jhs%Mitre-Bedford@d3unix.UUCP (05/11/85)
------- Forwarded Message Date: 9 May 1985 0731-EST (Thursday) From: bccvax!rachiele@nadc Received: from nadc by SIMTEL20.ARPA with TCP; Wed 8 May 85 13:43:49-MDT To: info-hams@simtel20.arpa Subject: request for info Two elementary questions from a novice: 1. What is a good reference book giving characteristics of various types of transisters? ******* Most transistor manufacturers publish reference books on their own products, and often include cross-reference lists to type numbers which they don't supply so you can pick one of their products to substitute. Often these books have fairly hefty prices printed on them, but don't be fooled: they really are intended to be given away to good customers. If you work for a company that buys electronic components, or if you KNOW someone who does (and what ham doesn't!) you can probably wheedle a free copy of such books with a little diplomatic effort. And don't feel guilty, the goodwill and future sales of the product are worth it to the transistor manufacturer! After all, YOU might work for Zenith or DEC or IBM some other day even if you don't now! You might also write or call Semiconductor Specialists in Elk Grove Village, Illinois. They probably will send you some useful catalogs, cross-reference lists, etc. Other sources can be found by browsing at your local radio store's bookshelf. Also, the Radio Amateur's Handbook always used to have tables of tube and transistor parameters at the back. Is this still true??? ****** 2. Is there an outfit (there must be) from which you can mail order components (I particularly need some small coupling transformers, Radio shack doesn't seem to have them in their catalog)? Is there a way to measure the inductance of a home-made transformer? ****** Try to get hold of a catalog from Newark Electronics (I think they are still around, based in Chicago) or Schweber or Page or Gerber (in Norwood, MA) or many other places. Most of 'em will gladly ship a mail order to an individual. Can anybody suppply addresses and phone numbers? I can if you persist. As for measuring inductance, there are instruments (reactance bridge, Q-meter) designed to do it, but you can do it simply if you have an AC signal source by making an LC tuned circuit and finding its resonant frequency. For RF, the easy way to do this is with a "dip meter" (formerly called a grid-dip meter until they did away with grids). It's worthwhile to buy a dip meter if you can find one at a price you like, maybe at a flea market or something. You will get a lot of use out of it if you are inclined to experiment with circuits and antennas etc. If you don't have a dip meter and don't have a tuneable signal source, you can still make a measurement by getting an AC current to flow through the transformer winding and measuring the current and the AC voltage across the winding while the current is flowing. This will let you calculate the winding's reactance and from that its inductance. From your question, I suspect you are talking about audio transformers. For this case, you can use 60 Hz from the power line as a signal source if you have nothing else. However, you should be careful how you go about it, as the voltage is hazardous. If you have ANY other option, such as borrowing or buying an audio signal generator, I would recommend you do so. If you HAVE to use the AC line as your signal source, one way that can be used safely IF YOU ARE VERY CAREFUL would be to run one wire to a known ground, either at the AC outlet or elsewhere, and then make up a test cord with ONLY ONE PRONG connected. When you plug this in, you either get ground (no voltage) or the hot lead. You reverse it until you get voltage. By connecting your ground lead separately DIRECTLY TO GROUND, you make sure you won't ever get voltage on what you thought was your ground lead and therefore onto the cases of voltmeters etc. Run the hot lead to a voltage divider that can be something like a 10 K Ohm, 2-Watt resistor on top and a 1 K Ohm resistor from the bottom of this to ground. This will take the 115 volts down to about 12 volts measured across the 1 K Ohm resistor to ground. Be careful to do all this in such a way that you are unable to touch the 115 volt points after you have wired it all up and before plugging it in. Electrical tape could be used to cover up points that will be "hot" before plugging it in, for example. Now the 12-volt rms signal taken from across the 1 K Ohm resistor can be fed into the inductor for measurement purposes, and you can work safely with this signal without fear of a shock. Run a wire from the 12-volt output of the voltage divider to one side of the transformer winding whose inductance you wish to measure, and run the other side of the winding through a resistor or other known impedance to allow you to measure the current. (If your meter will measure AC current directly, then you can just wire the meter in the circuit instead of the resistor. But it is best to make sure that the current doesn't change when you include the meter in the circuit! If you can leave the current meter wired in and measure the voltage across the inductor with a separate meter, that's best.) Once you know the voltage V across the inductor with a known current I running through it, you find XL by computing V/I. Then you can find L by computing L = XL / (2pi f). PLEASE NOTE THAT THE PROCEDURE DESCRIBED CAN BE HAZARDOUS IF YOU ARE AT ALL CARELESS IN CARRYING IT OUT. PLEASE NOTE THAT I RECOMMEND YOU TRY TO FIND ANOTHER WAY SUCH AS BORROWING EQUIPMENT DESIGNED FOR THE TASK. PLEASE ADVISE YOUR HEIRS BEFORE THE EXPERIMENT THAT I WILL TAKE NO RESPONSIBILITY FOR ANY HARM TO THE EXPERIMENTER! (Whew, I hope that clears me if you goof!) Good luck -John S., W3IKG ******