logajan@ns.network.com (John Logajan) (07/20/89)
In article <536@winnie.fit.edu>, acs60222@zach.fit.edu ( ENRIQUEZ) writes: > I began to think about putting another, smaller > transformer is parallel with the other to increse the driving current. Multiple parallel transformers do work (as long as you get all the polarities correct!) In fact, internal to many transformers are parallel windings tied together to get just that effect. They thus gain the advantage of using smaller guage wire, but end up with higher current carrying capability. There are two sources of "resistance" to high current capability of transformers -- the winding wire guage and the core cross-sectional area. Usually xformers optimize both, so that you can't increase one without needing to increase the other. In any event, multiple xformers in parallel do both jobs, increase the wire guage (by parallelism) and increase the core cross-sectional area (also by parallelism.) -- - John M. Logajan @ Network Systems; 7600 Boone Ave; Brooklyn Park, MN 55428 - - logajan@ns.network.com / ...rutgers!umn-cs!ns!logajan / john@logajan.mn.org -
gpz@bridge2.ESD.3Com.COM (G. Paul Ziemba) (07/20/89)
acs60222@zach.fit.edu ( ENRIQUEZ) writes: >I have encountered a problem in which a expensive solution has been >recommened. I have a 120v/12v transformer driving about 25 12v, 20 >watt halogen bulbs (I am not sure of the output current of the trans- >former). The problem is that after the 12th light is installed, >the system begins to dim, and by the 20th bulb, almost no light is >present. Let's see, that's about 42 amps at 12v you want. Are you sure that the house wiring can handle that amount of current? For that amount of current, I'd use at least 8 gauge copper wire. Anything smaller is probably inviting disaster! >The solution (given by an electrician) is to buy an large, expensive >transformer which will supply more current. Simple enough, I say, but >I began to think that there might be a more elegent solution (i.e. >cheaper) A reasonable solution. You can get 50 amps at 12v transformers from surplus places (granted, you might have to mail-order them. The rest of the world is not filled with electronics surplus places like the santa clara valley). >Since this is for a pre-wired house, not a lot of rearranging of bulbs, >wires, etc can be done. Being a *lowly* undergrad in CP and having some >slight EE experiance, I began to think about putting another, smaller >transformer is parallel with the other to increse the driving current. >However, I doubt this simple idea can work and would enjoy hearing why >it would not. Well, that could be dangerous if the two transformers are not identical. Let's say one transformer has a slightly higher output voltage than the other. You would burn up a lot of energy just between the two transformers even with no load. If the voltages were V1 and V2, and the impedances of the secondary windings were Z1 and Z2, you would have |V1-V2|/(Z1+Z2) amps roaring around just between the two transformers. It could work if the transformers were identical; that way you would minimize the power lost "internally" to heating. Another possibility would be to partition your circuit into two halves. It probably would involve some hunting around the various fixtures, disconnecting the wires on each side and finding out where to break the circuit. Then you could buy a 25 amp transformer and use it on the other half. (I am assuming a topology like this, initially:) break here V ------------------------------- | | | | | | | | | | | | | | | T O O O O O O O O O O O O O O | | | | | | | | | | | | | | | ------------------------------- ^ Good luck! ~!paul -- ---- Paul Ziemba ...!pyramid!zapi!gpz gpz@bridge2.3com.com (415)940-7671 (w)
jewett@hpl-opus.HP.COM (Bob Jewett) (07/20/89)
> I have a 120v/12v transformer driving about 25 12v, 20 > watt halogen bulbs (I am not sure of the output current of the trans- > former). ... > Since this is for a pre-wired house, not a lot of rearranging of bulbs, > wires, etc can be done. 12V and 20W implies about 2 amps per bulb, times 25 bulbs gives 50 amps. Check your wires out. If they are not rated for 50 amps, you are risking starting a fire. One solution that requires at least a little rewiring is to put 10 bulbs in series rather than parallel. > I appreciate any and all discussion about my problem... What does the electrical code in your area permit? Bob
tomb@hplsla.HP.COM (Tom Bruhns) (07/21/89)
acs60222@zach.fit.edu ( ENRIQUEZ) writes: > > ... > > slight EE experiance, I began to think about putting another, smaller > transformer is parallel with the other to increse the driving current. > However, I doubt this simple idea can work and would enjoy hearing why > it would not. > Actually it should work just fine, _IF_ the turns ratio is identically the same. There is an effective series resistance associated with each transformer's output that will cause them to share the load as the inverse of the ratio of the resistances. It would be best to use two identical transformers; then they would share the load equally. You can also put a small autotransformer in to account for slight voltage differences between two equal-VA but slightly different-voltage transformers. The autotransformer would be small compared with the other two if the voltage difference is small. But an even better way: you may also be able to locate a large transformer that will handle the whole thing, at a very reasonable price, if you spend some time looking. I've picked up 500 VA (nominally 500 watt) transformers for as little as about $2.00, in the voltage range you are interested in. > I appreciate any and all discussion about my problem... > > Thanks me > ----------
john@frog.UUCP (John Woods) (07/22/89)
In article <536@winnie.fit.edu>, acs60222@zach.fit.edu ( ENRIQUEZ) writes: > I have encountered a problem in which a expensive solution has been > recommened. [ A transformer is not powerful enough to drive 25 halogen > light bulbs. Proposed solution: a large, expensive transformer which will > supply more current. ] Paralleling transformers works quite well. It helps if you match the transformers so they put out exactly the same voltage, but there are ways to balance them if they aren't quite close. First, you need to get the polarity right. A quick way to determine the correct polarity is this: Wire the primaries in parallel. Wire the secondaries in series, with a resistor (~ 1K) also in series. Plug it it. Measure the AC voltage across the resistor. If it is double the transformer output voltage, reverse the secondaries; if it is zero, you're all set -- remove the resistor and parallel the secondaries; if it is small but non-zero, the transformers aren't well matched. A scheme for matching transformers can be found in The Radio Amateur's Handbook (by the ARRL), involving using a center-tapped secondary of another transformer (which must be able to handle the current but need not have any particular voltage rating, in particular can be quite small). I don't remember it exactly, but I believe it is T1 ---------------------------- | | T3 |||S |||S |||S |||S------------o voltage out |||S |||S |||S | | T2 -------------------- | | | |||S | |||S | |||S | |||S | | ----------------------------------------o voltage out T1 and T2 are your power transformers, T3 is any low-voltage transformer capable of handling the current of either T1 or T2, plus some small margin. (Primaries are not shown; T3's primary is left unconnected. If you feel really adventurous, you could probably unwind a junk transformer and wind a T3 yourself) You should try to find a Handbook to check this yourself; I will post a correction if I find that I am horribly wrong on my memory here. -- John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101 ...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu People...How you gonna FIGURE 'em? Don't bother, S.L.--Just stand back and enjoy the EVOLUTIONARY PROCESS...
sukenick@ccnysci.UUCP (SYG) (07/28/89)
>Multiple parallel transformers do work (as long as you get all the >polarities correct!) In fact, internal to many transformers are parallels Wouldnt you have to find matched transformers (close in real voltage)? What if the voltages are slightly different? (ie: there may be variations in voltages of 12.6 V transformers) Wouldn't one act as a sink and heat up if the voltages arent matched? What is a good rule of thumb for the max diff in volts? Where is Theremagoochymin's rule when you need it?
cees@maestro.htsa.aha.nl (Cees Keyer) (07/28/89)
In article <2541@ccnysci.UUCP> sukenick@ccnysci.UUCP (SYG) writes: >>Multiple parallel transformers do work (as long as you get all the >>polarities correct!) In fact, internal to many transformers are parallels > >Wouldnt you have to find matched transformers (close in real voltage)? >What if the voltages are slightly different? >(ie: there may be variations in voltages of 12.6 V transformers) >Wouldn't one act as a sink and heat up if the voltages arent matched? >What is a good rule of thumb for the max diff in volts? > >Where is Theremagoochymin's rule when you need it? A rule of tumb is that the secundary voltage of two parallel transformers must be with in 0.5% equal to each other for small <1 KW trans. The difference between the power output off the transformers must be within a factor 3 So if 1 trans. is 1 KW the other must be smaller than 3 KW. Last but not least the factor e = Vshort/Uprimary * 100% must be the same. Vshort is the voltage aplied to the primary coil with the nominal current flowing trough the primary coil and the secundary coil is shortend, like this -------) || (----------- Inom > ) || ( | ) || ( | ) || ( | ) || ( | --------) || (----------- So these are the rules for transformers parallel. The terms I used are poor translations from dutch so don't flame me about the language used. #include disclaimer.h #include language.h Cees Keyer. -- DISCLAIMER: I am not insane, I am a plane. Cees Keyer, Algemene Hogeschool Amsterdam. department of electrical engineering. UUCP: {backbones}!tamtam!cees cees@htsa.aha.nl cees@tamtam.htsa.aha.nl
acs60222@zach.fit.edu ( ENRIQUEZ) (08/16/89)
I have encountered a problem in which a expensive solution has been recommened. I have a 120v/12v transformer driving about 25 12v, 20 watt halogen bulbs (I am not sure of the output current of the trans- former). The problem is that after the 12th light is installed, the system begins to dim, and by the 20th bulb, almost no light is present. The solution (given by an electrician) is to buy an large, expensive transformer which will supply more current. Simple enough, I say, but I began to think that there might be a more elegent solution (i.e. cheaper) Since this is for a pre-wired house, not a lot of rearranging of bulbs, wires, etc can be done. Being a *lowly* undergrad in CP and having some slight EE experiance, I began to think about putting another, smaller transformer is parallel with the other to increse the driving current. However, I doubt this simple idea can work and would enjoy hearing why it would not. I appreciate any and all discussion about my problem... Thanks me
ankleand@mit-caf.MIT.EDU (Andrew Karanicolas) (08/26/89)
In article <536@winnie.fit.edu> acs60222@zach.UUCP ( ENRIQUEZ) writes: >I have encountered a problem in which a expensive solution has been >recommened. I have a 120v/12v transformer driving about 25 12v, 20 >watt halogen bulbs (I am not sure of the output current of the trans- >former). The problem is that after the 12th light is installed, >the system begins to dim, and by the 20th bulb, almost no light is >present. > >The solution (given by an electrician) is to buy an large, expensive >transformer which will supply more current. Simple enough, I say, but >I began to think that there might be a more elegent solution (i.e. >cheaper) > >Since this is for a pre-wired house, not a lot of rearranging of bulbs, >wires, etc can be done. Being a *lowly* undergrad in CP and having some >slight EE experiance, I began to think about putting another, smaller >transformer is parallel with the other to increse the driving current. >However, I doubt this simple idea can work and would enjoy hearing why >it would not. > >I appreciate any and all discussion about my problem... > >Thanks me If you use a single transformer, it needs to have something like a 500VA rating to drive all of the bulbs to a reasonable brightness. If you use multiple transformers to do this, the effective rating also has to be in the 500VA range. This IS the 'elegant' solution, assuming you wish to have each of the bulbs draw 20W or so. I would imagine that a single transformer to do the job is less costly than using many smaller transformers placed in parallel. A difficulty with placing tranformers in parallel is that mismatches between output voltage magnitudes and phases will result in a secondary loop current, possibly large. If you used two transformers and could be sure that the loop current is small enough that the sum of the transformer's current share (load-current/N, for N transformers) and the loop current is smaller than the transformer's rated current then you have no problem. This condition will be harder to guarantee for larger number of transformers placed in parallel or with larger uncertainty in output matching. Depending on how much you were quoted for the transformer you need, it might be cheaper to use a transformer out of a battery charger. Battery chargers are commonly available and the cost of the whole charger could be less than the cost of buying just the transformer alone. The transformer in a 50A charger would easily illuminate your bulbs. I would skip the rectifiers since they are not needed in your application. You could use a car battery as well, but that's not too practical if you plan to use these lights often. I think the safest bet would be to get a hold of a UL rated transformer for the job at hand so you won't have to worry about overheating and possible fire; depending on how the cost of transformers go, this may be the cheapest way to go and you would not have to incur the cost of rewiring. Be sure to use heavy guauge wiring at the input to this lighting system. Andy Karanicolas MIT Microsystems Laboratory ankleand@caf.mit.edu
irwin@m.cs.uiuc.edu (08/28/89)
/* Written 12:07 pm Aug 15, 1989 by acs60222@zach.fit.edu in m.cs.uiuc.edu:sci.electronics */ /* ---------- "Transformer Troubles" ---------- */ >I have encountered a problem in which a expensive solution has been >recommened. I have a 120v/12v transformer driving about 25 12v, 20 >watt halogen bulbs (I am not sure of the output current of the trans- >former). The problem is that after the 12th light is installed, >the system begins to dim, and by the 20th bulb, almost no light is >present. >The solution (given by an electrician) is to buy an large, expensive >transformer which will supply more current. Simple enough, I say, but >I began to think that there might be a more elegent solution (i.e. >cheaper) >Since this is for a pre-wired house, not a lot of rearranging of bulbs, >wires, etc can be done. Being a *lowly* undergrad in CP and having some >slight EE experiance, I began to think about putting another, smaller >transformer is parallel with the other to increse the driving current. >However, I doubt this simple idea can work and would enjoy hearing why >it would not. >I appreciate any and all discussion about my problem... >Thanks me /* End of text from m.cs.uiuc.edu:sci.electronics */ Since you are using a transformer, one can assume that all of the wiring for this was done by you, external to the house wiring. At a rating of 20 watts, the current per bulb is slightly under two amps. The solution is to power 5 bulbs from your present transformer and to series the other 20 bulbs in two groups of 10. Since each bulb requires 12 volts, 10 of them in series will consume 120 volts at slightly under 2 amps. Two groups 10 ten will do it. This is the manner in which most low cost Christmas tree light strings are wired. If you wire the series string in a clean and safe fashion, I see no problems in doing it. See to it that there is no frayed connections, insulate all splices. Wire nuts work good for this sort of thing. Of course, the restriction is that you will have to energize 10 bulbs at a time and will not be able to turn them on one at a time.