Hi Peter,
I didn't see the original question for some reason.
I wouldn't use PINs for a high-powered bandswitch on the hi-Z end of the
plate tank, and I doubt that many have done so successfully; although of
course in solid state transmitters it's quite common.
The 1N400x used as an RF switch similar to PIN probably should fail, since
it's still a rectifier, albeit one with a long trr. Recovery losses can
cause the junction to rupture without necessarily generating any noticeable
heating; so can avalanche failure which ultimately renders the silicon
intrinsic. The dopants can literally just boil away and then you have a
resistor instead of a semiconductor!
As you probably know, the PIN was "invented" by accident...but then, so was
the LED and many other devices. I worked for Unitrode back in the
1980's...acquired by Microsemi Corporation in about 1991 or 92, and MSC has
sustained the PIN operations. When I left MSC in 1993, luckily I retained
my PIN sample kit!
73 de Steve WB2WIK/6
"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill
> -----Original Message-----
> From: peter.chadwick@Zarlink.Com [SMTP:peter.chadwick@Zarlink.Com]
> Sent: Thursday, June 19, 2003 12:39 AM
> To: Steve Katz
> Cc: amps@contesting.com; amps-bounces@contesting.com; on4kj
> Subject: RE: [Amps] electronic switching
>
>
> The original question was about replacing ceramic switches - which in our
> context are usually used in the tank circuit. PINs used at 50 ohms are a
> totally different kettle of fish, and of course can handle very high
> powers. With a 3kV HV supply and 1500 watts out, there's around 5kV peak
> to
> peak to handle, and between 10 and 15 amps in the tank circuit - which is
> why Rich is always advocating some cooling air around the tank coil. I was
> reading in an old IEE HF Conference report of the '60's about about the
> problems with variable inductors in a 80kW tx - they used 1/2 inch copper
> tube and forced air down it for cooling, while the contact was made up of
> 80% silver and 20% carbon, which is apparently self lubricating. The
> circulating current there was said to be 80amps.
>
> PINs do have a long recovery time, and use this to be able to handle a
> much
> higher RF current than the DC through them. The reason you need a long
> trr
> is so that when the RF current reverses through the diode (on the other
> half cycle) the carriers can't be swept out of the junction, so the RF
> flow
> is not interrupted. There's various equations relating maximum RF current
> to bias current and trr and frequency. There's some excellent material on
> PINs and their applications available from MACOM, much of which originated
> when the major supplier was Unitrode. There's also a fair bit on the IMD
> performance of PINs when used in attenuators - a lot of that work was done
> for the cable TV industry. Because the way the PIN works is to have a long
> recovery time, I've found that some brands of 1N4007 do well as switching
> diodes (Motorola are good). The trr is about 8 microseconds. However, at
> the 100 watt level, they tend to fail after a few months - strange, as
> they
> don't get hot.
>
> There is one way of getting around the ceramic switch problem, and that is
> to make your own switch, using fibreglass. Mind, it's not really a
> project
> for kitchen table construction - I think you ideally need some machinery -
> at least a vertical drill ( drill press in American) and doubtless a lathe
> and mill would make things easier, depending on whether or not you were
> stealing the indexing mechanism from somewhere else. Desirable would be
> 3/16 or 1/4 inch fibreglass, but you can strip PC board material and glue
> pieces together. The copper on PC board material isn't thick enough to be
> of any use as a conductor at the sort of powers I presume we are talking
> about - at 160m, skin depth is about 2 thousands of an inch, and you need
> a
> conductor at least 5 skin depths thick if skin effects are to be
> neglected.
>
> You would of course need to be very generous on clearances, since if you
> had an arc for any reason, the fibreglass board will burn, producing vile
> smelling fumes and black smoke........been there, done that in my remote
> antenna tuner.
>
> Fair Radio sales <http://www.fairradio.com/tapswi.htm>still have the
> ceramic switches from the BC375 TU units at a low price - $3.95 each, 10
> for $32 - they are very chunky and good, and can be ganged with a bit of
> work, With a bit more work, you can motor drive them, although direct
> drive
> from a stepper motor would need a big motor unless you removed the
> indexing
> mechanism.
>
> 73
>
> Peter G3RZP
>
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