jgk@cbnewsc.att.com (joseph.g.klinger) (07/24/90)
I've been looking for JFET current limiter diodes and have come to two conclusions: 1) they're too expensive, $5 ea for a 1N5314, down only to $0.85 in thousands for a J557 (not cheap for a quantity price). 2) they're hard to find, not much bigger than 10 mA, and not common enough to ever get cheaper or more available. Are there cheaper devices? Part numbers? I could deal with maybe $0.25 in quantity; after all, it's only a diode :^) How can I build the equivilent to a 10 mA AC (I want to pass both halves of the AC waveform) current regulator, using cheap JFETs, resistors, and perhaps zeners? Thanks, -- Joe Klinger att!iexist!jgk Disclaimer - AT&T may not aggree with the opinions expressed in my comments.
whit@milton.u.washington.edu (John Whitmore) (07/31/90)
In article <1990Jul23.222401.26115@cbnewsc.att.com> jgk@cbnewsc.att.com (joseph.g.klinger) writes: >I've been looking for JFET current limiter diodes > 1) they're too expensive, $5 ea for a 1N5314, down only to $0.85 > in thousands for a J557 (not cheap for a quantity price). > 2) they're hard to find, not much bigger than 10 mA, > and not common > >How can I build the equivilent to a 10 mA AC (I want to pass both halves >of the AC waveform) current regulator, using cheap JFETs, resistors, >and perhaps zeners? > Any gate-shorted-to-source junction FET acts as a current regulator; the parts you've quoted are good for a very wide range of bias voltages (2 V to 50 V; some go to 300V), but do you need that? They come pre-calibrated, or at least pre-sorted (the assembly line doesn't permit high accuracy here); do you need that? If you can stand worse output impedances, consider using an RF JFET (like J304) and sorting them yourself. Other possible solutions: PTC thermistors (they heat up enough at 10 mA to raise their resistance...and choke the current off), or ballast tubes (these are incandescent bulbs, making use of the positive temperature coefficient of a heated wire), or even a current-limited op amp (use it to buffer your signal). What frequency range are you interested in? What impedance (or equivalently, voltage range)? How accurately need this limit be maintained? Can you tolerate distortion (a volt or so of distortion is to be expected from a diode bridge/FET combination)? John Whitmore