In Reply to: teflon caps posted by Raj on February 07, 2003 at 09:43:45:
I would not use a teflon as a bypass cap...I like Rel R/T which are polystyrene/tin for bypassing.....Don't use a large cap for bypassing op amps, just a stacked film to keep the leads short....Trim caps on op amps can be made using double sided teflon board cutting to size until the proper value is reached and hooking a lead to either side......
Where do you think would benefit from the use of teflon caps, especially at this size?
Teflons are wonderful for signal use only, in RIAA or to roll off the bass in main amps when using subwoofers or as input caps....They are normally too small for coupling and so much faster than anything else that they don't work well for bypassing other (ppn or styrene) DC blocking caps as you will always hear a discontinuity in the highs.....Interestingly enough teflons also show discontinuity when bypassing DC supplies so stick with styrene/tin or ppn/tin....The styrene/tin works well as a bypass for the Muse electrolytics BTW in high current supplies...
Hi Bob. You say double sided. This indicates to me that this is a pc style board with teflon coating on both sides? What about solid sheet teflon? Thanks for the idea which should greatly reduce the cost.
Actually, the board is teflon with copper on both sides. The area of the capacitor plates (copper) and thickness of the dielectric (teflon) determine the capacitance. You would solder leads to each side of the copper faces.
C = eA/d
where C is the capacitance, e the permittivity of the dielectric (teflon) between the plates (copper layers), A the area (of the copper) and d the separation between plates (i.e. thickness of teflon).
Could you do the same thing using silver rod and teflon sheeting? The rod, say 16 guage, would be inserted into the body of the teflon and a second would be placed at a specific distance. The rod could be fastened with conductive epoxy (?)
Mike, double sided teflon board material is great for making up trim caps 5-20 picofards or so is all.....Demian Martin used this twenty years ago when he was at Spectral and tough to find really low picofarad values these days in anything decent (sytrene or ppn) other than silver mica so this will be a nice alternative to low stocks of styrene and ppn caps....Just wait 'til you price double sided teflon board material as it ain't cheap nor is sheet teflon....Always ask for "drops", a portion that would normally be thrown away if it was fiberglass...
Bob, where would one look for teflon pcb? I'm in need of a pair of 15pF caps and have looked everywhere for them, can't find anything except for the silver mica, as you mentioned. How big would a 15pF teflon board cap be approximately?
You can also build "gimmick" caps from teflon CAT5 wire for small pf value caps. Take two teflon wires, say 4" long, and twist them together tightly - do not connect the far ends, keep them insulated from eath other of course. The two leads are your caps leads. I usually make a few and connect them in parallel. I fold them up then wrap them with white teflon plumbers tape. You need a meter to measure them to get the correct value. These work great in feedback circuits for me.
for the above calculation, e can likely be taken as 2.5 times Eplsilon-zero the permittivity of vacuum or k( dielectric constant) times E0.
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