Toroidal transformers are better - how?

[Craig Sawyers]

Originally Posted by G6Tanuki

These days rather than kludging around with relays and series-resistors I use an off-the-shelf inrush protector, like these:

https://www.ametherm.com/inrush-curr...t-limiter.html

There is problem with these in audio power amp applications. A typical current limiter for a 500VA - 1000VA has a cold resistance of 220 ohms, and a hot resistance of 1.4 ohms at maximum current (ie when the Tx is supplying maximum VA). The temperature of the limiter in the hot state is around 150C.

The problem is that quiescently an audio power amp is taking a very small current (that if it is not class A), and the transformer draw will only peak up during transient events - kick drum, orchestral crescendo etc.

So most of the time the current limiter will be sitting at close to maximum resistance.

As an example, the Crown XLS602 professional sound reinforcement amp uses two PTC thermistors in parallel, in series with a 47 ohm 23W resistor. This is all shorted out with a relay contact fired by (you guessed it) a semiconductor based timer.

If a transformer or power supply is operating flat out all the time, then the current limiter (Brimistor of old) is definitely a strong contender.

[MotorBikeLes]

Are there any NTC thermistors with current carrying capacity in the 1 to 5 amp range? I am still thinking of my M/C dynamo current limiter from a previous thread.
Les.

[Argus25]

Yes David, it is interesting that looking at the transformer in a steady state condition after the power is applied the flux wave peaks when the applied voltage is zero because its 90 degrees late. However if say the transformer is organised to switch on at zero crossing (no applied voltage) the inrush current is a maximum because no flux is established yet. And counter-intuitively the inrush current is minimal when the transformer is switched onto the AC supply when the voltage has reached its peak, because that is when the flux wave is normally zero under steady state conditions anyway. If the transformer switches on at zero crossing the peak flux is double the steady state flux which can saturate the core or at least move to a very flat section of the B-H curve.

So a peak crossing switch would solve the transformers inherent inrush current, we still have the downstream rectifier/capacitor.

[Argus25]

Originally Posted by MotorBikeLes

Are there any NTC thermistors with current carrying capacity in the 1 to 5 amp range? I am still thinking of my M/C dynamo current limiter from a previous thread.

Les, Silicon Chip here in Australia published a 10A rated "soft Starter" kit in April 2012 to plug in series with mains appliances with high inrush currents:

http://archive.siliconchip.com.au/cm...3/article.html

(you would need to be a subscriber to see the whole article)

It uses a large NTC thermistor part number SL32-10015. The thermistor they used has a nominal resistance of of 10R at 25 deg C and 0.048R at 228 deg C. But their circuit cleverly shorted it out after 2 seconds. The time delay simply provided by a simple RC network and a couple of transistors and a relay & some diodes. They also made it so the pcb could be added inside an appliance or in a separate box with mains connectors.

The Ametherm brand thermistor was Element 14 part number 1653459:

http://au.element14.com/ametherm/sl3...rfnonsku=false

[turretslug]

Originally Posted by MotorBikeLes

Are there any NTC thermistors with current carrying capacity in the 1 to 5 amp range? I am still thinking of my M/C dynamo current limiter from a previous thread.

Loadsa, loadsa. Ametherm and Epcos are two major sources, and there are lots of permutations of cold resistance/current/running dissipation. I've long used Siemens (as were)/Ametherm 220 ohm 1A types in the various thermionic radios/other devices here, they do need space and firm, heat-proof support (e.g. ceramic/Melamine terminal strip, etc) as they stabilise around 120-160 degrees C- no more threatening than a B7g output valve, really, but important to consider. It's difficult to statistically prove the benefit, but they're cheap'n'easy to fit and the slow fade-up of dial-lights is at least reassuring. The AR88 is said to have a slightly weedy ganged power switch and it can't harm other components like DH rectifiers to have a gentle wake-up. It also means that a tightly-rated QB primary fuse can be fitted. No, I don't come from that school of thought that demands that all fuses, breakers and other protection should be eliminated and all my audio gear be directly connected to the incomer by thick, Litz-weave silver rope....

[G6Tanuki]

The only potential downside I've ever found with NTC inrush-protectors is if your supply repeatedly 'bounces' [due to a trip/reset/trip/reset cycle on a recloser] faster than the cooldown-time of the NTC.

In this case the hot NTC doesn't provide any protection. But suitably-related fusing or circuit-breakers should catch the problem.

[MotorBikeLes]

I went of "half cock" with my question earlier. In fact it is a PTC that I am interested in, not NTC. More info herehttps://www.vintage-radio.net/forum/showthread.php?t=126414
It is something I need to get sorted shortly, so I have something else to think about.
Les.

[joebog1]

I regularly design "low flux density" toroids for use in studio preamps and control circuits.
An example is : 160 watt toroid ( ratings @ about 1.5 Teslas)
I overwind the transformer 2:1 so the maximum flux density is half of that, or about .75 T.
This of course results in a massive inductance, in both primary and secondaries.
( 240 volt primary and up to 750 volt secondary). To keep running temperatures down I use big wire ( about 1250 cm/amp) So now I have massive inductance with low resistance windings.
Inrush current can be massive ( in my experience ) and is ALWAYS much higher with toroids over EI designs.

The biggest problem I have experienced is flashover in the neighbouring turns. This due to enamel wire ratings of 500 volts.
I found it essential to fit VDR,s or GEMOV's to both primaries and secondaries ( high voltage windings at least) to stop internal arcing. This can also take out switches regardless of their size, even when subbing caps are fitted across the contacts to try and quench any residual arcing that the VDR "misses".

As far as transmitted noise from primary to secondary goes, I use copper tape electrostatic shielding, which comprises two layers of insulated copper tape wound over the primary and very well insulated with mylar tape between both primary, and over the screen. This is terminated to earth, as is any normal electrostatic screen. There is almost NO noise passed through the transformer.

Around the outside of the transformer I fit 3 turns of mumetal screening tape, specifically manufactured to prevent ANY external flux escaping the whole assembly.

It all adds cost of course, but if you want the best you must pay for it.

Just my take on toroids, although I do design, wind and use many EI designs as well. Especially for output transformers. I have tried toroids and sectioning a toroid is NOT as easy as it sounds. PLUS I dont need response down to earthquake level, or to annoy the fruit bats that may be passing.
A BIG EI core, with generous wire sizes, and maximum flux densities of perhaps 1T results in reasonably cheap and efficient output transformers.

Regards
Joe

[Argus25]

Originally Posted by joebog1

so the maximum flux density is half of that, or about .75 T.........Inrush current can be massive ( in my experience ) and is ALWAYS much higher with toroids over EI designs.

This is interesting because even at 1.5T the core is not at magnetic saturation, and if the running max flux density of your design is only 0.75T , then in theory at least, the peak flux density you would get at turn on, even in the worst case where the mains waveform was zero crossing and the flux peaked shortly after that, would only be 1.5T max. Versus say running the design at 1.5T and getting a 3T peak at turn on which is probably too much for the core material.

Generally most mains power transformers, of the E-I type are run around 1T, though sometimes they get pushed depending on the designer.

I think most switches are destroyed due to turn-off arcing so those MOV's sound like a good idea and maybe help save the transformer & wire insulation too.

[Radio Wrangler]

Those figures are for running flux density where the primary inductance and the mains voltage/frequency set the magnetising current. The turn-on grunt isn't just a function of the AC component of the mains, but also of the DC voltage transient created by the abrupt turn-on of the sinewave. This has a slower decay and can really push the flux density over the edge.

David

[Craig Sawyers]

One of the major problems is dual standard transformers from the US, with the primaries wired in series for 230V. US designers often have a blind spot regarding mains frequency, rating their design for 60Hz. Of course at 50Hz the flux in the core is increased by 20%, and leads to mechanical buzzing through non-linearity in the core.

I have had to site amps from the US remotely from the rest of my audio rack because the mechanical hum was getting into the a record being played.

[Argus25]

Originally Posted by Craig Sawyers

I have had to site amps from the US remotely from the rest of my audio rack because the mechanical hum was getting into the a record being played.

The American transformer thing on 50Hz seems more than the 20% factor, for old transformers. For example in a 1939 vintage Meissner TV I have here in AU, the rms primary current on the power transformer off load, was over one amp with 115V 50Hz. The radiated magnetic field so severe that the CRT beam was deflected and the scanning raster grossly modulated. When I replaced this power transformer with a modern Hammond multi-voltage one (same power rating but still wired for 115V ) the rms primary current off load was only 47mA and there is negligible radiated magnetic field. I had exactly the same problem in another set, the RCA621TS again cured with a new Hammond transformer. This made me wonder if in some way the old 60Hz American power transformers have a deteriorated iron core (but I have no proof of that) or if they were just run closer to core saturation in the first place.

[Argus25]

Originally Posted by Radio Wrangler

The turn-on grunt isn't just a function of the AC component of the mains, but also of the DC voltage transient created by the abrupt turn-on of the sinewave. This has a slower decay and can really push the flux density over the edge.

The graphs and equations for the turn on rush current are here; its the product of the integral of two sine functions and peaks on the next zero crossing, assuming the transformer was switched on at zero crossing(worst case):

http://www.electrical4u.com/magnetiz...r-transformer/

For a power transformer with a low steady state peak flux of say 0.75T, I think it is unlikely the 1.5T peak value at turn on would push it too far along the saturation curve.

[joebog1]

I should have added: The GEMOVS/VDR,s are for switch off current. It is VERY easy to achieve massive inductances with toroids. The specific example I quoted has over 250 henries ( 10 volts AC ) so when the unit is switched off, its exactly similar to a sporting type Kettering ignition system (points and coil system in old fashioned cars) Its this that punches holes in enamel insulation and switches.
Apologies mods
Joe

[kalee20]

Originally Posted by Argus25

...if say the transformer is organised to switch on at zero crossing (no applied voltage) the inrush current is a maximum because no flux is established yet. And counter-intuitively the inrush current is minimal when the transformer is switched onto the AC supply when the voltage has reached its peak, because that is when the flux wave is normally zero under steady state conditions anyway.

Absolutely! Switching on at a voltage peak is kindest to a transformer, though it may not be kindest to any load connected to the transformer.

Originally Posted by merlinmaxwell

So a peak crossing switch would solve the transformers inherent inrush current, we still have the downstream rectifier/capacitor.

Yes. Sometimes as engineers, we just can't win! You feel like never switching the flipping thing on at all... there just isn't a good time...

There is another effect with toroids, that with good, low-loss, high permeability, grain-oriented steel cores, the fact that they are a continuous strip means that there's effectively zilch of any air gap. And that means that there are no demagnetising forces.

So even with one of joebog1's very conservatively-rated transformers (nice notes there, Joe, thanks for sharing!), if you switch off at the wrong time, you could be left with 0.75T of remenant flux in the core. And if you switch on at the wrong time, you add another 0.75T to the already existing 0.75T... saturation looms!

It's instructive to demonstrate remenance, if you have a toroidal transformer and an Avometer. Measure the primary resistance, see hoe long the needle takes to creep to its final figure. Measure it a few times to get a feel for it. Then swap the leads over and see how the delay compares. You'll find that after a reversal, the needle takes much longer (I demonstrated this to someone, with a 1,000V toroidal transformer about 6" diameter, the delay was several SECONDS!!)

This is of course the basis of magnetic core memories.

EI transformers show the effect much less - firstly, even with interleaving, there is more of an air gap which serves to introduce demagnetising forces; secondly, in an EI transformer, some of the flux goes in a less-optimal direction metallurgically, so effective permeability is less and again, demagnetisation happens more readily. The inductance is less, magnetising current is more, but it's not always a bad thing!

[GMB]

The reason I really like toroids is that it is so easy to add extra windings. I do this all the time.

The lower turns-per-volt just adds to the ease of doing this.

[Argus25]

Originally Posted by kalee20

Sometimes as engineers, we just can't win! You feel like never switching the flipping thing on at all... there just isn't a good time...

It's instructive to demonstrate remenance...This is of course the basis of magnetic core memories.

Yes, on the can't win side it is the SSR, solid state relay for AC applications. Looks attractive on the face of it, but they switch on zero crossing, the absolute worst case for switching a transformer !

The remanence thing once played a very important part in car dynamos. The residual field allows dynamos to self excite. If you swap the polarity of the battery in the car, you have to re-polarize the dynamos field to get it to work.

And of course it is the basis of all permanents magnets, the core memory you mentioned and tape recordings, floppy discs etc etc. But there is more......

The most intriguing (and I think brilliant) example of it is an SCR based CDI distributor-less ignition designed by Lucas. It used the remanence in Alnico toroids to act as a magnetic memory to create a magnetic ring counter. The remanent fields result in a sequential firing system. It is a little known masterpiece of engineering where "necessity is the mother of invention" that I could have only achieved with TTL flip flops or an electronic shift register. It is a testament to the creativity of Lucas's engineers. I have attached this little known circuit.

[G4_Pete]

“Better” appears to be very dependent on the angle you approach this from.

From my perspective as someone who now has no professional involvement in electronics E and I winding is far easier , except for those horrid welded things, and using the circuit attached it is always easy to visually confirm if you have got the right primary inductance. ( I appreciate you can measure amps also)

On the times I have used this circuit on toroid’s I have noticed qualitatively that when compared with E and I the saturation is far more abrupt suggesting that if you do saturate a toroid it is very abrupt with no cushion.

REF: Electronic Design December 17 1999
Pete

[Argus25]

Originally Posted by G4_Pete

On the times I have used this circuit on toroid’s I have noticed qualitatively that when compared with E and I the saturation is far more abrupt suggesting that if you do saturate a toroid it is very abrupt with no cushion.

There was an option on the Tek466/464 scope (probably 465 too) to run the scope from an external low voltage DC supply. The oscillator for this is based on a small toroidal transformer running in a Royer oscillator configuration (which relies on core saturation to determine the operating frequency) at I recall about 400Hz. It drives switching transistors for additional primary windings on the main power transformer.

In any case this small toroidal transformer has a core with the most abrupt saturation point I have ever seen. So clearly there are some core materials that have this very abrupt saturation point. It was so impressive for this transformer that I took a recording of it where I applied a square wave with a longer half period than it took to reach saturation, look at how abrupt this is... see attached photo.