isolation transformer needed
in [Repair]
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From: GregS on 4 Dec 2009 09:45 In article <50c415e195dave(a)davenoise.co.uk>, "Dave Plowman (News)" <dave(a)davenoise.co.uk> wrote: >In article ><8122d63d-b1ef-41f7-bf49-24e4d16c165c(a)g12g2000yqa.googlegroups.com>, > whit3rd <whit3rd(a)gmail.com> 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. >
From: GregS on 4 Dec 2009 10:30 In article <%llRm.94141$gg6.27160(a)newsfe25.iad>, sbnjhfty <fgsdfgrr(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. 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
From: David Lesher on 26 Dec 2009 21:45 sbnjhfty <fgsdfgrr(a)asfffwer.com> 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 coast.................wb8foz(a)nrk.com & 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
From: Phil Allison on 26 Dec 2009 22:36 "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
From: David Lesher on 27 Dec 2009 20:19
"Phil Allison" <phil_a(a)tpg.com.au> 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(a)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 coast.................wb8foz(a)nrk.com & 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 |