[RCaffin]

quote]when the smaller 110 volt trannie I have for the oven comes on it gives a loud THUNK sound.....that's a sign it's not current inrush protected.[/quote]
Well ... The 'thunk' is magneto-striction of the steel: the steel actually contracts a tiny bit when highly magnetised. The hum you here from a large trannie is the steel actually contracting and expanding at 100 Hz. To be sure, with a slow turn-on the thunk would be very small.

the label on the side of the toroid it's rated at 400VA.....220 volts at 1.8 amps?????.....seems small for a toroid that size.

400 VA - that's not a big toroid. Yes, toroids do LOOK small: they are more difficult to make but they are far more efficient.
I wouldn't worry about longevity on a trannie - they don't die very often.

Cheers
Roger

[vegipete]

Justfor an example, if I connect a string of 100 X 100 watt light bulbs in series to the 10 amp supply the breaker does not trip but the light bulbs only glow dimly.........that indicates to me that the combined resistance of 100 light bulbs now passes so little current when in series that the breaker will not trip even though the load is quite huge .

No, the load will still be 100 watts.

If the light bulbs are wired in parallel the supply only see the resistance of a single light bulb but the current flow can trip the breaker because the combined wattage is now 10,000 watts which needs a current flow of 41 amps to light them to full brightness and so the breaker trips.

The resistance of 100 bulbs in parallel will be 1/100th the resistance of one bulb.

Add light bulbs in parallel until the breaker current is reached. Put this in series with the transformer. The bulbs will function as a sort of load meter: brighter = more load. But they will also swallow some portion of the power, allowing less wattage through the transformer. So they ought to be shunted out when not needed.

[vegipete]

However, the light bulbs will suffer from inrush current problems too, precisely because they have lower resistance when cold and higher when hot. So they are not great as in-rush protection.

This looks like an interesting read for learning about current inrush. Section 6 about half way down that page deals with passive inrush limiting.

[handlewanker]

However, the light bulbs will suffer from inrush current problems too, precisely because they have lower resistance when cold and higher when hot. So they are not great as in-rush protection.

This looks like an interesting read for learning about current inrush. Section 6 about half way down that page deals with passive inrush limiting.

Hi thanks for the link.....very interesting.........it confirms what I'm thinking........a resistor or NTC device in series with the mains supply to the primary of the transformer with a bypass switch will work and simply too.

All of the other solutions are complicated and totally impractical, mainly from the point of view that they are also too overkill and vulnerable to expiring before their use by date.

When I have the time I'll give the NTC device with a by pass switch a try as it's simplicity in it's simplest form and if it prevents the breaker from tripping at switch on............problem solved.

BTW.....in the link above, the circuits are mainly dealing with inrush factors from a direct mains application to a device......this is not applicable to the toroid problem being discussed which is at the toroid primary windings stage and can be cured by the NTC and bypass switch solution.

BTW....the NTC will get hot if left in circuit as the link also stated, and will add heat to the inside of the cabinet, so it needs to be switched out of circuit once the inrush has dissipated to allow full supply potential to be realised.
Ian.

[Eldon_Joh]

BTW.....in the link above, the circuits are mainly dealing with inrush factors from a direct mains application to a device......this is not applicable to the toroid problem being discussed which is at the toroid primary windings stage and can be cured by the NTC and bypass switch solution.

why not?

BTW....the NTC will get hot if left in circuit as the link also stated, and will add heat to the inside of the cabinet, so it needs to be switched out of circuit once the inrush has dissipated to allow full supply potential to be realised.
Ian.

usually its not significant. an NTC for a 1kw toroid could probably be sized to dissipate 1 watt, i'll have to check the recently available datasheets.

[RCaffin]

an NTC for a 1kw toroid could probably be sized to dissipate 1 watt

Perzacly.
Extremely appropriate technology.

Cheers
Roger

[handlewanker]

Hi, I would definitely have the NTC switched out of circuit when not in use......hot things in confined spaces bother me especially if they have mains voltage on them.

In the link I referred to, the design application had the NTC attached to the live wire going directly to the device being powered.

This is different to having it in the primary of the toroid, as when the toroid is switched on all that happens is the current flows into the primary and waits for something to be connected to the secondary.......this is only a very short on time for the NTC which gets switched out once the primary is up to the mains potential.

It's not the wattage that is being consumed that is the bother it's the heat that could impact on the rest of the circuitry and the fact that the NTC is a bare device with live wires exposed.........insulating the lead in wires to a hot NTC needs something like glass fibre insulation and after a period of time that becomes very brittle and can flake off.

In most cases, when you do trouble shooting in the mill cabinet the power needs to be on, any presence of bare live contacts at mains potential and hot devices is not user friendly......everything after the toroid is at low voltage levels.

In the SVM-0 mill electronics cabinet there are 2 mains voltage user sockets that have mains voltage connections to them exposed, but they are shielded to a degree by having the wires insulated at the crimped on connections, but none the less, the screws that hold the wires to the sockets are exposed anyway so extreme care needs to be exercised when the cabinet is open and the power is on.

It's ironic to think that anything at mains potential in a household is completely sealed from user interference and needs by law a qualified electrician to work on them.....but in the mill cabinet there are exposed mains potential items that are not covered by this regulation as it's not part of a household wiring.......fitting an NTC would need a qualified electrician's work invoice if an insurance company claim was in the offing.

There's a strong case here to switch the dang thing out of circuit once it's done it's job, otherwise it needs to be as large as the full amp capacity of the mill when running and that means heat.
Ian.

[RCaffin]

the design application had the NTC attached to the live wire going directly to the device being powered.
This is different to having it in the primary of the toroid,

Um ... unless we are talking about a toaster or pump motor, it is highly likely the the front end of the 'device being powered' will have a transformer there - which could be a toroid. which means there is no difference.

when you do trouble shooting in the mill cabinet the power needs to be on, any presence of bare live contacts at mains potential and hot devices is not user friendly

And who has exposed bare wires at mains potential floating around in their cabinet? Blimey - I make damn sure any mains wiring inside is properly shielded. Quite often this means putting a small clear plastic shield over the mains bits. I was under the impression that any (every) competent engineer did that. (Keeps the swarf off too!)

Apart from that, I think that any further explanations of the correct (and common) use of NTCs will have to be made by someone else.

Cheers
Roger

[handlewanker]

Hi....the circuit design application I referred to in the link showed a bridge rectifier and capacitor wired directly to the mains........this is not what we're discussing as the thread is about connecting a toroid primary to the mains via an NTC.

Wherever do you find a bridge rectifier and capacitor attached to a mains input.........connecting to a toroid or any form of transforemr is not possible as the output from the bridge is then DC to the toroid which is designed to work on AC and cannot work on DC.

In another circuit design application, in the link, it shows the NTC connected to the transformer secondary winding which means it's on line all the time........also not what were talking about.

For my specific use the NTC will be attached to the transformer primary and switched out when the current is on par at the primary..........I expect that this is applicable to the toroid too........who wants a hot device at mains potential burning away all the time in an enclosed cabinet.

BTW......the inside of the electronics cabinet IS sealed from the ingress of any swarf or coolant, but it's a lethal area with exposed mains connections........screwed terminal connections, specifically the backs of the user sockets are not shielded from user accidental contact......this is something that SKYFIRE.COM has overlooked when the wiring was installed.

I refer to the small black panel at the back of the mill electronics cabinet where the mains cable enters and the user sockets are attached to......photo attached......the screwed connections are in the right hand lower side of the photo.
Ian.

[G59]

Such a long discussion about a problem that was solved decades ago. Timer relay says it all. Cheap, effective and dead reliable. I won't go into detail as it's already been covered previously in this thread.