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isolation transformer needed
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. |
isolation transformer needed
On Dec 1, 9:41*pm, 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. Do you want to power the 1000 watt supplies with the isolation transformer, or use the transformer to power your soldering iron that you use to repair the supplies? |
isolation transformer needed
On Dec 1, 9:41*pm, 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. Do you want to power the 1000 watt supplies with the isolation transformer, or use the transformer to power your soldering iron that you use to repair the supplies? |
isolation transformer needed
sbnjhfty wrote in message
... 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. Try builders supplier for "site transformer" and also get a variac and a RCCD/ELCB/GFCI . -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
isolation transformer needed
In article ,
N_Cook wrote: sbnjhfty wrote in message ... 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. Try builders supplier for "site transformer" and also get a variac and a RCCD/ELCB/GFCI . Site transformers (UK) are 240 to 110 volt. And you pay for the rugged box and outlets - not needed in the workshop. I'm not sure if a similar thing is used in the US. Where I suppose the OP is. Do wish people gave their location when asking about this sort of thing. ;-) A good quality 1000va isolating transformer isn't going to be cheap new. -- *How many roads must a man travel down before he admits he is lost? * Dave Plowman London SW To e-mail, change noise into sound. |
isolation transformer needed
In article , 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. Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection. greg |
isolation transformer needed
In article ,
GregS wrote: In article , 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. Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection. That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer? -- *I'm already visualizing the duct tape over your mouth Dave Plowman London SW To e-mail, change noise into sound. |
isolation transformer needed
Dave Plowman (News) wrote:
GregS wrote: Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection. That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer? Seems to be a US safety requirement to ground one leg. http://en.wikipedia.org/wiki/Isolation_transformer I've got here a Farnell 500W 240V/240V isolating transformer that looks and weighs similar to a typical building site transformer, the outputs are floating. -- Adrian C |
isolation transformer needed
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isolation transformer needed
In article ,
Adrian C wrote: Dave Plowman (News) wrote: GregS wrote: Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection. That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer? Seems to be a US safety requirement to ground one leg. http://en.wikipedia.org/wiki/Isolation_transformer Interesting. I've got here a Farnell 500W 240V/240V isolating transformer that looks and weighs similar to a typical building site transformer, the outputs are floating. Indeed. They used to be very common for feeding things like electric guitars in a TV studio, etc. And grounding one leg of the secondary would negate the whole purpose of them. These days it's more usual to do a quick PAT (test) and supply the mains via an RCD. -- *It's this dirty because I washed it with your wife's knickers* Dave Plowman London SW To e-mail, change noise into sound. |
isolation transformer needed
In article , "Dave Plowman (News)" wrote:
In article , GregS wrote: In article , 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. Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection. That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer? I have been thinking about it. The MAIN purpose of an isolation transformer is to make it non isolated by making a NEW neutral close to the device being powered. The main purpose of an isolation transformer is noise control. Now we have us tecks who all our lives have learned isolation and that stuff. I taliked to the people at Tripplite and verified ALL their isolation transformers ARE grounded. Read the paragraph, and yet it says above, "Complete line Isolation" http://www.tripplite.com/en/products...txtModelID=230 Also look through http://www.tripplite.com/en/keyword-...%20transformer The isolator you buy from MCM is very unlikely to have the secondary grounded, but I really can't verify that. In reading PC Power Protection by Mark Waller, he pointed out their is a NEC requirment for this grounding. I have not found an exact description in the code. There is about 75 references in that book, but no index marks. Man, I should sell that book !!!! Big Bucks. greg |
isolation transformer needed
In article , Adrian C wrote:
Dave Plowman (News) wrote: GregS wrote: Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection. That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer? Seems to be a US safety requirement to ground one leg. http://en.wikipedia.org/wiki/Isolation_transformer I just put that there. it was not there this morning. I've got here a Farnell 500W 240V/240V isolating transformer that looks and weighs similar to a typical building site transformer, the outputs are floating. Thats probably true, and correct. its up to the contractor to use it in the right way. greg |
isolation transformer needed
On Tue, 01 Dec 2009 22:41:38 -0500, 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. Spending too little isn't cost effective either if you manage to fry a scope probe or other piece of test gear. Still, the 30 KVa three phase is serious overkill for your needs. Give a little thought as to how you are going to be using this. If you want to isolate the input side of the power supply from the line so you can look at the control ICs a relatively low capacity isolation transformer will suffice. If you want to isolate 1000 watt power supplies while testing at full load, you'd better have an isolation transformer rated at something over 1000VA. I would say something rated at 250 VA is the minimum you want. The medical isolation transformers or the BK Precision TR110 would be better. PlainBill |
isolation transformer needed
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isolation transformer needed
Adrian C wrote:
wrote: Give a little thought as to how you are going to be using this. If you want to isolate the input side of the power supply from the line so you can look at the control ICs a relatively low capacity isolation transformer will suffice. If you want to isolate 1000 watt power supplies while testing at full load, you'd better have an isolation transformer rated at something over 1000VA. I would say something rated at 250 VA is the minimum you want. The medical isolation transformers or the BK Precision TR110 would be better. PlainBill Could perhaps the figure of 1000W be taken from what might be repaired as a hobby interest - surround sound home theater amplifiers, where the actual power draw from the mains is considerably less than that hyped 1000W - and if it's that item, ye wouldn't really (if you value your ears) be testing at full load/volume anyway after fixing something that perhaps failed after moderate use. A good source of pretty large 1 to 1 isolation trannies is old slot machines. I used to be in that business and still have several. Find a company who rents out gaming machines and ask, they scrap lots of obsolete machines. On the subject of earthing (grounding) the transformer frame and (if metal) the enclosure is earthed, but neither side of primary or secondary. Ron(UK) |
isolation transformer needed
Dave Plowman (News) wrote:
In article , GregS wrote: In article , 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. Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection. That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer? My bench Isolation transformer, 2KW rating, has a 240v primary with a centre tapped secondary that has provision to tie the centre tap to the case which is also earthed. FWIW it weighs about 100 LB its also marked as (Noise Suppressing) ! -- Best Regards: Baron. |
isolation transformer needed
In article ,
GregS wrote: That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer? I have been thinking about it. The MAIN purpose of an isolation transformer is to make it non isolated by making a NEW neutral close to the device being powered. The main purpose of an isolation transformer is noise control. Now we have us tecks who all our lives have learned isolation and that stuff. I taliked to the people at Tripplite and verified ALL their isolation transformers ARE grounded. Read the paragraph, and yet it says above, "Complete line Isolation" http://www.tripplite.com/en/products...txtModelID=230 Also look through http://www.tripplite.com/en/keyword-...%20transformer The isolator you buy from MCM is very unlikely to have the secondary grounded, but I really can't verify that. It could be down to how power is distributed in our different countries. Effectively in the UK as regards power outlets there is only one voltage (240v) and all will be on the same phase. And the neutral is at ground potential. Without any form of protection, touching the neutral is safe. Touching the live isn't. With an isolating transformer, you can touch *either* safely, as there is no potential to ground. The only power outlet allowed in a UK bathroom is a shaver socket - and that is fed via an isolating transformer. -- *Aim Low, Reach Your Goals, Avoid Disappointment * Dave Plowman London SW To e-mail, change noise into sound. |
isolation transformer needed
On Dec 1, 7:41*pm, 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? Yes, they are. In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, and get the same kind of result. The best isolation transformers are made for RF passthrough protection, or medical equipment uses, and will work well for the less-demanding electronics workbench even when they're 40 years old... so buying a used one makes sense. |
isolation transformer needed
On Wed, 2 Dec 2009 17:13:20 -0800 (PST), whit3rd
wrote: Yes, they are. In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, Do they make plug-in GFI's? Like they make plug in surge supressors and plug-in adapaters from 3 pin plugs to 2 slot receptacles, it should be easy to make a plug-in, point of use GFI, but I havent' come across one. |
isolation transformer needed
On Tue, 01 Dec 2009 22:41:38 -0500, in sci.electronics.repair you
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. Now you've got me going and I want one too, but the biggest I saw today was 385 va, and it's listed as new, not surplus (which is what I thought you meant. They're 23 or 30 dollars plus 12 dollars shipping) I see one for 8 dollars but it's only 100va. Maybe I misunderstand your requirements, or my own :) . I'll wait as long as you tell me to, and I'm not going to bid against you, but you say there are loads of them. Can you give me a url or item number so I can see what you're talking about. I'd appreciate it. Mike |
isolation transformer needed
"sbnjhfty" wrote in message ... 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. So what do you want it for, to cut down on noise when you contact the "live" when working on the PSU or to avoid shocks whilst working on the gear ?? Regards......... Rheilly P |
isolation transformer needed
Rheilly Phoull wrote:
"sbnjhfty" wrote in message ... 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. So what do you want it for, to cut down on noise when you contact the "live" when working on the PSU or to avoid shocks whilst working on the gear ?? Regards......... Rheilly P To avoid shocks. |
isolation transformer needed
mm wrote in
: On Wed, 2 Dec 2009 17:13:20 -0800 (PST), whit3rd wrote: Yes, they are. In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, Do they make plug-in GFI's? Like they make plug in surge supressors and plug-in adapaters from 3 pin plugs to 2 slot receptacles, it should be easy to make a plug-in, point of use GFI, but I havent' come across one. I have a 'short extension cord with a built in GFI'. I don't remember where I bought it but you should be able to find such. A google search for extension cord with GFI brings a bunch of hits. -- bz please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. |
isolation transformer needed
whit3rd wrote:
On Dec 1, 7:41 pm, 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? Yes, they are. In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, and get the same kind of result. This must probably again be a voltage US/UK thing - or maybe me - but if I was working on 240V live equipment I'd rather it was ALL floating through an isolation transformer than rely on the complexities and effectiveness of a GFI/RCD to prevent shocks at the bench. -- Adrian C |
isolation transformer needed
sbnjhfty wrote:
Rheilly Phoull wrote: So what do you want it for, to cut down on noise when you contact the "live" when working on the PSU or to avoid shocks whilst working on the gear ?? To avoid shocks. In which country are you? -- Adrian C |
isolation transformer needed
Adrian C wrote:
sbnjhfty wrote: Rheilly Phoull wrote: So what do you want it for, to cut down on noise when you contact the "live" when working on the PSU or to avoid shocks whilst working on the gear ?? To avoid shocks. In which country are you? USA |
isolation transformer needed
On Wed, 02 Dec 2009 19:35:30 +0000, Adrian C
wrote: wrote: Give a little thought as to how you are going to be using this. If you want to isolate the input side of the power supply from the line so you can look at the control ICs a relatively low capacity isolation transformer will suffice. If you want to isolate 1000 watt power supplies while testing at full load, you'd better have an isolation transformer rated at something over 1000VA. I would say something rated at 250 VA is the minimum you want. The medical isolation transformers or the BK Precision TR110 would be better. PlainBill Could perhaps the figure of 1000W be taken from what might be repaired as a hobby interest - surround sound home theater amplifiers, where the actual power draw from the mains is considerably less than that hyped 1000W - and if it's that item, ye wouldn't really (if you value your ears) be testing at full load/volume anyway after fixing something that perhaps failed after moderate use. Well, the OP said 'Power supplies'. I have a bad habit of taking people at their word. The last time I worked on an amplifier I noted the power supply was an classic 'heavy iron' design, with the AC line feeding a transformer. Still, I wouldn't be surprised to find a SMPS in something rated at a pretend 1000 watts. IMHO, it's a fool who repairs a power supply, then does not test it with a dummy load BEFORE hooking it up to it's intended load. And again, the OP has to decide if he wants an isolation transformer capable of handling the full rated load. Personally, that is always my recommendation. It avoids nasty surprises when one is in a hurry to test and forgets to switch the power cord to an isolated source. PlainBill |
isolation transformer needed
On Dec 2, 8:55*pm, mm wrote:
On Wed, 2 Dec 2009 17:13:20 -0800 (PST), whit3rd wrote: Yes, they are. *In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, Do they make plug-in GFI's? Yes, of course; more important, one can get a GFI socket and wire it into a handy box (or a string of boxes with multiple plugs) and make a power distribution panel. There are (expensive, alas) sockets that offer 'isolated ground' so that the metal case(s) of the distribution panel can be grounded while the special sockets can be floated. Thus, you can wire sockets with ground-optional to the isolation transformer and have only the distribution panel on the workbench (the transformer can live on the floor or wherever). |
isolation transformer needed
On Dec 3, 6:20*am, Adrian C wrote:
whit3rd wrote: ... *In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, This must probably again be a voltage US/UK thing - or maybe me - but if I was working on 240V live equipment I'd rather it was ALL floating through an isolation transformer than rely on the complexities and effectiveness of a GFI/RCD to prevent shocks at the bench. Working on live 120V or 240V, you can always touch two points and get a shock. Either an isolation transformer, or a GFI interrupter, prevents the shock if one of those points is GROUND. I'm not sure why you would distrust a GFI, but they have a test button. Use it, and be reassured. The main plan, always, is not to touch the live wires. |
isolation transformer needed
In article
, whit3rd wrote: This must probably again be a voltage US/UK thing - or maybe me - but if I was working on 240V live equipment I'd rather it was ALL floating through an isolation transformer than rely on the complexities and effectiveness of a GFI/RCD to prevent shocks at the bench. Working on live 120V or 240V, you can always touch two points and get a shock. Either an isolation transformer, or a GFI interrupter, prevents the shock if one of those points is GROUND. If one of those points is ground touching the other gives you a shock. If neither is ground you can touch either one without a shock. The whole principle of using an isolating transformer for safety. Makes for better odds. I'm not sure why you would distrust a GFI, but they have a test button. Use it, and be reassured. The main plan, always, is not to touch the live wires. That is the best safety advice. But have a backup plan for if you do. -- *A snooze button is a poor substitute for no alarm clock at all * Dave Plowman London SW To e-mail, change noise into sound. |
isolation transformer needed
On Thu, 3 Dec 2009 13:57:06 -0800 (PST), whit3rd
wrote: On Dec 2, 8:55*pm, mm wrote: On Wed, 2 Dec 2009 17:13:20 -0800 (PST), whit3rd wrote: Yes, they are. *In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, Do they make plug-in GFI's? Yes, of course; more important, one can get a GFI socket and wire it into a handy box (or a string of boxes with multiple plugs) and make a power distribution panel. There are (expensive, alas) sockets that offer 'isolated ground' so that the metal case(s) of the distribution panel can be grounded while the special sockets can be floated. Thus, you can wire sockets with ground-optional to the isolation transformer and have only the distribution panel on the workbench (the transformer can live on the floor or wherever). Very good. Thanks to you and bz. |
isolation transformer needed
In article , "Dave Plowman (News)" wrote:
In article , whit3rd wrote: This must probably again be a voltage US/UK thing - or maybe me - but if I was working on 240V live equipment I'd rather it was ALL floating through an isolation transformer than rely on the complexities and effectiveness of a GFI/RCD to prevent shocks at the bench. Working on live 120V or 240V, you can always touch two points and get a shock. Either an isolation transformer, or a GFI interrupter, prevents the shock if one of those points is GROUND. If one of those points is ground touching the other gives you a shock. If neither is ground you can touch either one without a shock. The whole principle of using an isolating transformer for safety. Makes for better odds. Its also necesary to make certain differential oscilloscope measurments if you don't have a differential plug-in. I also tend to use a battery scope for that instead. greg I'm not sure why you would distrust a GFI, but they have a test button. Use it, and be reassured. The main plan, always, is not to touch the live wires. That is the best safety advice. But have a backup plan for if you do. |
isolation transformer needed
In article , 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. One thing you don't hear mentioned too much about is voltage. We all know transformers are rated for a voltage with load. The same thing can apply to an isolation transformer. i have ordered custom wired transformers, and its NOT 1:1 with no load. They increase the voltage to make up for the loss. In some applications the voltage may be too high after this. Good to use a variac along with an isolation transformer and monitor voltage and current. greg |
isolation transformer needed
sbnjhfty writes:
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. Try all the usual surplus suspects. Of course the secondary power rating will be the limiting factor. -- A host is a host from coast to & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 |
isolation transformer needed
"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: 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. 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. The iron core losses will far exceed that. However, if you try to make it work in reverse to deliver 120 volts at 500VA from the primary - things get nasty. 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%. So now it is say 2 amps, referred to the primary. 2 amps at 120 volts = 240VA and that HAS to be *continuously supplied* by the first tranny in the pair. So, the result is that the pair of trannys can only deliver half the VA into the load that one is capable of. ..... Phil |
isolation transformer needed
"Phil Allison" writes:
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. I recall doing it with a pair of plate transformers [600V at 200-300 mA]; and later with some weird 60V@6A ones. The idle currents were not egregious, and given the broad range of "120 volt line" the loss unobtrusive. It was surely safer than working on hot-chassis equipment without same. -- A host is a host from coast to & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 |
isolation transformer needed
neet"Phil Allison" wrote in
: "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/m...1/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. |
isolation transformer needed
bz wrote:
neet"Phil Allison" wrote in : "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/m...1/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? |
isolation transformer needed
"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/m...1/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 |
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