root@Clio.Uiuc.ARPA (11/25/85)
berger@clio.UIUC.EDUC I'm surprised that nobody mentioned it before. You can make a cheap isolation transformer by connecting two filament transformers back to back (one is step-down, other is step-up). The result is a lot cheaper than a conventional isolation transformer.
levy@ttrdc.UUCP (Daniel R. Levy) (12/03/85)
In article <3500001@Clio>, root@Clio.Uiuc.ARPA writes: >berger@clio.UIUC.EDUC > >I'm surprised that nobody mentioned it before. You can make a cheap >isolation transformer by connecting two filament transformers back to >back (one is step-down, other is step-up). The result is a lot >cheaper than a conventional isolation transformer. FILAMENT transformers??? What the dickens are THOSE? Oh, for TUBES you say? Gee whiz what're they??? You mean like in my father's WWII radio?? :-) More to the point, yes it's cheaper. And less efficient and it has less power handling capability, perhaps on the order of a 10-20% loss in voltage (due to resistive voltage losses in the windings which are not made up in an increased secondary turns count since the two transformers are obviously symmetrically ratioed) and perhaps ~10 watts power handling capability for a garden variety 1.5-ampere @ 6.3v output "filament" transformer a la Radio Shock. If the load draws a net DC current from the line this magnetically biases the core, which will even further drag down the efficiency of a garden-variety "filament" (or similar) transformer. So you'd need a really horsey pair of such "filament" transformers (even for a modest 80 watt draw) and your picture would be fairly shrunken (if the circuitry worked at all on the reduced volt- age). -- ------------------------------- Disclaimer: The views contained herein are | dan levy | yvel nad | my own and are not at all those of my em- | an engihacker @ | ployer or the administrator of any computer | at&t computer systems division | upon which I may hack. | skokie, illinois | -------------------------------- Path: ..!ihnp4!ttrdc!levy
crs@lanl.ARPA (12/03/85)
> More to the point, yes it's cheaper. And less efficient and it has less power > handling capability, perhaps on the order of a 10-20% loss in voltage > (due to resistive voltage losses in the windings which are not made up in > an increased secondary turns count since the two transformers are obviously > symmetrically ratioed)... This is not to disagree with Dan's article but merely to add an idea for those applications where one can stand all of the disadvantages except the voltage loss. [See caveat at end of article.] A filament transformer should provide its *rated* output voltage at its *rated* current. Thus a 6.3 volt, 10 ampere transformer should supply 6.3 volts when it's load is 10 amps. To compensate for IR drops in the windings, I assume that the turns ratio is juggled slightly. It is here that trouble occurs because the "juggling" that compensates when the transformer is used normally will work against you when used backwards. Since in the output transformer both the IR drop and the compensation are *decreasing* "output" voltage, at rated load, I'd guess the "system" will act as though neither transformer is compensated and so there will be a voltage loss as suggested. Suppose that, instead of making the configuration symmetrical, you make the input transformer (operated normally, as a step down transformer) a 6.3 volt filament transformer. Now, making suitable allowances in power rating, make the output transformer (connected "backwards" as a step up transformer) a 5 volt filament transformer. If all goes well, under load the 6.3 volts out of the input transformer will be about 26 percent more than the 5 volts "expected" by the output transformer. Assuming the 20% figure suggested by Dan and assuming that half of it is lost in the turns ratio compensation and the other half in IR losses: 117/5 = 23.4 0.9 * 23.4 = 21.06 21.06 * 6.3 = 132.7 0.9 * 132.7 = 119.4 volts out Naturally, this output transformer must be *over* *rated* because with 6.3 volts across a 5 volt winding dissipation will be higher than normal. Naturally we aren't limited to filament transformers. In these days of solid state circuits, power transformers are also, typically, step down transformers. A *first* *approximation* of output current rating for the combination would be the rated secondary current (when the transformer is used normally) divided by the turns (or voltage) ratio. [Note: I haven't tried this -- it's just an idea.] -- All opinions are mine alone... Charlie Sorsby ...!{cmcl2,ihnp4,...}!lanl!crs crs@lanl.arpa
mike@cucca.UUCP (Mike Ardai) (12/03/85)
Making an isolation transformer from a pair of back-to-back filament transformers will work, but you have to watch the current ratings... /Michael L. Ardai (Why do we still call them filament transformers? I haven't used them to run tube filaments in years... :-)
blarson@oberon.UUCP (Bob Larson) (12/04/85)
In article <3500001@Clio> root@Clio.Uiuc.ARPA writes: > >berger@clio.UIUC.EDUC > >I'm surprised that nobody mentioned it before. You can make a cheap >isolation transformer by connecting two filament transformers back to >back (one is step-down, other is step-up). The result is a lot >cheaper than a conventional isolation transformer. Also note that: it is less efficiant than a conventional isolation transformer expect it to put out less voltage than you put in -- Bob Larson Arpa: Blarson@Usc-Ecl.Arpa Uucp: ihnp4!sdcrdcf!oberon!blarson __________________________________ Unix: An operating system similar to Os-9, but with less functionality and special features designed to soak up excess memory, disk space and CPU time on large, expencive computers. -- Os-9/68000 operating system users manual
crandell@ut-sally.UUCP (Jim Crandell) (12/17/85)
>> More to the point, yes it's cheaper. And less efficient and it has less power >> handling capability, perhaps on the order of a 10-20% loss in voltage >> (due to resistive voltage losses in the windings which are not made up in >> an increased secondary turns count since the two transformers are obviously >> symmetrically ratioed)... Not to be a wet blanket or anything, but I vaguely recall that the ``standard'' isolation transformer of years past actually did have a slight step-down; seems like 117:110 was the most common spec. In most areas, a voltage reduction is not really such a bad idea -- the line voltage at my place averages about 123, for example. For equipment using tubes (forgotten what those are? So you really enjoy editing on that Decwriter, eh?), it's particularly unstupid, in fact. (This topic leads rather naturally into a discussion of classic methods for extending tube filament/cathode life expectancy. One of the most common of these is particularly obvious by now, I would think.) Besides, if you're gonna shell out for a pair of 60-watt filament transformers, you might as well buy an isolation transformer. It probably won't cost any more; it occupies less space; and it emits less heat. -- Jim Crandell, C. S. Dept., The University of Texas at Austin {ihnp4,seismo,ctvax}!ut-sally!crandell