QRZ! Ham Radio 3/QRZ Ham Radio Callsign Database - Volume 3.iso/digests/homebrew/940050.txt

Internet Message Format | 1994-06-04 | 16KB

Date: Sat,  5 Mar 94 04:30:49 PST
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Subject: Ham-Homebrew Digest V94 #50
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Ham-Homebrew Digest         Sat,  5 Mar 94       Volume 94 : Issue   50

Today's Topics:
  Converting cheap-o walkie-talkies (stabilizing crystals?) (2 msgs)
                         GPS Receiver Boards
             Looking for a keyboard for a HaL Telereader
                      Paralleling Power Diodes ?
         Series Diodes (was Re: Paralleling Power Di (2 msgs)
     Series Diodes (was Re: Paralleling Power Diodes ?) (2 msgs)
                  Source for "heat Diodes/Semi's" ?

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We trust that readers are intelligent enough to realize that all text
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policies or positions of any party.  Your mileage may vary.  So there.
----------------------------------------------------------------------

Date: Thu, 3 Mar 94 15:29:02 GMT
From: mnemosyne.cs.du.edu!nyx10![e-mail]
Subject: Converting cheap-o walkie-talkies (stabilizing crystals?)
To: [e-mail]

I've been thinking about how most walkie-talkies work at 49.9xxMHz and it
occured to me that 6m is 50.1 and higher...  So - I grabbed a cheap-o
walkie-talkie (an AM and an FM) from RS, ripped it open and started to see
what I could do to it.  I found that I could tune to a 6 MHz range using
the little tuning coil inside and my freq. counter.  However, I couldn't
get a stable signal once I deviated very much from the frequency it was
designed for.  Is this due to the crystal?  If so, can I just replace the
crystal and maybe some other misc. parts?  I'd love to be able to take a $10
walkie-talkie and convert it to work in the amateur band.  Any ideas?

------------------------------

Date: Fri, 4 Mar 1994 03:17:12 GMT
From: ihnp4.ucsd.edu!agate!library.ucla.edu!csulb.edu!csus.edu!netcom.com![e-mail]
Subject: Converting cheap-o walkie-talkies (stabilizing crystals?)
To: [e-mail]

In article <[e-mail]> [e-mail] (Jonathan  Sherman) writes:
>I've been thinking about how most walkie-talkies work at 49.9xxMHz and it
>occured to me that 6m is 50.1 and higher...  So - I grabbed a cheap-o
>walkie-talkie (an AM and an FM) from RS, ripped it open and started to see
>what I could do to it.  I found that I could tune to a 6 MHz range using
>the little tuning coil inside and my freq. counter.  However, I couldn't
>get a stable signal once I deviated very much from the frequency it was
>designed for.  Is this due to the crystal?  If so, can I just replace the
>crystal and maybe some other misc. parts?  I'd love to be able to take a $10
>walkie-talkie and convert it to work in the amateur band.  Any ideas?

I've done this some time ago, yes you need to change the crystal.  I got
some crystals at hamfests and such for cheap.  If you wanted a specific
frequency, you may have to order it.  May be too expensive, though.

------------------------------

Date: Fri, 4 Mar 1994 16:31:44 GMT
From: news.acns.nwu.edu!math.ohio-state.edu!howland.reston.ans.net!noc.near.net!ns.draper.com!news.draper.com!jwy1294a.draper.com![e-mail]
Subject: GPS Receiver Boards
To: [e-mail]

I've followed various GPS threads as they've come and gone.  In order to pose
volume/price arguments to folks in the business, what would be the size of the
amateur market for GPS engines?

Presume an engine consists of the entire RX, minus antenna, including a data
port (probably NMEA 0183), on a PC board.  Examples are currently manufacured
by Rockwell, Magellan, Canadian Marconi, Plessey, and Trimble, among others.
Marketed in the $500 class for single-unit quantities.

Skip, K1NKR

------------------------------

Date: 28 Feb 94 14:05:19 GMT
From: nprdc!ihnp4.ucsd.edu!swrinde!gatech!news-feed-1.peachnet.edu!umn.edu!mr.net![e-mail]
Subject: Looking for a keyboard for a HaL Telereader
To: [e-mail]

My dad has had a HaL Telereader for years (decodes CW, RTTY, etc). The
keyboard encoder has partially failed (one row of keys doesn't work).

We traced the problem to the encoder and called HaL. They don't make it
any more. I called the manufacturer of the chip (used to be General
Instruments, now is MicroChip), any they said they don't make it anymore.

We did discover that a "standard parallel keyboard" will work. Looking
at the schematic, it looks like uses 8 bits of data, a strobe, and
an ACK line.

I could make a serial -> parallel converter and use an IBM style keyboard,
but I would rather pick up another Telereader or parallel keyboard.

Hints?

Paul Anderson - N0RIK
Stillwater, MN
[e-mail]

ps. Thanks to all who responded to my request for vendors of ferrite
rod. I will post a summary of my experiences after I have had a chance
get some product from some of the sources.

------------------------------

Date: Sat, 5 Mar 94 04:20:09 GMT
From: btree![e-mail]
Subject: Paralleling Power Diodes ?
To: [e-mail]

In article <[e-mail]>, Alan Bloom <[e-mail]> wrote:
>Alan Anderson ([e-mail]) wrote:
>: >
>: >I wouldn't count on 3A with 3 diodes using that technique,
>: >but you should get something better than 1A.
>: >
>: >AL N1AL
>: >
>
>:  What would you recommend using for 3A at 4KV, I have seen some
>: 2Amp 10000 volt "double diode" bridges in Nebraska surplus. These are
>: setup for heat sink mounting. 2 in parallel ?
>
>I would just use a series string of lower-voltage 3A diodes.
>
>I hate to bring up the next point because, since it violates conventional
>wisdom, it is bound to draw some flames.  But what the heck...
>
>     You don't need to put resistors in parallel when you string
>     diodes in series.
>
>There, I've said it.
>
>Think about it.  When you over-voltage a diode, it acts like a high-voltage
>Zener and clamps at its breakdown voltage.  So long as the current is
>limited, no permanent damage occurs.  Since all the diodes in the string
>are in series, the current through each is the same.  The current will be
>limited to the leakage current of the lowest-leakage diode in the string,
>typically a few microamps.  Unless you exceed the SUM of the breakdown
>voltages of all the diodes in the string, no catastrophic breakdown
>will occur.
>
>AL N1AL

Commercially available high voltage rectifier modules consist
of a lot of conventionla silicon rectifer dice stacked in
cascade _without_ any equalizing components in parallel with
the individual rectifiers.  Manufacturers can do this because the
dice all come from the same wafer and are therefore very well
matched, both in terms of capacitance and in terms of leakage.

There is a real problem in series stacks of _unmatched_ rectifiers.
When a recifier is avalanched a lot of charge carriers are created
and injected into the junction.  It takes time for these carriers
to be swept out by the field.  This can cause significant amounts
of current to flow in the rectifer at the wrong time, and that
leads to unexpected heating of the device.  Whether this is a problem
depends on the luck of the draw for most of us.

Bob Hale     [e-mail]

------------------------------

Date: 3 Mar 1994 13:01:51 GMT
From: ihnp4.ucsd.edu!agate!howland.reston.ans.net!EU.net!sunic!sics.se!eua.ericsson.se!euas12d01![e-mail]
Subject: Series Diodes (was Re: Paralleling Power Di
To: [e-mail]

For a string of diodes in series without a parallel resistor string,
the reverse voltage across individual diodes will depend on the leakage
current of each of the diodes as they approach breakdown.

This is not particularly nice, but I guess the current in diodes which
have broken down is limited to the leakage current of diodes which
have not broken down, so the diodes should survive.

However, the parallel string of capacitors, (which should be present
for transient protection), may not be so happy about the uneven voltage
distribution unless each capacitor has a voltage rating equal to the
peak voltage for the whole string.

73, Stuart

------------------------------

Date: Fri, 4 Mar 1994 03:24:04 GMT
From: ihnp4.ucsd.edu!sdd.hp.com!col.hp.com!srgenprp![e-mail]
Subject: Series Diodes (was Re: Paralleling Power Di
To: [e-mail]

Stuart Hale ([e-mail]) wrote:
: For a string of diodes in series without a parallel resistor string,
: the reverse voltage across individual diodes will depend on the leakage
: current of each of the diodes as they approach breakdown.

: This is not particularly nice, but I guess the current in diodes which
: have broken down is limited to the leakage current of diodes which
: have not broken down, so the diodes should survive.

Right.

: However, the parallel string of capacitors, (which should be present
: for transient protection), may not be so happy about the uneven voltage
: distribution unless each capacitor has a voltage rating equal to the
: peak voltage for the whole string.

Funny you should mention that.  I don't think the capacitors are needed
either.

I can think of two reasons for the capacitors.  One is as you said:
for transient protection.  However this could be done with a single
capacitor (or two for center-tapped secondaries) across the secondary.
For that matter, seems to me that a transient suppressor on the
primary should take care of the matter.

The only other reason I have heard for putting cap's across the diodes
is in case different diodes in the string turn off at different times
due to different carrier lifetimes.  But if you use all diodes from
the same manufacturer's batch they will be very well matched.

But even if they aren't, remember that the voltage across the diode
string is close to zero at the moment of turnoff.  For a normal
capacitor-input power supply, the turnoff point is near the top of the
60-Hz sinewave where the voltage is changing very slowly.  It will
take a good portion of the 1/60 second cycle time (several milliseconds)
before the diodes are reversed-biased significantly.

But let's take a worst-case condition:  A diode rectifier feeding a
resistive load or a choke-input filter.  With a 60-Hz line, the reverse
voltage across a string of, say, 5 diodes won't reach 1/5 of peak for
1 / (5 * 60 * 2 * PI) sec = .53 milliseconds after turn-off.  So the
slowest diode would have to turn off >= .53 ms after the next-slowest
diode to even potentially cause a problem.  That would take some awfully
slow (and mismatched) diodes!

AL N1AL

------------------------------

Date: Thu, 3 Mar 1994 16:56:29 GMT
From: ihnp4.ucsd.edu!pacbell.com!sgiblab!sdd.hp.com!hpscit.sc.hp.com!cupnews0.cup.hp.com!news1.boi.hp.com!hp-pcd!hpcvsnz![e-mail]
Subject: Series Diodes (was Re: Paralleling Power Diodes ?)
To: [e-mail]

Alan Bloom ([e-mail]) wrote:

: Yes, I have seen degradation of a transistor's beta when the base-emitter
: junction is reverse-biased for a relatively long period of time.
: Frankly, I'm not sure what causes that effect.  But I have never heard
: of degradation of diode characteristics from reverse-biasing as long as
: the current is held to a low value.  Don't forget, we're talking only a
: few microamps here, since the current flow will be limited by the
: lowest-leakage diode in the string.

I'd like to add a couple comments basically in support of this.

First, another thing that degrades with asking the BE junction to handle
a reverse current is noise figure; this can be dramatic with relatively
short pulses current.  My point of reference on this is very low noise
audio amplifiers; noise figures around 0.5dB, optimized for 50k ohm
source impedances.  A "trick" to include in your circuit designs for
low noise amps is some way to insure that on power-on and power-off,
no capacitors try to charge/discharge through the BE junctions of your
input stages.

Second, how about a simple diode balance tester?  Someone with an
interest in this can go check some diodes and let us know quantitatively
how much of a problem it really is.  Tester:  feed a transformer with a
center-tapped high voltage secondary from a variac, so you can control
the voltage.  Ground the center-tap.  On each end of the winding, put a
capacitor, perhaps 0.1uF, rated at the peak voltage of the secondary
(RMS*1.5 or so).  Connect two diodes in series between the remaining
ends of the caps.  Measure the voltage with a scope at the junction
between the diodes.  If things are prefectly balanced, that voltage
will always be zero.  You should be able to tell from the shape, phase
and amplitude of the waveform you observe there just what the
imbalances are.  You could use this jig to sort (match) diodes, if you
really feel a need to.

73, K7ITM

------------------------------

Date: Sat, 5 Mar 94 04:29:44 GMT
From: btree![e-mail]
Subject: Series Diodes (was Re: Paralleling Power Diodes ?)
To: [e-mail]

In article <[e-mail]>, Alan Bloom <[e-mail]> wrote:
[ about series-connected rectifier strings ]

>Yes, I have seen degradation of a transistor's beta when the base-emitter
>junction is reverse-biased for a relatively long period of time.
>Frankly, I'm not sure what causes that effect.  But I have never heard
>of degradation of diode characteristics from reverse-biasing as long as
>the current is held to a low value.  Don't forget, we're talking only a
>few microamps here, since the current flow will be limited by the
>lowest-leakage diode in the string.


Am I seeing a "mixed metaphor" here?  Leakage current flows in
a reverse biased rectifier at voltages _below_the_avalance_voltage_.
When a good junction avalanches, its dynamic resistance becomes
very low and the current will rise to as much as the source can
provide.  There is some similarity to a spark gap switch - the
resistance is very high until it is triggered, and then the
resistance becomes very low.

Avalanching a junction does degrade it.  The first thing noticed
is usually that the leakage increases.  If it's a low noise
transistor the noise figure degrades.  If it's a bipolar transistor
the beta degrades.

Bob Hale     [e-mail]

------------------------------

Date: 4 Mar 1994 03:23:34 GMT
From: ihnp4.ucsd.edu!swrinde!cs.utexas.edu!howland.reston.ans.net!math.ohio-state.edu!hobbes.physics.uiowa.edu!news.uiowa.edu!franklin.uhl.uiowa.edu![e-mail]
Subject: Source for "heat Diodes/Semi's" ?
To: [e-mail]

Don

  Here are a couple of companies


Melcor
Materials Electronics Products Corporation
1040 Spruce Street
Trenton, New Jersey  08648
Ph.  609 303 4178
FAX  [phone]


Marlow Industries, Inc.
10451 Vista Park Road
Dallas, Texas  75238 1645
Ph  214 340 4900
FAX [phone]


   Have ordered from Marlow. Their  standard single stage tecs run from $12
to $55.50.  Special single stage tecs run $23 to $150.  Multi stage tecs
run $43 to $810. (tecs = thermoelectric coolers)

  Both companies will send you a very nice application booklet if you give
them a call.  The book gives enough info to design simple cooling circuits.
Hope this all helps , I've got my eye on a used set for a special coffee
cup coaster that would heat with the handel in one direction and for tea
cool with the handel in the other direction.    John WA0VYZ

------------------------------

End of Ham-Homebrew Digest V94 #50
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