to start off of

    Does you storage scope external trigger, triggers the sweep and the storage start button?

    At work we don't use counters because my manager said you can just use a stop watch , its the same thing he said. Is this true? or what can counters do that a stop watch can't?
     
  10. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    The merit to using a counter would be better uncertainty, however, if the accuracy of a counter isn't needed then using a stopwatch is fine. If for example I want to measure 4 seconds and +/- 0.5 seconds is acceptable then a stopwatch is fine. If I want to measure 4 seconds +/- .002 seconds then a counter is the better choice. Just a matter of how accurate the measurement needs to be. If you are working from a procedure that calls out the use of a stopwatch then we can assume a stopwatch and the method used is adequate. Time interval measurement is time interval measurement, how we do it is a matter of the results we expect. We can use a counter, scope or even a stopwatch.

    Again, as far as scope setup it depends on the scope used. For measuring long time periods or most of what has been mentioned as to signals I would likely use Single Sweep mode which just about all scopes have, both analog and DSO types.

    Ron
     
  11. Billy Mayo

    Billy Mayo Member

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    not all scopes have single sweep, i have normal sweep and auto sweep.

    tthe storage has a start button that i have to press at the same time i'm probing an output or input that i'm measuring so the start event starts at the same time as im pressing the storage start button
     
  12. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Not familiar with the scope you are using. Good example though that while similar there are differences between scopes.

    Ron
     
  13. Billy Mayo

    Billy Mayo Member

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    What I'm saying is that the storage start button can't be triggered or sourced off of channel#1 what iim measuring
     
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    If you are looking at the cap charge with the scope then you dont need auto trigger. You'll see the wave START to rise (or fall) when the event starts. It wont rise much at first, but it will rise a little from what the flat line zero line looks like. So even if you start the sweep first, you should still be able to measure the rise time to the threshold.
     
  15. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Also a counter means you can set it up press go/start, and then you are free to Destroy test another circuit. So from a time point of view a timer is more productive
     
  16. 4pyros

    4pyros Well-Known Member Most Helpful Member

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  17. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    What is the scope as in make & model?

    Ron
     
  18. Billy Mayo

    Billy Mayo Member

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    Tektonix 2212 , I have to press the Storage "ON" button at the same time I want to trigger an event

    If I don't press the Storage "ON" time at the right time of the Start event , it will add "Dead lead time" which I don't want because my timing measurement will be off

    This Scope doesn't have a single sweep or how to Trigger the Storage " On" Button so it starts when the start event you're measuring

    https://www.google.com/search?q=tek...out-punta-garantia-sonda__5B22EA.html;440;330

    If I don't use auto trigger, what should I use? normal or single sweep?
     
  19. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    I actually took the time to download and read the manual on your scope. Billy, the manual has an entire section dedicated to making storage measurements. Your problem is that you still do not understand what the scope is doing or more important triggering.

    That isn't true at all. That is what you are reduced to doing because you don't understand triggering concepts or how this scope works.

    Store Mode offers up RECORD MODE and ROLL MODE. Record mode is 50 mS Div to 20 uS Div, and Roll mode is 50 S to .1 S Div. In Roll Mode triggers are disabled except SINGLE SWEEP! Read the section on SGL SWEEP, Roll Mode.

    Ron
     
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  20. Billy Mayo

    Billy Mayo Member

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    Ok I will sorry
     
  21. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Billy, no need for sorry. Just give the suggestions in the book a shot. Once you get a good working handle on the scope you will be amazed at how easily this stuff works out. Just a matter of time and patience with the learning curve.

    Ron
     

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