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Question for the Oscilloscope Experts

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rsfoto, Oct 10, 2015.

  1. rsfoto

    rsfoto Member

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    Hi,

    Thanks. I will play around with those values.

    I guess I can use also lower resistor values, right ?
     
  2. rsfoto

    rsfoto Member

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    audioguru, Thanks for the info and yes I know what that means but as I said in my first post I just wanted to see how an oscilloscope works, if I really need one ? and therefore I decided to get a simple one before braking the bank and then find out I really do not need a Ferrari when a small beetle is more then enough.

    I am not going to make for the moment big measurements, Just looking at the curves for me at the moment is enough. Perhaps if I dig deeper into electronics I will change my mind.

    :cool:
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My 'scope is about 52 years old. It was my second electronics project (the first was a multimeter kit), the 'scope was also a kit and it still works. It is analog so it does not have the digital artifacts of a low bit rate like yours has.
    I think the opamp will thank you (by surviving) if you add a Schottky diode at its input to ground to clamp negative inputs from the sensor to less than -0.3V. Also add a zener diode at the input with a voltage rating less than your power supply voltage.
    My son married a Mexican girl and we went to their wedding in Leon, Mexico last December. The 13 persons Mariachi band was wonderful. Adios.:)
     
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  4. dave

    Dave New Member

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  5. rsfoto

    rsfoto Member

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    Hi,

    Thanks. The zener diode at which input ?

    Yes, León is one of the nicer cities in Mexico.
     
  6. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Sure. Although, I'd leave the 47k as is.

    Once you've determined the optimum gain, it would be best to replace the pot with a fixed, 1% resistor. Choose a value as close to that equaling the pot's current setting resistance.

    You don't want to "bump" or otherwise accidentally alter the pot's value and undo your calibration efforts :banghead:.
     
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  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I have another suggestion. Place back to back diodes at pins 2 and 3 of the OP amp. Then add a series resistor from 1K to 10K to the diode protected inputs.

    The two back to back silicon diodes basically act as ~ +- 0.6V clamps. I didn't look at the datasheet, but usually you can find what you need to limit the current to an input too to prevent damage. With no power, you piezo sensor generates a voltage and the op amp power is 0 V, so a current tries to flow to the inputs.
    Inherently there is a diode drop to the substrate. This is why you usually see -0.3 to Vdd+0.3 in spec sheets. Vdd happens to be zero when power is off. 0..3 V is a "good number" for a Shockley diode. It's made with a metal and a semiconductor rather than a p-n junction.

    Series resistors to the input add thermal noise. The lower the value the less noise. The lower the temperature, the less noise. In the ideal case, no current flows to the inputs, so a resistor in series doesn't alter the original circuit.

    So where pins 2 and 3 are break the circuits. Add a resistor in series with the break and a diode to the other input. Make sure the diodes are back to back in the end.
     
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  8. rsfoto

    rsfoto Member

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    Hi Kiss,

    Thanks. Let me see if I understood correctly o_O

    Something like below ?

    MS100_3_circuit_DDR.JPG
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your HUGE negative schematic with a black background was hard to look at so I resized it and made it a positive with a white background (but it still has your chicken pox dots).
    You had NO negative feedback so I fixed it.
    10M for R3 is too high for an LM358 so I reduced it to 100k.
    R4 did nothing so I replaced it with wire.
    I provided part numbers for the diodes and corrected their ground connection.

    How will you disable the second opamp in the LM358?
     

    Attached Files:

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  10. rsfoto

    rsfoto Member

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    Hi audioguru,

    Thanks a lot for the circuit correction. Will try it when I get the parts.

    Sorry for the black background. Will fix it next time I post a schematic of a circuit.

    Again Sorry for the black background. :sorry: :sorry: :sorry:
     
  11. rsfoto

    rsfoto Member

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    Hi All,

    Thanks a lot for all your help.

    Working on the LM3914 and having thought that I burned one I went into the catalog of the store where I buy and by pure coincidence I saw that there are LM3915 and LM3916.

    Looking at the datasheets of both I saw in the LM3915 the following schematic and I just do not know what the value of that capacitor is, connected between source voltage and pin 3, marked with " 2.2 ". From the + marking I see it is an Electrolyt one. Could it be 2.2μF ?

    Could somebody help me to fiddle out that value ? I tried different capacitors I have but non did work. I took all my capacitors marked with 22xxx.

    Thanks in advance :cool:

    LM3925_vibrationmeter.JPG
     
  12. rsfoto

    rsfoto Member

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    H
    Hi,

    Question is answered :)

    Taking reference to the prior posting I found a 2.2μF Electrolyt capacitor and tested now the circuit and it works nicely.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The datasheet says,
    "Long wires from VLED to LED anode common can cause oscillations. Depending on the severity of the problem 0.05 μF to 2.2 μF decoupling capacitors from LED anode common to pin 2 will damp the circuit. If LED anode line wiring is inaccessible, often similar decoupling from pin 1 to pin 2 will be sufficient."
    I think a 22uF capacitor might have too much inductance and less that 0.05 uF is not enough capacitance.

    The LM3914 is a voltmeter. The first LED lights with 10% of the full range input voltage. The second LED lights with 20%, the third LED lights with 30% etc.
    The LM3915 is a power meter. The first LED lights with -30dB of full range. The second LED lights with 3dB more (double the power of the first LED), the third LED lights with another +3dB (double the power again) etc.
    The LM3916 might be obsolete. Its LEDs lighted at various spacings.
     
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  14. rsfoto

    rsfoto Member

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    Thank you. I read the data sheet a few times but that must have escaped me ...

    :cool:
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    SIMILAR to this http://www.google.com/url?sa=t&rct=...dia/en/training-seminars/tutorials/MT-069.pdf application note. Figure #1. D1, D2 and RLimit. The figure is a special type of OP amp called a differential amplifier, but the protection method is the same.

    To find the most appropriate value, one could look at the datasheet.

    Some sort of protection method should be employed because the transducer is a voltage source. I've actually used values to 100 K, but that's too high for your circuit.

    I don't know if power supply reverse polarity protection is required either.
     
  16. rsfoto

    rsfoto Member

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    Hi,

    Here is the datasheet of the Minisense 100 which I am using and so far I have not seen anything about pruced current in mAmp ...

    http://www.meas-spec.com/downloads/MiniSense_100.pdf

    So my question is why do we worry about this variable if I understood you all correctly about inserting diodes from the positive input down to ground, or is it for protecting the LM358 against to high negative voltages entering the positive input.

    What would happen if I connect the negative pin of the MiniSense 100 to the Inverting input ?

    As you see I have no idea about how an Op Amp works, as what I have beein doing is just a mere copy & paste circuits thinking that they work as I need :oops:

    Thanks in advance
     
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    This is not likely to do any damage to the OP amp so protection is probably unnecessary.

    As I said earlier, the damage can result (with certain OP amps) if the voltage either exceeds the supply voltage by a small amount and the current is not limited in some way.
    When an OP amp is UNPOWERED the SUPPLY is 0 V. So, the sensor could have been generating a voltage at sufficient current to damage the OP amp when it is turned off. That appears not to be the case.
     
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  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The old original LM358 datasheet from National Semiconductor says the maximum allowed input was -0.3V and 50mA.
    Since Texas Instruments recently bought National then all Texas Instruments datasheets for National devices have been updated. The latest datasheet of the LM358 from Texas instruments says the maximum input is 0.3V and 1mA and the output phase is undefined. They recommend an input as low as 0V, not a negative input.

    Since they say to limit the input to 0.3V then I think a Schottky diode is needed in this application since we do not know how much current the piezo can supply.
     
  19. rsfoto

    rsfoto Member

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    Hi audioguru,

    It will be used for a second piezo sensor, The whole enchilada will have a horizontal an a vertical sensor in order to measure vertical and horizontal vibrations, The final stage will ahve a total of 3 sensors similar to an accelerometer eg. I will measure X, Z and Y axis.

    Somewhere I said that I have also purchased an accelerometer with which I will start my test for the same purpose but have not yeet analized how to set the X, Z and Y outputs of the accelerometer to 0 (zero). Here a spec http://www.robodacta.mx/index.php?dispatch=products.view&product_id=85

    As you see I still have a long way to go with my poor electronic knowledge ;)
     
  20. rsfoto

    rsfoto Member

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    Hi audioguru,

    I will contact MEAS and ask them about the current this MiniSensors do produce ... Hopefully we will get an answer ...

    BTW, the vibration measuring and optical showing using the LM3915 works great. Later this afternoon I will post the bread board circuit and measure the excitation of the sensor to get the LED 10 lit up. I used the exact same circuit I posted in message #50 and the LM3915 did not blow up :happy:
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I forgot that the datasheet for the LM3915 shows a vibration meter circuit and a piezo transducer feeding a 1M resistor that works fine with the lM3915. Also, the LM3915 does not become damaged by negative inputs like the LM358 opamp. Then is the opamp needed anymore?
     

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