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Recently in another thread the topic of capacitor effective series resistance (ESR) was briefly discussed but it is best to start a new thread on this rather than to stray off topic there.
Recently in another thread the topic of capacitor effective series resistance (ESR) was briefly discussed but it is best to start a new thread on this rather than to stray off topic there.
I have read on other threads which focus on crystal radio technology where coil Q can be measured using air variable capacitors since they have “low” dissipation. With this in mind I measured the D factor of a 160 pF air variable I have in my parts bin using a Fluke 710-A Impedance Bridge which gave a factor of 0.07 which is very high. The bridge gives a typical and reasonable reading of 0.003 for dipped mica caps so I have no reason to suspect a faulty bridge.
A D of 0.07 yields an ESR of 44 ohms for 160 pF at 1.68 MHz which would be a “Q killer” in a resonant circuit. It could be that something is wrong with this cap. and a search for information about air variable cap. D was not useful. Before spending time rummaging through my junk and measuring a bunch of air variables I though I would ask here if anyone has either measured this or knows of typical values to expect? I would expect the D factor to be very low for these, certainly lower than what I measured.
Maybe my cap. needs to be run through the dishwasher.
Neil
Total posts : 45366Neil, I don’t have any data for you, but I do
have a question.Did you try the variable capacitor at different
settings when you ran the test? And, who
knows? Maybe something IS dirty or gunked
up in there, but I sure don’t know what it
would be.Hopefully, the more experienced guys will
come out of the woodwork.Best Wishes,
Bruce, MICRO1690/1700
Total posts : 45366Bruce,
No, I only tried the measurement at full capacitance. I suspect the connection to the rotor may be dirty and not making good contact with the wipers and bearing. I have others I could test but time is a bit short now and if this is “normal” I won’t bother.
Neil
Total posts : 45366Hi Neil,
I did a little web checking on air variables and I am surprised that the manufacturers don’t specify the Q or DF. Search “Q of air variable capacitors”. Mostly they just claim the Q is high. One guy has a site that describes his results from measurements. I believe a properly working, good quality air variable should have a Q of at least 1000 (DF=1/Q=0.001), and likely significantly higher. Heck, even a ceramic NP0/C0G capacitor has a DF=.001, so you would think any sort of air variable should be better.
I use a Johanson 8050 .6pf-6pf air variable piston trimmer (http://www.johansonmfg.com/pdf/Air-Capacitor.pdf) in the forthcoming AMT5000. Johanson specifies the Q to be >10,000 (DF<.0001). I think that's about as good as it gets. BTW, not that it has anything to do with air variables, one of the best application guide and technical analysis I have seen is found on pages 4-9 of this document from Kemet: http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfilesname/F3101.pdf/$file/F3101.pdf
Total posts : 45366Thanks Phil.
I checked three more air vars. from my stock ranging up to 400 pf and they all had a D very close to 7%. I also checked a glass piston (6 Pf) and it measured just below 7%. This isn’t making sense to me since I used the original 160 I mentioned to measure the Q of my loading coil using Tongue’s method and it was just over 150 with a calculated Rs = 15 ohms. This would not be possible if the cap ESR is 44 ohms.
I checked the bridge again using a CDE dipped mica (1000 pF) and the D was 0.3% and Y5P 3900 pF ceramics measure 0.6% which gives me some confidence that the bridge is OK. I will try a mica with a know series R added and see if it measures as expected.
I suspect there is something wrong with how I am measuring this but can’t figure what it is.
Neil
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