They can stand 120 and 240 and transients reliably already. It takes a huge voltage to punch through enameled magnet wire.
A one-off home project doesn't need to pass safety standards.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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.** 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
** 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
** 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
** Mostly they are bifilar wound.
When used in 240V mode, this means there is 120VAC between adjacent wires o ver the whole length of the primary.
A tiny manufacturing error ( like a kink in the wire) or defect in the ena mel leads to early failure of such transformers - then they will normally b low 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 ge tting them exactly right for ever and a day - this is very important for us e 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
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.
Silly fears sure aren't.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Transformer winding machines count turns exactly.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Actually, we don't know what he wants it for.
Your attitude is fraidy-cat.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
True for bobbin types. Not so for Chinese specials that are just enamel wire on enamel wire. The slightest scratch...
NT
** I'm sure they do.
But counting errors still happen for other reasons.
Fuckhead.
.... Phil
The last design I ordered from China has a split bobbin (PA66 94V-2 plastic from Dupont or an alternate Chinese supplier, both UL file
UL thermal fuse included.
No quality problems whatsoever.
I've seen some pretty scary stuff from India, not so much from China these days. We're showing them how to make 1st world quality. Of course if you go onto Ali or worse, you might find some shockingly bad product.
--sp
-- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
wire on enamel wire. The slightest scratch...
** The last toroidal tranny I replaced was an 800VA job from China. The pri mary had failed, layer to layer and would blow an AC fuse at switch on.
Some time later, I decided to unwind the tranny, take a look at the damage and get some free copper wire to boot.
This was very tedious as it meant removing the secondary first, lots of pol y tape and then most of the primary to find the problem area. The primary w as bi-filar wound as was the secondary.
I finally found carbonisation of adjacent turns and traces of melted copper concentrated near the inside corner of the core, where winding pressure wo uld naturally accumulate.
After unwinding all the wire, I got down to the steel core where a nasty su rprise was waiting for me.
Unlike any other toroidal I had ever seen, the core was not wound with a si ngle strip of tape nor had its sharp edges covered with plastic caps. About twenty scrap pieces of core material of slightly varying widths had been s pot welded together, end to end, to make the final core. It looked like shi t.
The centre hole was dead round, having been fitted onto a mandrill for wind ing, but the outside diameter varied by 3mm or 4mm around the circumference .
This precluded the use of end caps so the maker has simply ground the sharp edges down a bit by hand and then applied adhesive cloth tape along each o f them. Grinding the inner hole was more difficult, so not done very evenly .
The tranny was bound to fail and I understand a large percentage of them di d.
If you think it must have come fitted inside some flea-bag Chines product - it did not. The tranny was in a Crown Audio (of the USA) professional powe r amplifier model XLS602. See internal pic.
Understandably, I was very reluctant to buy another example and luckily had on hand a suitable toroidal of Italian manufacture that was close enough i n ratings to do the job.
.... Phil
Yeah, got caught by that one once on a Hammond isolation transformer. Plugged a scope into the isolation transformer to float it, hooked up the ground lead of the scope probe and shorted my circuit to ground. As per CSA, the ground pin in the receptacle of an isolation transformer must be connected to ground (which kinda ruins the "isolation" function of the device).
wire on enamel wire. The slightest scratch...
they smile politely, promise quality, and deliver it at first. Then the cor ner cutting happens and you get crap. All nicely polished of course.
The thing with Chinese goods is that while a lot is good, much is lousy qua lity, too much is noncompliant, and the only way to know is a complete tear down.
NT
Partly true
When it comes to the direct winding, yes tape or triple insulated wire is used (beware UL recent rules does not allow for usage of triple insulated wire alone)
But when the wire exit the bobbin and is wound onto the pins clearance and creepage distances comes into play. The wire insulation reduces the requirements for clearance distances
For triple insulated wire the wire must be tested during wire production for 5.5kV, and finding a wire that can comply to that is difficult (I have never found one that does)
Cheers
Klaus
If he does what you say, he is still required to do it according to the standards. What happens if he gets a visitor that is electrocuted?
Cheers
Klaus
wn do you get with the 2 primaries wouund one directly on the other?
the two primaries?
When you don't know, you should not recommend something that is potentially deadly for the OP!
Yes, for a one off test. The reason for the insulation is due to what over voltage category you have for the equipment. For over voltage category II s urge pulses (lightning overvoltage) of several kV is normal, and the insula tion needs to be able to survive that. Also, the wire insulation degrades o ver time, and with temperature
Not really an argument
Cheers
Klaus
Yes, but several kV surges are normal
One other reason for not doing it like that is that during production the wire takes some punishment when winding on the bobbin. The surface can have a lot of minute cracks, which develop over time
Cheers
Klaus
It is legal to test to compliance. See for example EN60335 section 29
But UL and VDE will be in close control of what you are doing
Cheers
Klaus
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