isolation transformer needed

To avoid shocks.

mm wrote in news: snipped-for-privacy@4ax.com:

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.

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

In which country are you?

--
Adrian C

USA

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

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).

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.

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.

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        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

Very good. Thanks to you and bz.

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

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

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@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

"David Lesher"

** 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

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 coast.................wb8foz@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

"David Lesher" "Phil Allison"

** 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

neet"Phil Allison" wrote in news: snipped-for-privacy@mid.individual.net:

I am sorry to have to say this, but that statement is wrong.

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'.

The magnetic core doesn't care which winding induces the magnetic field.

At what phase? With no load, it is NOT in phase. It DOES cause IR losses in the primary, however.

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.

Shouldn't. The iron core losses are PART of the total losses seen.

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.

The identical but reversed 2nd transformer 'expects' that higher voltage and exactly compensates for it.

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.

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.

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.

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.

Best Regards

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

Nice vanity call you have there Mr. N5BZ..

For some reason I have a vision of seeing you on slow scan?

"bz"

** I'll give this very confused radio ham just one try.

** You are a brave man - and a very foolish one too. ** Not relevant.

The issue is the magnetising CURRENT !!

** 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.

** Fraid it does - pal.

** Fraid they do get nasty.

Even if nobody ever told you about 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.

** Fraid it is quite true - pal.

** The discussion is still about the no load situation.

You are miles away from any understanding of the issue.

** Fraid it is correct - pal.

** 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.

** 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