Date: Sat, 5 Mar 94 04:30:49 PST From: Ham-Homebrew Mailing List and Newsgroup Errors-To: Ham-Homebrew-Errors@UCSD.Edu Reply-To: Ham-Homebrew@UCSD.Edu Precedence: Bulk Subject: Ham-Homebrew Digest V94 #50 To: Ham-Homebrew 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" ? Send Replies or notes for publication to: Send subscription requests to: Problems you can't solve otherwise to brian@ucsd.edu. Archives of past issues of the Ham-Homebrew Digest are available (by FTP only) from UCSD.Edu in directory "mailarchives/ham-homebrew". We trust that readers are intelligent enough to realize that all text herein consists of personal comments and does not represent the official 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!jsherman@uunet.uu.net Subject: Converting cheap-o walkie-talkies (stabilizing crystals?) To: ham-homebrew@ucsd.edu 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!wa2ise@network.ucsd.edu Subject: Converting cheap-o walkie-talkies (stabilizing crystals?) To: ham-homebrew@ucsd.edu In article <1994Mar3.152902.9597@mnemosyne.cs.du.edu> jsherman@nyx10.cs.du.edu (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!jyoungberg@network.ucsd.edu Subject: GPS Receiver Boards To: ham-homebrew@ucsd.edu 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!usenet@network.ucsd.edu Subject: Looking for a keyboard for a HaL Telereader To: ham-homebrew@ucsd.edu 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 pja@wrmed.com 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!hale@network.ucsd.edu Subject: Paralleling Power Diodes ? To: ham-homebrew@ucsd.edu In article , Alan Bloom wrote: >Alan Anderson (alana@u018.me.vp.com) 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 hale@brooktree.com ------------------------------ 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!konstu@network.ucsd.edu Subject: Series Diodes (was Re: Paralleling Power Di To: ham-homebrew@ucsd.edu 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!alanb@network.ucsd.edu Subject: Series Diodes (was Re: Paralleling Power Di To: ham-homebrew@ucsd.edu Stuart Hale (konstu@eua.ericsson.se) 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!tomb@network.ucsd.edu Subject: Series Diodes (was Re: Paralleling Power Diodes ?) To: ham-homebrew@ucsd.edu Alan Bloom (alanb@sr.hp.com) 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!hale@network.ucsd.edu Subject: Series Diodes (was Re: Paralleling Power Diodes ?) To: ham-homebrew@ucsd.edu In article , Alan Bloom 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 hale@brooktree.com ------------------------------ 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!jstroppe@network.ucsd.edu Subject: Source for "heat Diodes/Semi's" ? To: ham-homebrew@ucsd.edu Don Here are a couple of companies Melcor Materials Electronics Products Corporation 1040 Spruce Street Trenton, New Jersey 08648 Ph. 609 303 4178 FAX 609 393 9461 Marlow Industries, Inc. 10451 Vista Park Road Dallas, Texas 75238 1645 Ph 214 340 4900 FAX 214 341 5212 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 ****************************** ******************************