i need help varicap(varactor)

Discussion in 'Test & Measurement Forum' started by michael1978, Feb 11, 2018.

  1. michael1978

    Thread Starter Member

    Jun 29, 2014
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    hello to everyboyd , can somebody help, how to connect varicap to make 500 of 1000 pf
    i connect from this example

    and i connect and i get .300 to .400µ
     
  2. MisterBill2

    Member

    Jan 23, 2018
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    Forward biasing the diode will get you greater capacitance but the operation will be different. My suggestion is don't meter across the varicap diode, it won't give the right answer. Put the diode in your tuned circuit and bbserve the change in resonant frequency. AND, check the diode maker's website for application information. and a test circuit. Usually the published test circuit will deliver the published specification performance.
     
  3. michael1978

    Thread Starter Member

    Jun 29, 2014
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    Do you mean that i put forward biased the diode, i will check one more time
    i want to measure the capacitance of varicap
    http://electronics-diy.com/varicaps.php
    i try also like this, but i dont see nothing change
     
    Last edited: Feb 12, 2018 at 3:31 AM
  4. MisterBill2

    Member

    Jan 23, 2018
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    When the diode is forward biased the control characteristics change a lot. But that was with a different model varicap diode. It may not work with the diode that you have.
    Also, it is quite possible that your diode is not capable of that much capacitance. So once again I am suggesting to use the test circuit suggested by the manufacturer. In addition, not all capacitance measuring meters are able to check varicap diodes. My suggestion of checking it using a tuned circuit still stands. Or are you intending to use the varicap in some other type of circuit?
     
  5. michael1978

    Thread Starter Member

    Jun 29, 2014
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    Hi m model is BB205 varicap, so i have just simple multimter, i look in internet how to test it , i can find, of the varicap wrok for FM or AM
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The BB205 looks like a UHF varicap to me, as the capacitance range is from about 2.2 to 12.5 pF.
    See the attached pdf for more info.

    Bertus
     
  7. ian field

    AAC Fanatic!

    Oct 27, 2012
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    I've seen an AM preselector project using a couple of parallel TO220 style Schottky rectifiers - they almost managed the whole MW band.
     
  8. MisterBill2

    Member

    Jan 23, 2018
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    Such a small change in capacitance may be below the resolution of the measuring device. That would make it hard to see.
     
  9. MisterBill2

    Member

    Jan 23, 2018
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    I have replaced the Varicap in a CB radio frequency synthesizer with a 100 volt power diode, but it was only a 500Ma rated diode. Many diodes will work, some much better tghan others, and some are much more temperature stable. Idon't think that the application for this one has ever been mentioned, and the application is oten important in selecting a component.
     
  10. michael1978

    Thread Starter Member

    Jun 29, 2014
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    Hello
    for example i have bb205G i forget i ask it welke model, how i can adjust the size of capacitans, can somebody tell me what is the formula, i see
    i connect back to back to 9 v and is see 10p and i change he go to 22p, i need to make 150pf
     
    Last edited: Feb 13, 2018 at 6:25 AM
  11. michael1978

    Thread Starter Member

    Jun 29, 2014
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    but i go to electronics shop i told me the are for tunning
     
  12. michael1978

    Thread Starter Member

    Jun 29, 2014
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    ooh man i dont know i ask it in electronics shop he told they are for tunnig
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In the attached PDF you will find information on the varicap and how to use them in tuned circuits.

    Bertus
     
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  14. MisterBill2

    Member

    Jan 23, 2018
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    Thank You, Bertus! I was going to look those formulas up and send them. Tuning is certainly theprimary application of varicap diodes, the only other place I have seen them used is in a balanced modulator used for generating sideband signals as part of a transmitter. Also, the only way that I have ever been able to check them is with a tuned circuit, noting the change in resonant frequency.
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you want to know more about mixer applications, have a look at the attached PDF's.

    Bertus
     
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  16. michael1978

    Thread Starter Member

    Jun 29, 2014
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    thank you very much....;-)
     
  17. ian field

    AAC Fanatic!

    Oct 27, 2012
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    The varicap effect is due to higher reverse voltage making the depletion layer wider and causing lower capacitance - most diode types do it.

    Varicap diodes have guaranteed parameters when used for that purpose - power rectifiers that might give you the capacitance you want, likely have only vague references on the data sheet for reverse bias capacitance, probably only 2 or 3 measurement points.
     
  18. Audioguru

    Expert

    Dec 20, 2007
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    Michael, why are you using a BB205 varactor diode made for tuning UHF frequencies that has a maximum capacitance of only a few pF but you need 150pF?
    I think many years ago there were high capacitance varactor diodes made for tuning an AM radio.
     
  19. MisterBill2

    Member

    Jan 23, 2018
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    Ian F is certainly correct about power diodes not being specified very well for varicap service. But the one that I tried has worked very well in a CB radio for over ten years. And since I had no specifications for the one that I replaced, even a good spec sheet would not have helped me much. So tying and seeing is a way to go if no adequate data is available. And in my case the power diodes were on hand and free.
     
  20. michael1978

    Thread Starter Member

    Jun 29, 2014
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    is for FM/TV tun ing i test the diode i read old book, abc of varactor,and i make a circuit like in book to measure capacitance, but hi change the value i dont understang nothing, some times is good tot 300 450pf, soms to 1000 to 1500p, i dont know maybe my multimter is not working
     
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