Mains transformer goodness

could

I thought the disjunctures between the laminations of conventional Tx set up miniature magnetic circuits in opposition to the magnetising field and so reduced the current inrush. Toroidal formers don't have these.

True, but effective in 'most' instances according to application. Another solution is to place a time-delayed relay across the surge-gard so it cools quickly again.

Never tried the triac idea. With the relay shunt mentioned above, some people also used fixed value power resistors. Fine as long as no appreciable current is drawn before the relay closes.

Graham

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

More useful for answering questions:

They're called air gaps between the E and I laminations and cannot be completely avoided.

They alter the permeability of the core.

Indeed, toroids don't have air gaps.

Graham

completely

Yes yes yes. If you want something special you can get it.

It isn't what N Cook was talking about though. Stop obfuscating. It makes you look silly.

Graham

The Wiki says otherwise. And I have had a lot of experience with high power toroids and have seen the effects of high inrush current.

I have personally observed this in our circuit breaker test sets. We take care to apply voltage at about 70 degrees phase angle to obtain an initial current peak equal to those that follow, and adjust the timing so that there are equal numbers of positive and negative half-cycles. Inrush current on successive pulses is minimal. But if there is an unequal number, and a net DC component, there is always a much higher peak inrush current. This is easily seen with short duration pulses of several cycles. In breaker testing, of course, the breaker may trip at any time, sometimes causing net DC, and the next operation usually exhibits high inrush.

Some other references:

Paul

A Massive Prong was removed, forming the hole in the donut.

Paul

remanent

power

number,

Can a toroidal former become permanently magnetised and cause excessive current draw. ? One, in front of me from an amp, drawing excess current , does not attract a small piece of iron. I don't know what the main secondaries are supposed to provide, but can give 300 watts into 4 ohms. I've wound off the primaries (yes external to the secondaries) to try and understand what is going on with this transfornmer. Applying 70 volt ac , from a variac, across series secondaries is ok but increase to 75 volt and the current draw goes up to 1/4 amp. Should be able to take twice that voltage. Absolutely no visible problem , to the wiring or the coating of the toroid and both secondaries measure DMM the same DC. Tomorrow will apply something a bit over 70 volt with temperature sensitive labels fitted around. I will even try a TV degaussing coil.

"Paul E. Schoen"

The rather higher inrush surge of a toroidal mains tranny is explained by the nature of the wound strip core itself - not on past history.

Normal users do NOT do the weird S*IT to transformers that you do - yet still suffer from large inrush surges.

So YOU ARE 100% WRONG

- YET AGAIN.

..... Phil

"N_Kook"

Normal use on an AC supply very quickly demagnetises the core - if it ever becomes magnetised.

..... Phil

Once you apply some AC voltage to the primary the remanent magnetism should be neutralized, and you should be able to apply full rated voltage without saturation. If the winding were made of the wrong material, such as perhaps hardened steel, perhaps a powerful DC pulse could essentially make a permanent magnet which would take a reverse pulse to neutralize. That seems unlikely, but you might see the effect if you observe the current waveform on a scope. I would think you would see peaking on one set of half-cycles and not the other.

Most likely you have a shorted turn, and as the voltage rises, it starts to break down. Again, if you observe current waveform on a scope, you might see spikes near the peaks.

If you are talking about an audio transformer, 300 watts into 4 ohms is about 35 volts. But it has to be able to work down to about 20 Hz, so it would handle 100 volts at 60 Hz.

If this is a power transformer, it probably needs to generate a +/- DC supply for direct drive into 4 ohms, which will require about 50 volts DC. Two windings of 35 to 50 VRMS would be about right. So two in series might be 70 VAC, which corresponds to your readings.

Paul

While it gives good background on PCBs it does not inform on the regulatory situation. Moreover some of the background is missing, like the presence of PCBs in varnishes and potting compounds used for ballasts and transformers. Also regulation wise very nearly all uses of PCBs are now banned. .

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

The wiki DOES mention the use of PCB as plasticizes in insulation, and gives a time-line for the production and use, thus gives us a good idea of which products might contain PCBs.

As a chemist (BS 1970), I have some familiarity with PCBs and the hazards and regulations associated with them. As a TEA Certified Electronic Technician, I have experience servicing electronic equipment. I also have practice in reading regulations. However I still found the Wiki much more useful than the regulations.

The simple fact that REGULATION WISE, the use of PCBs are now banned, in almost all cases, is easy enough to make without the reams of [almost] unreadable regulations.

If there is a particular SECTION of the regulations that you found particularly illuminating and informative, PLEASE point it out to us. My quick skim of the regulations did NOT give me any useful information other than PCBs are banned and must be disposed of carefully and under carefully controlled and regulated conditions. What point were YOU trying to make by citing that reference???

Actually, the moment you put a gap there, the core is no longer geometrically a toroid any more.

Graham

Depends on the working flux Phil. If one had been designed for very low radiated field, it's entirely possible that effect would occur.

Graham

This has zero relevance to the OP.

Not to mention the possibility of overtorqueing on top of that. What goes on in some places is unreal. I've even seen steel TO-3 flanges dimpled by the mounting bolts pulling into the clearance hole !

Graham

Hilarious. ;~)

Graham