Home made 5 pf trimmer capacitor

Discussion in 'The Projects Forum' started by joemayer, May 26, 2013.

  1. joemayer

    Thread Starter New Member

    May 26, 2013
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    I am in need of a 5 pf trimmer cap and was thinking whether it would be possible to make one from small washers and mica?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Make a gimmick capacitor by twisting two pieces of ordinary hookup wire.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Typically you get 2-3pF/inch of twisted wire that way.
     
    absf and #12 like this.
  4. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Length matters
     
  5. ErnieM

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    Apr 24, 2011
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    As does using insulated wire. ;)
     
  6. Shagas

    Active Member

    May 13, 2013
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    won't that make a mini inductor?
     
  7. LDC3

    Active Member

    Apr 27, 2013
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    No, because the wires are not connected together. There is a very large resistance between the wires, just like a capacitor.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    No. You use two separate pieces of insulated wire twisted together. No connection between them. It's a simple two-plate capacitor with the wires being the plates.
     
  9. joemayer

    Thread Starter New Member

    May 26, 2013
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    Thank for responding guys. I need the 5 pf trimmer to offset the vfo of my QRP cw transceiver about 1 kHz below the transmit frequency on receive, I wonder if the twisted hookup wire will do the job?
     
    Last edited: May 27, 2013
  10. crutschow

    Expert

    Mar 14, 2008
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    We've given you the info. I suggest you give it a try and let us know if it works. You shorten it and change the twist to tweak the value. Start with a longer wire then you think you need and keep snipping off a short piece of the ends until you get the right value.
     
  11. atferrari

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    Few weeks ago I did that with a pair of tightly twisted wires.

    If I recall right, 8 cm measured 8pF. After pruning it little by little I found the relationship to stay quite consistent down to the last 2 cm.

    I wonder how stable the actual value could be. Besides temperature, what else could ruin the value?
     
  12. LDC3

    Active Member

    Apr 27, 2013
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    Degradation of the insulation; although that may take several years.
     
  13. crutschow

    Expert

    Mar 14, 2008
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    If you use wire with teflon insulation, the value will likely be stable for the life of the equipment.
     
  14. edwardholmes91

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    Feb 25, 2013
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    I personally think it would be easier to buy one
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Unfortunately, the OP doesn't know exactly what value he needs but is likely somewhere near 5pF and the wants to adjust/ tune his circuit. The lowest value is 3.5pF and, after soldering, may be outside the range the OP needs. The twisted pair method can be used for very fine tuning. Wires with varying thickness insulation can be used for varying degrees of control.
     
  16. edwardholmes91

    Member

    Feb 25, 2013
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    Fair do's, the link I posted though had ones down to 1.5pF ;)
     
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