EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: antenna on October 16, 2022, 07:34:54 pm
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I am just starting to get into RF oscillators and wanted to buy some trimmer caps. The only ones that appear to be priced for experimentation are the PP film types like this one
https://www.mouser.com/datasheet/2/427/bfc2808_5mm-1762950.pdf (https://www.mouser.com/datasheet/2/427/bfc2808_5mm-1762950.pdf)
but the datasheet says the neitherthe electrical or physical characteristics are guaranteed after just 10 rotations. Is the rotational life of these really that bad or can they typically go many more turns and be ok?
Also, If anyone knows a good source of cheaper trimmers that work ok at HF/VHF, I would appreciate recommendations. I would like to have on hand a dozen or so each of three different ranges. Maybe 1-10pF, 4-20pF and 40-120pF or something close.
Thanks!
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Look for a local version of something like this (the style of tuning cap used in crystal sets and basic RF discovery kits)
https://www.jaycar.com.au/60-160pf-miniature-tuning-capacitor/p/RV5728?pos=1&queryId=e8c6afe515512269e70f33f7b5b6684b (https://www.jaycar.com.au/60-160pf-miniature-tuning-capacitor/p/RV5728?pos=1&queryId=e8c6afe515512269e70f33f7b5b6684b)
DS says 10,000 rotations
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Capacitors have an adjustment range of less than 360° whereas inductors have cores which can be rotated over several turns, making them easier to adjust precisely.
To add, in relation to your question, the range would mean a certain movement would have more effect on a capacitor than an inductor. The sliding plates of capacitors would I guess be more susceptible to movement but how much would depend on the construction.
Once they're adjusted, you can put a blob of cement on to annoy the person who has to repair it. router login (http://"https://routerlogin.ltd/")
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Piston trimmer capacitors, with a lead screw, are easier to adjust than 180o rotation units.
They are somewhat expensive, however.
They also can have extremely low loss and parasitics.
New prices can outrageous, but there are some reasonable surplus sources, e.g. https://www.surplussales.com/Variables/PistonTrimmers/index.html (https://www.surplussales.com/Variables/PistonTrimmers/index.html)
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Tektronix scopes and plug-ins used the 180 deg trim caps, single disc céramique are quite stable but the thin metalized ceramic rotors easily break after decades.
The modern multi layers plastic dielectric are often junk quality from China, thus the low number of rotation
99% of trim caps are set once and forget.
Finally the normal variable capacitors used in RX, TX and old radio use last forever
Jon
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If you dig around ebay and can live with the size, surplus air variable capacitors in trimmer sizes (a couple dozen pf, give or take) can be had at reasonable rates.