isolation transformer needed
in [Repair]
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From: Phil Allison on 28 Dec 2009 23:29 "mike the madman " > You can certainly come up with a reason to have an isolation transformer. ** Like being able to safety ground the chassis, speakers and tuner box of a TV set that otherwise all sit at lethal voltages. > But for most things, it's just a false sense of safety/security ** Yep - never use one if the unit already has one. > What matters is the DIFFERENTIAL voltage between two points. > If you float the mains, you still have that differential > and have gained nothing. ** Pure insanity. > 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. ** Any sane tech will simply ground the common ( negative line ) in the high voltage part of the PSU. Then proceed as usual for units that have internal isolation trannys. > What's worse, the normally isolated secondary side > may have a lot of common-mode volts on it. ** Totally insane BOLLOCKS. > Well, we can't have that. Let's cut the ground pin on the scope power > cord. ** Something only rabid lunatics like mike do. Yawnnnnnnnnnnn........ > 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. ** Used examples sell for around $US1000 through on-line dealers. More utter insanity. > I suspect they're substantially more from a dealer. ** Wot a posturing, bullshit artist. > 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. ** Wot an obvious pile of crapology. Typical of mug, 5V engineers way out of their depth. ...... Phil
From: mike on 29 Dec 2009 04:14 Phil Allison wrote: > "mike the madman " > >> You can certainly come up with a reason to have an isolation transformer. > > > ** Like being able to safety ground the chassis, speakers and tuner box of a > TV set that otherwise all sit at lethal voltages. > >> But for most things, it's just a false sense of safety/security > > > ** Yep - never use one if the unit already has one. > > >> What matters is the DIFFERENTIAL voltage between two points. >> If you float the mains, you still have that differential >> and have gained nothing. > > ** Pure insanity. > > >> 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. > > ** Any sane tech will simply ground the common ( negative line ) in the high > voltage part of the PSU. > > Then proceed as usual for units that have internal isolation trannys. > > >> What's worse, the normally isolated secondary side >> may have a lot of common-mode volts on it. > > ** Totally insane BOLLOCKS. > > >> Well, we can't have that. Let's cut the ground pin on the scope power >> cord. > > > ** Something only rabid lunatics like mike do. > > Yawnnnnnnnnnnn........ > > >> 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. > > ** Used examples sell for around $US1000 through on-line dealers. > > More utter insanity. > > >> I suspect they're substantially more from a dealer. > > ** Wot a posturing, bullshit artist. > > >> 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. > > ** Wot an obvious pile of crapology. > > Typical of mug, 5V engineers way out of their depth. > > > ..... Phil > > Creative snipping eh?
From: Andy on 29 Dec 2009 07:29 On Dec 2, 12:03 pm, zekfr...(a)zekfrivolous.com (GregS) wrote: > In article <50c36de762d...(a)davenoise.co.uk>, "Dave Plowman (News)" <d...(a)davenoise.co.uk> wrote: > > > > >In article <hf5svh$5g...(a)usenet01.srv.cis.pitt.edu>, > > GregS <zekfr...(a)zekfrivolous.com> wrote: > >> In article <%llRm.94141$gg6.27...(a)newsfe25.iad>, sbnjhfty > > <fgsdf...(a)asfffwer.com> 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/model.cfm?txtSeriesID=325&EID=13... > Also look throughhttp://www.tripplite.com/en/keyword-search.cfm?q=isolation%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 On an isolation transformer (single phase), the primary side is always grounded because the neutral leg is always bonded in the distribution panel. This is a code requirement. If the secondary is also grounded (I am pretty sure this is also a requirement), the two sides are still electrically isolated. The current leaving the secondary hot leg must return to the neutral (grounded) leg of the secondary. There is no path from the secondary hot to the primary neutral. The secondary has to leave the hot leg and return on the secondary neutral leg. The secondary current cannot go any where except through secondary loop. Any noise on the high side (from nearby VFDs or other) must pass through the primary winding in order to induce a current onto the secondary. The winding itself is a low pass filter, so much of the noise on the primary is dissipated as heat.
From: Dave Plowman (News) on 29 Dec 2009 19:23 In article <hhbs06$hvi$1(a)news.eternal-september.org>, mike <spamme0(a)go.com> wrote: > You can certainly come up with a reason to have an isolation transformer. > But for most things, it's just a false sense of safety/security > that will let you convince yourself that it is reasonable to do > VERY UNSAFE things. > What matters is the DIFFERENTIAL voltage between two points. > If you float the mains, you still have that differential > and have gained nothing. To be safe, you still have to > put both hands in your pockets and leave them there. If your mains supply has one side grounded, then touching the 'live' side causes a shock. If you use an isolating transformer, you can touch either leg safely. That plus an RCD feeding it provides the best degree of safety in the workshop. I'm surprised you don't see this. -- *Wrinkled was not one of the things I wanted to be when I grew up Dave Plowman dave(a)davenoise.co.uk London SW To e-mail, change noise into sound.
From: mike on 30 Dec 2009 01:16
Dave Plowman (News) wrote: > In article <hhbs06$hvi$1(a)news.eternal-september.org>, > mike <spamme0(a)go.com> wrote: >> 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. > > If your mains supply has one side grounded, then touching the 'live' side > causes a shock. If you use an isolating transformer, you can touch either > leg safely. That plus an RCD feeding it provides the best degree of safety > in the workshop. I'm surprised you don't see this. > You amaze me. You're so bent on proving me wrong that you are not open to the possibility that applying logic to misguided advice might yield useful information. Normally, I'd not waste my time trying to train internet denizens. But in cases where a plethora of misguided advice presented vehemently and with AUTHORITY puts people at risk of DEATH, I feel compelled to speak up. This stuff hangs around for years. So, back to your statement...with logic... If you "touch" only one thing, it doesn't matter whether you have an isolation transformer or not. Problem is that touching one thing provides no information beyond what you could get using an insulated tool. You really don't need to "touch" anything. Electricity finds it's way to touch you through paths you didn't anticipate. How many of you woke up on the morning of your electrocution and said, "today, I think I'll electrocute myself"? Raise your hands high...oh, those of you who committed suicide by electrocution can put your hands down. This is the important part...my contribution to the thread... Pay attention now: The primary reason to want an isolation transformer to troubleshoot a power supply is to work on the primary side. Now, the KEY word is TROUBLESHOOT. What that means is that the power supply has a FAULT in the primary circuit. What's the fault? You don't know, or you'd just fix it. How safe is it to troubleshoot that particular fault? You don't know, 'cause you don't know what it is. You're gonna RISK ELECTROCUTION based on the misguided assumption that an isolation transformer keeps you safe. YOU'RE NOT SAFE. YOU DON'T HAVE ANY IDEA WHAT THE PRIMARY CIRCUIT IS, BECAUSE IT'S NOT AS DESIGNED. IT HAS A FAULT!!!!!!! The node that the designer called common may not be common at all. IT HAS A FAULT!!!! You should not arbitrarily ground ANY node. It has a fault!!! (I'm skipping over the obvious question, "what is ground anyway?") An isolation transformer is not inherently bad. It can provide a layer of protection. What is bad is the FALSE sense of security that the transformer makes it safe to poke around inside a supply WITH A PRIMARY FAULT. I'll say it one more time. I don't care if you design power supplies in your sleep. A FAILED power supply DOES NOT have the circuit that was intended. It has a FAULT. A safety analysis based on a working power supply and an isolation transformer DOES NOT APPLY. Maybe you've always been lucky. That's no cause to suggest to another that they can do what you've been doing and NOT DIE. |