Mains transformer goodness

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There's three issues with PCB oils. First, they're fireproof (a safety plus). Second, the oil can overheat and decompose, into dioxins and other toxins/carcinogens. This, is a safety minus. I've heard of a contaminated animal feed episode that got LOTS of political attention. The third one, is kinda scary: like DDT, the compound persists and spreads in the natural environment, so it can cause problems in future decades or centuries if it isn't collected and neutralized. No one wants to take the risk of causing the next species extinction and/or lawsuit flood.

PCBs are really dangerous stuff. They are carcinogenic, mutagenic bioaccumlative toxins. I must have waded through thousands of pages of technical and medical data on them. The parts i printed stacked up to a couple of inches. .

It ain't necessarily so.

Start with this for current US regulation:

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Anyone care to speculate on the timeline of failure? There was no local hotspot and no lacquer damage more than a mm from the "spot weld", and a bit of very localised smoke staining travelling a cm or so along the affected wires in each direction. Nothing to suggest that the initial bridge was between the 2 bifilar wires of the primary, so not running a 120 volt primary in effect on 240V ac for any time. For 240V use the 2 primaries seriesed to give about 2.8 ohm originally , after failure then about 0.4 ohm. The 2 primaries broke into 5 lengths

9.6,14.6,20.1,20.1 and 25.3m long , measured to about 0.2m accuracy. So originally probably 2 x 45m. Don't know for sure as did not think to check but the weld was probably 20.1m from one end, but I would suggest that bridge occured after an arc to another layer (higher pd) and then localised heating to bridge across to the bifilar fellow wire.

In summary , no evidence that bifilar wiring itself was the reason for failure but more due to the lack of any interlayer insulation. Because of the uneven wire spacing between inside and outside faces of the toroid it is too easy for the layers to be jumbled.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

It would also be helpful to know how the toroid was mounted, and if the failure occurred where pressure was applied. Toroids are usually mounted either flat with washers and rubber gaskets with a single screw through the hole, or vertically in an "Omega" bracket, with some rubber cushioning material around the periphery. But in either case there may be additional pressure on a "high spot" where two windings may be crossed, and softening of the insulation from overheating may also contribute to a short.

It is fairly easy to check for dielectric breakdown between the bifilar windings, but nearly impossible to detect a weakness between adjacent turns of a single winding. But it might be possible to use a higher frequency, or PWM pulses of higher peak voltage, to produce a higher potential between turns of the same winding, and observe spikes of current due to breakdown.

Paul

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But the primary is buried under the heavy gauge secondaries windings and then three layers of 0.05mm polyester film strip, each layer of that probably overlapped by a factor of 3. So I would have thought immune from any external mechanical effects, exccept vibration perhaps.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

This one is mounted by central coach bolt and dished disk plus rubber disk. In use the bolt is vertical and when amp is carried it is horizontal. I should say I've seen the result of an amp set for USA use used on 240V , extensive areas of bare copper due to lacquer melting or burning off and large amounts of smoke damage.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

"N_Cook"

** Now I know for sure you are a posturing BULLSHIT ARTIST.

The only thing that fails when a toroidal of several hundred VA is fed with double the rated primary voltage is the damn FUSE !!!!

..... Phil

Assuming the correct value/type of fuse was in place and hadn't been "repaired" by wrapping aluminum foil (aka ciggy paper) around it. In my experience not many 120vac transformers survive even momentary

240vac,fused correctly or not. Maybe you are talking about much larger transformers than I deal with. JC

** 100% WRONG !! ** So YOU only deal with flea powered trannys - right ? ** Did you bother to read what I posted ????

A dual-primary tranny of 300VA has 5 ohms of resistance if connected for

230/240 volts and 50 Hz operation

If the same tranny is connected for 120 volts - that 5 ohm value falls to a tiny 1.25 ohms !!!

When 240 is applied in error, the AC current draw is limited only by that

1.25 ohms of resistance - since the core is instantly driven into * total magnetic saturation* and effectively disappears.

The resulting AC current flow is in the order of **150 ** amps - so any tubular glass or ceramic fuse blows INSTANTLY.

Any domestic power circuit (ie thermal magnetic, 15 or 20 amp rated ) breaker will open in a few milliseconds under such a gross overload.

Get real pal.

...... Phil

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It was someone who brought their 160VA Alchemist APD22 Nemesis Amplifier over from the USA, transformer survived long enough for the electrolytics to short , then they overloaded the primary enough to blow the , still set for USA use, mains fuse.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Why should anyone bother?

So the fuse should be about 2 amp slow blow type, which should blow within

0.1 sec at 20 amps and within 1 or 2 cycles at higher current. For 240 VAC use, it would be about half that. But the parallel primary connection would also be able to handle twice the current, so the fuse should have been adequate.

I have tested some toroidal trannies that seem to saturate quite slowly with increased primary voltage. In a 2 kVA unit rated 240 VAC I had about

50 mA at 240V, 100 mA at 280V, and 600 mA at 320V. Of course it would have drawn a lot more current at 2x rating, but perhaps not enough to blow a properly rated fuse instantly. I have seen a rather severe failure in a toroidal 480 to 240 VAC autotransformer that was connected backward to a high capacity mains source, thus placing the protective fuse on the load side.

Paul

would

I was trying to find a table or formula for toroidal inrush current versus power rating, but nothing found. A toroidal Tx , power for power comparison to conventional Tx, has to be larger fuse rating, to avoid blowing at switch on or after a few switch-ons. If a toroidal Tx has nothing wrong with it at switch one then there is more headroom for some later failure inducing problem not to blow the fuse. But does this headroom increase with higher wattage ? does the ratio of practical fuse rating/ normal maximum power consumption rise with higher wattage toroidals ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

"N_Cook"

** Has a 3 ohm primary.

Current draw on a 240 volt supply is circa 70 amps !!!

** Pure fantasy.

** Pack of lies.

The supply side fuse would have blown in milli-seconds.

The electros would not have be over voltaged.

..... Phil

"Paul E. Schoen"

** Not true of any normal commercial toroidal sold for use with AC supply, ** Memory plays tricks and your test results are probably wrong.

..... Phil

Well of course you won't you mental retard. It depends on the exact construction of the transformer, the load and the instant at which it is switched on in the mains cycle.

Why do you have to clog up these groups with your infantile questions ?

If in doubt fit a Surge-Gard in series.

Graham

Toroidal trannies do seem to have higher inrush currents due to remanent magnetism. If there are an unequal number of positive and negative half-cycles applied, the effect is a net DC component that magnetizes the core. Depending on the phase of the next application of voltage, this could add to the normal surge and cause very high inrush for a couple of mSec, which can trip some sennsitive breakers or pop fuses.

I have tried adding series thermistors, but they only work when they are cold, and in our application we applied multiple surges that heated them and reduced their effectiveness. In normal applications where power is switched infrequently, they work well. Another method is to apply soft turn-on or gradual turn-off using a triac with phase modulation to demagnetize the core or reduce the turn-on surge.

Paul

Where did the trannie get the sex change?