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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Few days ago I finally got my R/C meter; I measured capacitances of few TC toploads and concluded that calculations of capacitance are totally wrong. Small ping-pong ball sized sphere had to be far under 1 and has 2pF, when I put it as away as possible from grounded objects. One bigger sphere has 4pF instead of 1, and my DRSSTC toroid that supposed to have 10pF and be in exact 1/4 wavelength frequency had 44pF!
I could never get capacitance to be even close to 10pF as calculated, and as much as I tried to get toroid away from ground I never reached under 40pF.
There is something obivously very wrong in calculations and CAD programs, my meter measures small caps very precisely and it obivously isnt lying. What is happening there?
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
How are you measuring capacitance? You need to calculate capacitance by measuring the resonant frequency of a secondary and vary the topload. I've done this experiment already with a number of toroids for my coils and the capacitances come out PERFECTLY to the equations.
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
I always use my R/C meter. What I do is suspend the toroid on a piece of PVC and away from everything. The measurements are strikingly close to the formula. Field effect has a HUMONGOUS impact on measurement, so having it suspended is imperative. Even moving around a foot or so away changes the value. Also, you must ground to RF ground the other test lead or measurement will be way off.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I aclculate capacitance using this formulae, obivously used in most web calculators, wintesla and similar programs as result is always around 10pF for my 220 * 80 mm toroid.
I measured it with RC meter, 1 meter above the ground and min 1 meter away from all objects. Meter was grounded to rf ground and despite all efforts capacitance is always above 40pF, seems funny for that small toroid but it is. RC meter measures all capacitances I tried correctly, and very precisely in pF range (2-3pf error at most, and it can be calibrated) so 30pF difference between measurement and calculations is odd.
It would be nice if I had oscilloscope (hope to get one soon) and could ring the secondary to finally see what is going on.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
When I used my homemade LC meter on TC topload I also got very high readings. I think the formulas are correct and our meters are not though, because the resonant frequency I get from the formulas is acutally what I see on my scope.
I have no idea if any frequency- or voltage-dependent effects come into paly here, but clearly there is a lot that can go wrong when measuring L or C in circumstances different from the actual application. I have seen a table somewhere comparing the readings of ten or so expensive lab LC meters on different calibrated caps and inductors, and they also differend in their readings by as much as 100%.
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Topload capacitance
Hmm... I don't know then I guess.
