| Home |
Ask The Editor, He Knows! Got a question or a problem with a project--ask The Editor. Please remember that The Editors' column is limited to answering specific electronic project questions that you send to him. Personal replies cannot be made. Sorry, he isn't offering a circuit design service. Write to: The Editor C&E HOBBY HANDBOOKS INC. P.O. Box #5148 North Branch, N.J. 08876, USA. Horns of a Dilemma I like your magazine and trust what you guys say, so I thought I'd write to you about my problem in choosing a DMM. I'm trying to stay in the $50-60 range. Although I realize my choices will be limited, I think I've found two good ones but can't decide between them. I am in school for electrical engineering and intend to use the meter for building projects and school work. I want reasonable reliability and accuracy, resistance measurement to around 10 megohms, and AC/DC current capability (should I worry about getting down to the micro-amp range?). Capacitance measurement would be a nice feature, but do I really need it? The two DMMs I'm vacillating between are the Radio Shack/Micronta Model 22-186 and the B&K Toolkit 2704A. The Micronta has auto-ranging and data-hold, as well as a 31-dot bar graph; but the B&K has capacitance, a micro-amp current range, and a transistor-checking feature (which they don't bother to explain). At the moment, I'm leaning toward the B&K 2704A because of the company's reputation, but the Micronta with auto-ranging is pretty tempting. Given my status as a student, which one do you think I should buy? --Mike Hardy; San Jose, CA You're obviously still a young fella, Mike. Some day you'll meet a nice girl, marry her, and she'll make decisions like this for you. Then, the best I can do is evaluate the various features of the two meters under consideration, and leave the ultimate decision to you. Here goes: In practice, you will need some means of measuring capacitance, if only to verify that a capacitor meets its specifications. You have two options: a DMM with capacitance-measuring capability, or a separate capacitance meter. It's cheaper to buy a DMM that can measure capacitance than to in- vest in both a DMM and a stand- alone capacitance meter. However, a DMM that can also measure capacitance will offer a limited range (typically, 1000 pf to 20 mf) and limited accuracy (2-4%). By comparison, you can expect a dedicated capacitance meter to measure from 1 pf to 2000 mf with an accuracy of 0.5% or better. The ability to measure current in the micro-amp range can be very handy at times. Transistor checking serves to measure a transistor's DC gain. It can be a desirable feature at times, particularly if you have no other instrument with which to make such a measurement. Autoranging saves time, and I'd recommend it to anyone working in a production environment. For a student, though, it is not a crucial feature. Data-hold is a feature you will seldom use. In theory, data-hold allows the operator to concentrate on the placement of his probes while the DMM stores the measurement. I suppose that situations exist where this might be important, but I can't think of one. A bar-graph display is offered in order to shore up an essential weakness of most DMMs: they cannot readily follow a changing signal because the display is updated roughly three times a second. So, if you are monitoring a voltage while adjusting a pot, you turn, wait for the display to settle, turn some more, and wait some more. It is a slow, sometimes exasperating process. One solution would be to use an analog meter, which has a fast, smooth response. The other alternative is to use the bar -graph display, which is up-dated much more rapidly than the digits. Check the specs of the bar-graph display. It should update 20-30 times a second. A bar -graph display is handy but not essential. B&K and Micronta both offer adequate warranties. You should realize that for $50 you are almost certainly getting a product manufactured in Taiwan or Korea. Furthermore, for all I know, B&K and Micronta could be using the same offshore facility to do their assembly work. So it is hard to differentiate between the two on the basis of reputation. Suffice it to say that both manufacturers sell a lot of DMMs, and I haven't heard any complaints about either company. I hope this information will be of some help to you. Incidentally, Volume 13 of Electronics Handbook features a review of the Triplett Model 2202, another nice low-cost DMM. The Taper Caper Could you please explain to me the difference between audio-taper and linear-taper potentiometers? I assume they are not interchangeable. --Brian Forbes, Houston, TX Your assumption is correct, Brian. They're not interchangeable. The resistance of a linear -taper pot is a linear function of the angle of rotation, while the resistance of an audio -taper pot varies as the logarithm of the angle of rotation. Often used as volume controls, audio -taper pots compensate for the ear's diminished sensitivity at low power levels. Finally, let us not forget the South American tapir. If you find one of these in your basement workshop, back out cautiously and call the SPCA. Balancing Act I am happy to have found a magazine that is at my level and still a challenge to read. Your articles are very clear and provide a great alternative to a textbook. Besides praise, I also have a question for you. I am currently involved in a hobby that requires the balancing of small motor armatures. The motors run to the tune of 100,000 rpm, and balancing is a must. One problem is that the armatures are about half an inch in diameter and one inch long. Any guidance you can offer regarding a no -frills dynamic balancer that I could make at home would be appreciated. --Geary Gaspord; Glenview, IL The dynamic balancers that I've seen, Geary, are complicated pieces of equipment costing several thousand dollars. I see no way of reducing them to anything even remotely resembling a no -frills home-brew project. Perhaps one of our readers has interests similar to your own and can offer further assistance. If so, we'll pass the information along to you. Looking For A Meter I am in the process of building a metal detector and need to find a reasonably priced panel meter with a full-scale indication of 50 microamps DC. The catalogs that I've checked either lack such meters altogether, or charge exorbitant prices for them. Doesn't anyone sell reasonably priced meters anymore? --Wilfred Detcheverry; Glace Bay, Nova Scotia Nothing is reasonably priced anymore, Wilfred. In this instance, though, you're fortunate because the meter in a metal detector need not be highly linear or accurate. Hence, feel free to use the cheapest meter you can find. After a fairly exhaustive search through a stack of electronics catalogs, I managed to locate a decent 50-microamp DC meter that sells for about $13. The source is Ocean State Electronics (PO. Box 1458, Westerly, RI, 02891). You might also wish to check out the surplus dealers, who routinely sell meters for as little as 3-4 dollars. Two good sources are Marlin P Jones & Assoc. ( PO. Box 12685, Lake Park, FL, 33403) and Hosfelt Electronics (2700 Sunset Blvd., Steubenville, OH, 43952). Depending on how lucky you are, you may find just what you need or something close to it. For example, you could probably get by with a 100-microamp meter; just use a series current -limiting resistor that is one-half the size of the one specified. I doubt that an additional 50 microamps of current will severely tax the capability of whatever circuit you are building. More on PCBs I just picked up the most recent edition of Electronics Handbook at a local supermarket and am quite impressed with the amount of information you've packed into the magazine. I especially enjoyed the article on PCBs. I am only a novice at electronics, but I feel that I learned more from this one magazine than I could from $100 worth of other books and periodicals. I do have one bone to pick with you, however. I wasn't all that thrilled when you advised novices not to start off with PCB-design programs, but to design their boards manually instead. I realize that you wanted to save newcomers a bit of money, but there are numerous inexpensive shareware PCB-design programs. I use one myself, and have obtained good results. --Aaron Waychoff; Coeur d'Alene, ID Glad you liked the magazine, Aaron. We didn't mean to imply that novices should avoid computer-assisted PCB design altogether, just that they should cut their teeth on manual methods first. How can you appreciate the wonders of computer design if you've never toiled over a manual layout? Crazy Cables I see advertisements in audio magazines for Monster Cables and other special wires for hooking up loudspeakers. The manufacturers claim their hook up cables will make the hi-fi system sound better. I thought wires connecting amplifiers or receivers to speakers were just that-wires. It sounds like Hype to me. What's going on here? What am I not up on? -J.R. Russel; Norcross, Georgia. You are up on this totally. This is one of the most popular, and profitable (for manufacturers and the store salesmen) hypes going. Some audio enthusiastics (audio nuts, if you prefer) will believe almost anything if it promises to make their stereo system sound better. One of the most respected audio experts in the business, Julian Hirsh (of Stereo Review) ran careful blindfold tests a couple of years ago, using several such special cables and comparing them with ordinary electric lamp cord. He proved what I have always claimed; that there is no difference in the sound. But it's easy to believe you hear differences if you want to believe it. That's how so many speaker makers manage to get along. +++++++++++++++++++++++ Also see: UNDERSTANDING ELECTRICITY--PART VII Understanding Electricity--Part III
adapted from: Electronics Handbook XV (1994) |
| PREV. | | NEXT | | More E-H articles | HOME |