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Help needed desperately with PIC LCF meter

Discussion in 'General Electronics Chat' started by bigal_scorpio, May 2, 2008.

  1. lynx

    lynx Member

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    hi eric :)

    yes i did..


    mine can't remain stable.. :/

    at you previous post you say it could be the PCB? how can the PCB cause such a problem? is it stray capacitance?...
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The stray capacitance on the PCB should be fairly constant, how long are your test leads.??

    As I recall you are not in the UK.??
     
  3. lynx

    lynx Member

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    the test leads are about 10cm long...

    no i'm not in the UK, by the way i got a small oscilloscope...
    and shorting the leads and measuring between the minus of the battery and the
    pin 12 of the uC the frequency is around 20KHz and fluctuates a little bit up
    and a little bit down
     
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    With the test leads shorted together, the frequency on pin 12 of the PIC should be about 300KHz, if its only 20KHz check that the inductor wired to the PCB is really 10microHenry.!

    Also look at this image, , is that pin hole soldered in correctly.??
    What are those long black wires fastened to the end of the R5 resistor.?
    Its hard to see on your PCB images.

    EDIT:
    I cannot see the 'L1' 10uH inductor on the PCB, could you mark it for me.?
     

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    Last edited: Apr 27, 2011
  6. lynx

    lynx Member

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    it's hard to tell right now because of HMA around the inductor..but as far as
    i remember the value of the inductor is correct. turning the pot to the right the frequency
    can go up to 500KHz turning it to the left goes close to 19KHz


    yes it is fine, i don't know why it looks like it's not soldered...maybe it wasn't soldered
    when i took the photo.


    check the new photo...the inductor is near the R5.
     

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    Last edited: Apr 27, 2011
  7. lynx

    lynx Member

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    Hi...

    i was wondering... other than ericgbbs nobody else can contribute
    with some knowledge?

    after all this talk and and tests..i believe the problem with my LCF meter got norrowed down to
    4011 gate, something is wrong with the oscillator..but what is wrong? can that be saturation of the
    gate which makes the oscillator not that steady?!

    i also want to add that when i tried to measure the inductance of a transformer
    the results where steady, so i'm very confused with all this issue.

    i appreciate ericsgbbs try to help me...also anybody else willing to give a farther help! :)
     
    Last edited: Apr 28, 2011
  8. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    If the project works OK on the transformer are you 100% sure that the project really has a problem.

    Test a number of known value inductors and post what you measure.
     
  9. lynx

    lynx Member

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    the instrument is not calibrated very well, since the readings aren't steady
    i can't calibrate it until i fix it.

    please check this video i believe it is helpful enough so you can understand
    the issue much better.

    ImageShack® - Online Photo and Video Hosting


    P.S can you please explain to me generaly what can cause such stability issues with oscillators built around a gate?
     
    Last edited: Apr 29, 2011
  10. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Watched the vid.
    The frequency of the oscillator is increasing with time [ which I guess you realise] this will give an apparent decrease in the inductor value.

    Have you monitored accurately the supply voltage on pin #14 of the 4011, while you are taking a inductor measurement, lets know what you see.

    EDIT:
    What are you using to power the PCB. ???
     
    Last edited: Apr 30, 2011
  11. lynx

    lynx Member

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    hi :)

    i'm using a 9V battery, i'll let you know for the rest later.
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi lynx,
    I assume that you do have a 7805 Vreg between the project and the 9V battery.??, if you do have the 5Vreg, check the battery and the +5V..

    Depending upon the battery type.?? it may not be 9v.
     
  13. lynx

    lynx Member

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    everything is exactly as in the schematic...the battery is a normal 6LR61
    9V battery (a little bit empty but still holds)

    at the pin #14 of the 4011 gate the voltage is 5.04V
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi lynx,
    Ref your PM's
    I have relooked at the project pictures and only error I can see is the way the SET switch is connected.
    All it does is link two pins #1 and #3 on the PIC.
    Pin #1 is supposed to go to +5V via the SET switch and if you are only using the inductor section pin #3 is connected to +5V.

    Let me know what you find.
     

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  15. lynx

    lynx Member

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    eric you say i should cut the track and relocate the "S3" and let "S2c" nc?
     
    Last edited: May 2, 2011
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    If you are not using the CAP mode, wire PIC pins #1 and #2 like this image.
     

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  17. mvs sarma

    mvs sarma Well-Known Member

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    I fear, too long wires connecting to switch might cause problems of instability. Check if you can reduce the wiring to bare minimum.
    Many other designs, in fact, recommend on board switch with suitable pcb artwork , let it be rotary or sliding.
     
    Last edited: May 3, 2011
  18. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Sarma,
    lynx is making a 'cut down' version of the LCF meter, he only requires the Inductor test function.
    So he has dropped off the Select switch, when doing this he has made a mistake of connecting PIC pins #1 and #3 together.

    BTW: he did make the full version which did work.!

    Eric.
     
  19. mvs sarma

    mvs sarma Well-Known Member

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    Thanks Eric. For Inductance meter, perhaps one design by Phil Rice of Australia is found to be best
    it also works well for C . resolution appears Good. for the concerned issue i would join back shortly

    PS: the values of C5a& b and C6 a&b are expected to be close tolerance such as 1%. This is one major cause for instability, once other things are attended to. See the component list of the article concerned. We had used such accurate caps buying from Farnell.Of course our design (phil Rice )used 1000pF 1%.


    I doubt whether LYNX has them and used them.
     
    Last edited: May 3, 2011
  20. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Paul,
    Ref your PM, is this what you are asking for.??
     

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  21. Ohmnium

    Ohmnium New Member

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    Hi. I'm trying to build the EPE LCF Meter, but the pic that i'm using is the 628A. I had problems to download the hex file which ericgibbs proposed. I probed some hex files from different forums, but any of them worked. In the mode of capacitor, the LCD display "Capacitor time out". Someone built the circuit an it worked fine?? What hex file used?? Please, I need help :(
     

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