isolation transformer needed
in [Repair]
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From: Phil Allison on 27 Dec 2009 21:22 "David Lesher" "Phil Allison" > >>> Buy 2 transformers with 120V primaries, and some voltage secondaries. >>> Hook them back to back. >>> >>> The weirder the secondary voltage, the lower the cost. > > >>** That is a very unwise suggestion: > >>AC supply transformers are NOT intended to be operated in reverse - cos >>the >>magnetising current is intended to be carried by the supply side winding >>and >>NOT the secondary. > > I've used this approach several times in the past, and never had > the catastrophe you imply. ** Try reading my post again. Only fuckwits snip posts out of site so they can write idiot comments like you just did. No "catastrophe" was predicted by me. Anyone can claim to have done something wacky and got away with it. Sorry to say that is ** ABSOLUTELY NO BASIS ** for advising others in a public forum. ---------------------------------------------------------------- Some years back, I tried connecting a pair of new 240V / 6.3V, 15VA transformers back to back to get an isolated 240 V output. The first tranny in the pair ran hot with no load on the second. A 15 watt load on the second dropped the voltage from 240V to 200V and the first tranny then got very hot. Testing revealed that the current drawn from the first tranny by the second nearly equalled to its 15VA rating. Useless. ..... Phil
From: bz on 28 Dec 2009 17:10 neet"Phil Allison" <phil_a(a)tpg.com.au> wrote in news:7po31lFjqcU1(a)mid.individual.net: > > "David Lesher" > >>>I'm looking for an isolation transformer for repairing small (less >>>than 1000W) power supplies. I see loads of them on ebay and some are >>>medical grade and inexpensive. Are these usable for what I want? >>>Other than that, any tips on where to get a good deal on one? I >>>don't want to spend a load as this is hobby work only. >> >> >> Buy 2 transformers with 120V primaries, and some voltage secondaries. >> Hook them back to back. >> >> The weirder the secondary voltage, the lower the cost. > > > ** That is a very unwise suggestion: I am sorry to have to say this, but that statement is wrong. > AC supply transformers are NOT intended to be operated in reverse - cos > the magnetising current is intended to be carried by the supply side > winding and NOT the secondary. The magnetic flux density depends on current AND the number of turns. Less turns and higher current gives the same flux density. This is why 'ampere-turns' are used in calculating flux density, not simply 'amps'. http://mysite.du.edu/~jcalvert/tech/transfor.htm The magnetic core doesn't care which winding induces the magnetic field. > > A standard 500VA transformer operating from 120 VAC may well draw 1.5 > amps with no load At what phase? With no load, it is NOT in phase. It DOES cause IR losses in the primary, however. > - no problem since as the primary winding has only > about 0.6 ohms of resistance and hence loses only 1.35 watts in heat. 1.5 amps at 120 volts = 180 W 1.5 amps at 120 volts at 89.57 degrees (or a power factor of 0.993) gives 1.35 watts. Slightly over 1% loss. That is a bit high for modern power transformers under no load, but a normal loss under max rated load. > > The iron core losses will far exceed that. Shouldn't. The iron core losses are PART of the total losses seen. > > However, if you try to make it work in reverse to deliver 120 volts at > 500VA from the primary - things get nasty. No. You should only, at worst (approximately) double the losses (assuming two identical transformers). Let us assume that the secondary of T1 is 12 vac. To deliver 500VA at 12 volts, the secondary is going need to see a 0.288 ohm load and will deliver 41.7 amps to the load. The DC resistance of the winding will be much lower than the load resistance, on the order of 0.03 ohms for an inefficient transformer. > First, the secondary will have to be fed with a ** higher voltage** > than the **off load** voltage by about 4% to cover voltage drop under > load. > > Naturally this increases the previously mentioned magnetising current > level by about 30%. The identical but reversed 2nd transformer 'expects' that higher voltage and exactly compensates for it. > > So now it is say 2 amps, referred to the primary. > Wrong. If the output of the second transformer is carrying 1 amp, the primary of the first transformer will carry 1 amp plus the iron and copper losses of the two transformers. A poor efficiency is about 95% so with two transformers, back to back, you might expect 10% losses resulting in 1.10 amps. A low voltage, high current secondary is commonly wound with heavier wire so it can stand more current and presents a much lower dc resistance. When it is driven, as when the windings are reversed, it will run just fine. > 2 amps at 120 volts = 240VA and that HAS to be *continuously supplied* > by the first tranny in the pair. > Wrong. Only the current to supply the 'no load losses' needs to be supplied 'continuously'. Under the 500VA load, the primary will present a 28.8 ohm load to the 110VAC line, drawing 4.2 amps of current while the back to back 12 volt windings will be carrying 42 amps of current. The no load losses will be quite low because the primary presents an essentially pure inductive load rather than a resistive load to the ac line. Only the core losses, on the order of .25 to .5% of the rating will need to be supplied. http://findarticles.com/p/articles/mi_m0BPR/is_10_21/ai_n6259812/ Under no load, the 12 volt windings would see a 'parasitic current' on the order of 0.02 Amps and the 110 V primary would see a current on the order of 0.1 Amp and present a 'resistive component' of the load of 115 ohms to the supply lines. > So, the result is that the pair of trannys can only deliver half the VA > into the load that one is capable of. > The combo should be able to supply close to the rated VA of _a_single_transformer. You will have ~twice the losses and both transformers will reach the temperature that one would have reached. So, de-rate the pair of 500VA back to back transformers to 450VA to give yourself plenty of safety margin and don't seal them into an airtight box together, and you should be just fine. > .... Phil Best Regards -- bz 73 de N5BZ k please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set.
From: Jamie on 28 Dec 2009 18:35 bz wrote: > neet"Phil Allison" <phil_a(a)tpg.com.au> wrote in > news:7po31lFjqcU1(a)mid.individual.net: > > >>"David Lesher" >> >> >>>>I'm looking for an isolation transformer for repairing small (less >>>>than 1000W) power supplies. I see loads of them on ebay and some are >>>>medical grade and inexpensive. Are these usable for what I want? >>>>Other than that, any tips on where to get a good deal on one? I >>>>don't want to spend a load as this is hobby work only. >>> >>> >>>Buy 2 transformers with 120V primaries, and some voltage secondaries. >>>Hook them back to back. >>> >>>The weirder the secondary voltage, the lower the cost. >> >> >>** That is a very unwise suggestion: > > > I am sorry to have to say this, but that statement is wrong. > > >>AC supply transformers are NOT intended to be operated in reverse - cos >>the magnetising current is intended to be carried by the supply side >>winding and NOT the secondary. > > > The magnetic flux density depends on current AND the number of turns. > > Less turns and higher current gives the same flux density. > > This is why 'ampere-turns' are used in calculating flux density, not simply > 'amps'. > http://mysite.du.edu/~jcalvert/tech/transfor.htm > > The magnetic core doesn't care which winding induces the magnetic field. > > >>A standard 500VA transformer operating from 120 VAC may well draw 1.5 >>amps with no load > > > At what phase? With no load, it is NOT in phase. It DOES cause IR losses in > the primary, however. > > >>- no problem since as the primary winding has only >>about 0.6 ohms of resistance and hence loses only 1.35 watts in heat. > > > 1.5 amps at 120 volts = 180 W > 1.5 amps at 120 volts at 89.57 degrees (or a power factor of 0.993) gives > 1.35 watts. > > Slightly over 1% loss. That is a bit high for modern power transformers > under no load, but a normal loss under max rated load. > > >>The iron core losses will far exceed that. > > > Shouldn't. The iron core losses are PART of the total losses seen. > > >>However, if you try to make it work in reverse to deliver 120 volts at >>500VA from the primary - things get nasty. > > > No. You should only, at worst (approximately) double the losses (assuming > two identical transformers). > > Let us assume that the secondary of T1 is 12 vac. > To deliver 500VA at 12 volts, the secondary is going need to see a 0.288 > ohm load and will deliver 41.7 amps to the load. The DC resistance of the > winding will be much lower than the load resistance, on the order of 0.03 > ohms for an inefficient transformer. > > >>First, the secondary will have to be fed with a ** higher voltage** >>than the **off load** voltage by about 4% to cover voltage drop under >>load. >> >>Naturally this increases the previously mentioned magnetising current >>level by about 30%. > > > The identical but reversed 2nd transformer 'expects' that higher voltage > and exactly compensates for it. > > >>So now it is say 2 amps, referred to the primary. >> > > > Wrong. If the output of the second transformer is carrying 1 amp, the > primary of the first transformer will carry 1 amp plus the iron and copper > losses of the two transformers. A poor efficiency is about 95% so with two > transformers, back to back, you might expect 10% losses resulting in 1.10 > amps. > > A low voltage, high current secondary is commonly wound with heavier wire > so it can stand more current and presents a much lower dc resistance. > When it is driven, as when the windings are reversed, it will run just > fine. > > >>2 amps at 120 volts = 240VA and that HAS to be *continuously supplied* >>by the first tranny in the pair. >> > > > Wrong. Only the current to supply the 'no load losses' needs to be supplied > 'continuously'. > > Under the 500VA load, the primary will present a 28.8 ohm load to the > 110VAC line, drawing 4.2 amps of current while the back to back 12 volt > windings will be carrying 42 amps of current. > > The no load losses will be quite low because the primary presents an > essentially pure inductive load rather than a resistive load to the ac > line. Only the core losses, on the order of .25 to .5% of the rating will > need to be supplied. > > http://findarticles.com/p/articles/mi_m0BPR/is_10_21/ai_n6259812/ > > Under no load, the 12 volt windings would see a 'parasitic current' on the > order of 0.02 Amps and the 110 V primary would see a current on the order > of 0.1 Amp and present a 'resistive component' of the load of 115 ohms to > the supply lines. > > >>So, the result is that the pair of trannys can only deliver half the VA >>into the load that one is capable of. >> > > > The combo should be able to supply close to the rated VA of > _a_single_transformer. > > You will have ~twice the losses and both transformers will reach the > temperature that one would have reached. > > So, de-rate the pair of 500VA back to back transformers to 450VA to give > yourself plenty of safety margin and don't seal them into an airtight box > together, and you should be just fine. > > >>.... Phil > > > Best Regards > > > Nice vanity call you have there Mr. N5BZ.. For some reason I have a vision of seeing you on slow scan?
From: Phil Allison on 28 Dec 2009 18:37 "bz" ** I'll give this very confused radio ham just one try. >> ** That is a very unwise suggestion: > > I am sorry to have to say this, but that statement is wrong. ** You are a brave man - and a very foolish one too. >> AC supply transformers are NOT intended to be operated in reverse - cos >> the magnetising current is intended to be carried by the supply side >> winding and NOT the secondary. > > The magnetic core doesn't care which winding induces the magnetic field. ** Not relevant. The issue is the magnetising CURRENT !! >> A standard 500VA transformer operating from 120 VAC may well draw 1.5 >> amps with no load - no problem since as the primary winding has only >> about 0.6 ohms of resistance and hence loses only 1.35 watts in heat. > > Slightly over 1% loss. ** Nonsense. The power loss is mostly from the iron core when there is no load. The previously mentioned 1.5 amps of magnetising current is inversely proportional to the number of turns on the core. > The iron core losses will far exceed that. > > Shouldn't. ** Fraid it does - pal. >> However, if you try to make it work in reverse to deliver 120 volts at >> 500VA from the primary - things get nasty. > > No. ** Fraid they do get nasty. Even if nobody ever told you about it. > Let us assume that the secondary of T1 is 12 vac. > To deliver 500VA at 12 volts, the secondary is going need to see a 0.288 > ohm load and will deliver 41.7 amps to the load. The DC resistance of the > winding will be much lower than the load resistance, on the order of 0.03 > ohms for an inefficient transformer. > >> First, the secondary will have to be fed with a ** higher voltage** >> than the **off load** voltage by about 4% to cover voltage drop under >> load. >> >> Naturally this increases the previously mentioned magnetising current >> level by about 30%. > > The identical but reversed 2nd transformer 'expects' that higher voltage > and exactly compensates for it. ** You have failed to see the issue of transformer "regulation", ie the *off load* and *on load* secondary voltages are different - the voltage always drops when load is applied. Ohms Law you know. All transformers are wound so as to give the desired secondary voltage/s when " on load" - ie the turns ratio is adjusted to compensate for the regulation factor. >> So now it is say 2 amps, referred to the primary. >> > > Wrong. ** Fraid it is quite true - pal. > If the output of the second transformer is carrying 1 amp, ** The discussion is still about the no load situation. You are miles away from any understanding of the issue. >> 2 amps at 120 volts = 240VA and that HAS to be *continuously supplied* >> by the first tranny in the pair. >> > > Wrong. ** Fraid it is correct - pal. > Only the current to supply the 'no load losses' needs to be supplied > 'continuously'. ** That is exactly what I said. Taking your example of a 12 volt secondary, the magnetising current drawn when used in reverse is 10 times that when used the normal way. So, instead of 1.5 or 2 amps of current - it is 15 or 20 amps. Transformers are always rated in VA rather than watts - cos it is possible to * fully load * a transformer with capacitance or inductance while drawing no real power. IOW - once the secondary *current rating* is reached, the game is up. > The no load losses will be quite low because the primary presents an > essentially pure inductive load rather than a resistive load to the ac > line. Only the core losses, on the order of .25 to .5% of the rating will > need to be supplied. > > http://findarticles.com/p/articles/mi_m0BPR/is_10_21/ai_n6259812/ ** How hysterically funny !!!!!!!!!! The radio ham has quoted a page that discusses megawatt transformers the size of houses. Then he blithely assumes all transformers have the same characteristics as these. Wot a hoot. ...... Phil
From: mike on 28 Dec 2009 22:10
sbnjhfty wrote: > I'm looking for an isolation transformer for repairing small (less > than 1000W) power supplies. I see loads of them on ebay and some are > medical grade and inexpensive. Are these usable for what I want? > Other than that, any tips on where to get a good deal on one? I > don't want to spend a load as this is hobby work only. You can certainly come up with a reason to have an isolation transformer. But for most things, it's just a false sense of safety/security that will let you convince yourself that it is reasonable to do VERY UNSAFE things. What matters is the DIFFERENTIAL voltage between two points. If you float the mains, you still have that differential and have gained nothing. To be safe, you still have to put both hands in your pockets and leave them there. So, you're bored standing there with your hands in your pockets. Let's do some unsafe things! Let's make a measurement with the oscilloscope. So, you grab the probe. Hey, what's this black wire dangling from the probe? Let's hook it "here" in the circuit. Doesn't matter where "here" is, you've now got a NOT FLOATING power supply. What's worse, the normally isolated secondary side may have a lot of common-mode volts on it. You're at risk of a shock...AND...simultaneously blowing up your scope and anything you're using for a load. Well, we can't have that. Let's cut the ground pin on the scope power cord. That'll fix it...wonder if they have WiFi in heaven so I can report my progress??? If you want to work on power supplies, get yourself an isolated scope probe. Or a portable scope with no metal parts designed for that type of measurement. Tektronix A6902 probe works well. I picked up mine at a garage sale for a buck. I suspect they're substantially more from a dealer. Using a DVM with well-insulated probes, you can tell if the diodes are open or input storage cap is defective or there's volts on the fet. Beyond that, you really need more than an isolation transformer if you expect to do it safely. mike |