ems@Apple.COM (Mike Smith) (11/23/89)
Wasn't the detector in old crystal radios made of 'galena'? Someone posted that the crystal was a germanium lump, but I seem to remember 'galena' as the stuff... If so, it raises an interesting point. Galena is PbS (Lead Sulfide). Taking a look at the periodic chart it looks like Pb and S are in about the right places to make a semiconductor (like Indium Phospide, Gallium arsenide, etc.) Is PbS a semiconductor (if properly doped)? If so, what are it's properties? I seem to remember some 'old timers' saying that a good crystal was far better than those new fangled diode things... -- E. Michael Smith ems@apple.COM 'If you can dream it, you can do it' Walt Disney This is the obligatory disclaimer of everything. (Including but not limited to: typos, spelling, diction, logic, and nuclear war)
mmm@cup.portal.com (Mark Robert Thorson) (11/24/89)
ems@Apple.COM (Mike Smith) says: > Is PbS a semiconductor (if properly doped)? If so, what are it's > properties? I seem to remember some 'old timers' saying that a > good crystal was far better than those new fangled diode things... Yeah, I bet the diode drop might be small er than for a silicon diode. Anybody know how much it is?
ISW@cup.portal.com (Isaac S Wingfield) (11/24/89)
Mike Smith writes: >wasn't the detector in old crystal radios made of 'galena'? >Someone posted that the crystal was a germanium lump, but I >seem to remember 'galena' as the stuff... If so, it >raises an interesting point. >Galena is PbS (Lead Sulfide). Taking a look at the periodic >chart it looks like Pb and S are in about the right places to >make a semiconductor (like Indium Phospide, Gallium arsenide, etc.) >Is PbS a semiconductor (if properly doped)? If so, what are it's >properties? I seem to remember some 'old timers' saying that a >good crystal was far better than those new fangled diode things... Nope, galena is right. Typically, a cubic crystal about 1/4" on the side was cast into a base of some low melting point alloy (anybody know what?). The "cat's whisker" (moveable lever with a spring wire on the end) hung over the galena; you moved it around 'till you found a "hot spot", a place where the junction rectified. It was (is) better than "new fangled diode things" because the forward conduction drop is much less (FCD is how much signal it takes to "break through" the junction); what you get to listen to is anything over this voltage. Silicon diodes are about 0.6 V, germanium are about 0.3 V, and as I remember it, galena is about 0.05 V; this means that you can listen to weaker stations with galena. BTW, if you are considering building a crystal radio, be aware that you *really* have to look for germanium. I know for a fact that many 1N34's (classic germanium diode) are now made from silicon; last time I required germanium was about 15 years ago, and out of all the 1N34 manufacturers, only ITT's were Ge. Nowadays, I dunno. There's a thing called a "backwards diode" that I always thought would be good for a crystal radio. It's a silicon diode that's doped so heavily that it leaks like a sieve in the reverse direction; it can't block more than about 25 - 50 mV. But oddly, the forward conduction drop is still 0.6 V! So, you just turn it around (backwards diode, remember?) and Hey, Presto! You have a diode with 25 mV FCD, and 0.6 V reverse blocking - plenty good enough for a crystal set. It used to be said that the oscillator and converter noise in a typical superhet AM receiver was responsible for considerable signal degradation, and a good TRF rig (just a crystal set with an RF amp) sounded much better. It would be fun to build a "modrun" receiver using filter synthesis techniques for optimum passband shape in the tuned circuits, and a low loss detector. Isaac isw@cup.portal.com
henry@utzoo.uucp (Henry Spencer) (11/25/89)
In article <24441@cup.portal.com> ISW@cup.portal.com (Isaac S Wingfield) writes: >...you *really* have to look for germanium. I know for a fact that many >1N34's (classic germanium diode) are now made from silicon; last time >I required germanium was about 15 years ago, and out of all the 1N34 >manufacturers, only ITT's were Ge. Nowadays, I dunno. Um, possibly this is a reflection of my ignorance, but I always figured that if it walks like a duck and quacks like a duck, you might as well treat it as a duck -- isn't there an agreed-on standard for the forward drop of a 1N34? (I don't have a spec sheet handy.) If so, do I really care whether it's germanium, silicon, or praseodymium in there? -- That's not a joke, that's | Henry Spencer at U of Toronto Zoology NASA. -Nick Szabo | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
ISW@cup.portal.com (Isaac S Wingfield) (11/26/89)
Henry Spencer writes: >Um, possibly this is a reflection of my ignorance, but I always figured >that if it walks like a duck and quacks like a duck, you might as well >treat it as a duck -- isn't there an agreed-on standard for the forward >drop of a 1N34? (I don't have a spec sheet handy.) If so, do I really >care whether it's germanium, silicon, or praseodymium in there? Good question: I suspect the real "don't really care" for almost all applications is the FCD, but many people want to replace a 1N34 with another "1N34", so that's what manufacturers print on the outside of it. I was trying to interface a Honeywell minicomputer to standard TTL; the Honeybucket (for obscure reasons) used standard DTL running on 6(!) volts, and I really wanted that 0.3V FCD. TI listed a 1N34, so I ordered up a bunch, but they all had 0.6V FCD. (This was for a military application, so everything had to be "worst case" designed.) I brought things to my boss's attention, and he observed that TI hadn't processed any germanium for years! After much looking around, and some *very careful* data sheet reading, we concluded that germanium diode specs were all kind of loose, and it was possible to duplicate (or even better) all specs *except* FCD using silicon, and that's what nearly everybody was doing! The ITT devices came in at 0.3V, and everything was fine. With respect to the second part of your question, "do you really care what's in there": it has been my impression that the FCD is pretty much defined by the material used in the junction (discounting peculiar dopings, or metal-semiconductor junctions, for example). So, if it's a certain FCD you need, then you most certainly do care. Isaac isw@cup.portal.com
vaso@mips.COM (Vaso Bovan) (11/26/89)
In article <1989Nov25.080724.19517@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >In article <24441@cup.portal.com> ISW@cup.portal.com (Isaac S Wingfield) writes: >>...you *really* have to look for germanium. I know for a fact that many >>1N34's (classic germanium diode) are now made from silicon; last time >>I required germanium was about 15 years ago, and out of all the 1N34 >>manufacturers, only ITT's were Ge. Nowadays, I dunno. > What is the nature of your evidence that many 1N34s are now made of silicon ? >Um, possibly this is a reflection of my ignorance, but I always figured >that if it walks like a duck and quacks like a duck, you might as well >treat it as a duck -- isn't there an agreed-on standard for the forward >drop of a 1N34? (I don't have a spec sheet handy.) If so, do I really >care whether it's germanium, silicon, or praseodymium in there? >-- 1N34 is a JEDEC-registered device. JEDEC would be interested to know if a manufacturer is substituting silicon for germanium.
eachus@aries.mitre.org (Robert I. Eachus) (12/02/89)
Once upon a time there were galena crystals. However the first
electronic virus struck and changed the crystalline structure of
galena crystals. The American manufacturer who was strck by this
plague sent samples to a German manufacturer who was still able to
produce good crystals, and his plant became infected. Soon no one was
able to make "working" galena crystals, and radios had to switch to
selenium or germanium.
With modern materials science techniques, it should be possible
to make galena crystals in an "uninfected" environment, and then
encapsulate them. Anyone want to try?
--
Robert I. Eachus
with STANDARD_DISCLAIMER;
use STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...