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Transformer Question

Started by Unknown February 2, 2017
John Larkin wrote:

> > > A dial-primary (120:240 to whatever) transformer makes a good > isolation transformer. Use the two primaries and ignore the secondary, > unless you have some use for it too. >
** Fraid that idea is not safe to recommend - as the two primaries may well be bi-filar wound. That is not adequate insulation for a mains isolation tranny, it could not pass safety standards anywhere. ..... Phil
dca...@krl.org wrote:

> > If you are making a 1:1 transformer using a couple of MWT's > ( MicroWave Tronsformer ), You do not use the secondary windings. >
** THAT is a COMPLETELY DIFFERENT story to what you posted previously. YOU need to be WAY more careful with what you post !!!!!!!!!!!!!!! W what you posted before was DANGEROUS advice - anyone reading it would think you meant to use the two transformers as they were. You need to go back and retract that post NOW. --------------------------------------------- .... Phil
On Saturday, February 4, 2017 at 11:58:11 AM UTC-8, tabb...@gmail.com wrote:
> On Saturday, 4 February 2017 18:30:34 UTC, John Larkin wrote: > > On Sat, 4 Feb 2017 09:32:13 -0800 (PST), tabbypurr wrote: > > >On Saturday, 4 February 2017 17:16:45 UTC, John Larkin wrote: > > > >> A dial-primary (120:240 to whatever) transformer makes a good > > >> isolation transformer. Use the two primaries...
> > >as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other?
> > Kilovolts, probably.
> that's at odds with my experience of hipot testing transformers. Enamel on enamel failed below 1kV. Unfortunately I don't remember what voltage it managed. Probably over 500v, but that's not a comfortable margin.
> > It will have more inter-winding capacitance than one of those > > extreme-expense isolation things. > > Yes. I don't know how much shock current that lets through.
Real isolation transformers are tested (briefly) at 3 to 5 kV (like, there might be a surge...) So, loosen the wallet and get one. It only hurts once. **note, Tripp-Lite sells "isolation transformers" with the neutral of the secondary bonded to ground/case. That is NOT what you want, if you might be poking into a live circuit. If the specs mention "Neutral to ground bonding at the secondary" it means you can get a shock from isolated HOT to ground.
On Sat, 4 Feb 2017 19:59:52 -0800 (PST), Phil Allison
<pallison49@gmail.com> wrote:

>John Larkin wrote: > >> >> >> A dial-primary (120:240 to whatever) transformer makes a good >> isolation transformer. Use the two primaries and ignore the secondary, >> unless you have some use for it too. >> > >** Fraid that idea is not safe to recommend - as the two primaries may well be bi-filar wound.
They can stand 120 and 240 and transients reliably already. It takes a huge voltage to punch through enameled magnet wire.
> >That is not adequate insulation for a mains isolation tranny, it could not pass safety standards anywhere.
A one-off home project doesn't need to pass safety standards. -- John Larkin Highland Technology, Inc lunatic fringe electronics
John Larkin wrote:

> > > > >Relying on testing insulation is venturing on a slippery slope > > > > > I assume the op wants to hack one isolation transformer for personal > use. I don't think he intends to get UL or VDE certification. >
** But a single point failure of the enamel, likely from excess pressure, a defect or continuous corona discharge over time would make the unit lethal. An isolation transformer for bench work on live electronics is a SAFETY device, protecting human life and it is not rational to make or use a hazardous one. Rational advice is not your forte - is it ? Cos you openly treat NG posting as a joke. .... Phil
John Larkin wrote:

> > >> > >> > >> A dial-primary (120:240 to whatever) transformer makes a good > >> isolation transformer. Use the two primaries and ignore the secondary, > >> unless you have some use for it too. > >> > > > >** Fraid that idea is not safe to recommend - as the two primaries may well be bi-filar wound. > > They can stand 120 and 240 and transients reliably already. It takes a > huge voltage to punch through enameled magnet wire. > > > > >That is not adequate insulation for a mains isolation tranny, it could not pass safety standards anywhere. > > A one-off home project doesn't need to pass safety standards. >
** Fraid that is a complete irrelevance. The OP needs the isolation to be there to protect him from electrocution. Your attitude is *criminally irresponsible*. Par for the course for an utter asshole that believes NG posting is a joke. ..... Phil
whit3rd wrote:

> > > Real isolation transformers are tested (briefly) at 3 to 5 kV >
** Which is not proof of an overall safe design. Things like clearances, creepage distances, amount and type of insulation and having thermal fuses built in are *crucial* to achieving acceptable safety. The topic is not suitable for arrogant fools like Larkin. .... Phil
dca...@krl.org wrote:

> > > Don't most dual primary transformers have a layer of insulation between > the two primaries? > >
** Mostly they are bifilar wound. When used in 240V mode, this means there is 120VAC between adjacent wires over the whole length of the primary. A tiny manufacturing error ( like a kink in the wire) or defect in the enamel leads to early failure of such transformers - then they will normally blow fuses at switch on. It is convenient for makers to do this cos it saves time & guarantees exact turns identity between primaries. OTOH, transformer makers have been winding centre tapped secondaries and getting them exactly right for ever and a day - this is very important for use with full wave rectifier / capacitor input PSUs so the ripple voltage is only at double the supply frequency and both halves of the secondary share load evenly. .... Phil
On Sat, 4 Feb 2017 20:28:31 -0800 (PST), Phil Allison
<pallison49@gmail.com> wrote:

>John Larkin wrote: > >> >> > >> >Relying on testing insulation is venturing on a slippery slope >> > >> > >> I assume the op wants to hack one isolation transformer for personal >> use. I don't think he intends to get UL or VDE certification. >> > >** But a single point failure of the enamel, likely from excess pressure, a defect or continuous corona discharge over time would make the unit lethal.
120 volts is no big deal. I wire everything hot. And 120v is not going to make corona. 120 volt transformers never arc over, so why would one do that when it's used as an isolation transformer? Don't be such a wuss.
> >An isolation transformer for bench work on live electronics is a SAFETY device, protecting human life and it is not rational to make or use a hazardous one. > >Rational advice is not your forte - is it ?
Silly fears sure aren't. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sat, 4 Feb 2017 21:23:27 -0800 (PST), Phil Allison
<pallison49@gmail.com> wrote:

>dca...@krl.org wrote: > >> >> >> Don't most dual primary transformers have a layer of insulation between >> the two primaries? >> >> > >** Mostly they are bifilar wound. > >When used in 240V mode, this means there is 120VAC between adjacent wires over the whole length of the primary. > >A tiny manufacturing error ( like a kink in the wire) or defect in the enamel leads to early failure of such transformers - then they will normally blow fuses at switch on. > >It is convenient for makers to do this cos it saves time & guarantees exact turns identity between primaries. > >OTOH, transformer makers have been winding centre tapped secondaries and getting them exactly right for ever and a day - this is very important for use with full wave rectifier / capacitor input PSUs so the ripple voltage is only at double the supply frequency and both halves of the secondary share load evenly.
Transformer winding machines count turns exactly. -- John Larkin Highland Technology, Inc lunatic fringe electronics