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Old 28th February 2008, 05:02 PM   #11
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Quote:
Originally posted by AndrewT
no.
the toroid holds the flux (or similar) that it was switched off at.

Only true if it's a superconductor and nothing else in the PSU is bleeding energy off. Neither is going to be true in practice.

Power is removed, and resistance and draw pull out all the energy stored in the magnetic field.
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Old 1st March 2008, 12:33 AM   #12
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Hi,

The inrush current is measured via a shunt resistance of 0.02 ohm connected to a digital scope.

The inrush will depend on the total primary resistance of the unit, if your transformer has 2 primary windings wired in parralel for 120Vac then theorically will be much higher then for the 230Vac setting which normally uses the 2 windings in series. For 120Vac the overall resistance is 1/4 the one of the 230Vac setting.

Normally you can easily get 80Apk for a 300VA transformer, of course it depends of the primary windings impedance. In order not to dammage your mains switch it is best to get a TV rated (ex. TV-5 or TV-8) switch, these are made specifically to handle high surge peak.

An easy fix is to install a NTC, they are cheap and do the trick.

Regards,
Eric
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Old 1st March 2008, 02:14 AM   #13
TheMG is offline TheMG  Canada
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Quote:
Originally posted by e_fortier
The inrush current is measured via a shunt resistance of 0.02 ohm connected to a digital scope.

And not forget to use an isolated probe or connect the transformer you are testing through an isolation transformer!

Bad things happen when a non-isolated scope ground gets connected to mains "hot" lead.
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Old 2nd March 2008, 08:49 AM   #14
zdr is offline zdr  Belgium
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thanks for all the replies, I guess I will have to make the measurements myself (in order to calculate NTC parameters.
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Old 2nd March 2008, 09:02 AM   #15
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Quote:
Originally posted by xiphmont
A toroid is a huge inductor. Inductors by their very narture resist changes in current. Why do you think the toroid has an inrush problem? A transformer is usually its own soft-start.
As Andrew has said the core has a residual flux. Upon start-up this flux is added to by the action of the current passing through the windings causing the core to saturate briefly until it reaches a kind of equilibrium where normal operation occurs.

Soft-starts are simply essential for the good design and engineering of a product with large power supply. Try it yourself - get a 500VA toroid or bigger and apply power to it whilst it shares the same power outlet as an incandescent lamp. Watch the lamp dim as the inrush current cripples the supply and stresses the switchgear and associated components.
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Old 2nd March 2008, 09:02 AM   #16
AndrewT is offline AndrewT  Scotland
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The NTC dominates the start up current.
You don't need to know the transformer only start up current.
The best bit of useful information is the primary winding resistance.
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Old 2nd March 2008, 11:47 AM   #17
Nuuk is offline Nuuk  United Kingdom
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I prefer dual mono supplies because they simply sound much better.

But an added benefit is being able to use two lower VA transformers and not having to worry so much about this sort of issue!
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Old 4th March 2008, 09:25 AM   #18
zdr is offline zdr  Belgium
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Quote:
Originally posted by Nuuk
I prefer dual mono supplies because they simply sound much better.

But an added benefit is being able to use two lower VA transformers and not having to worry so much about this sort of issue!
Akhm, I am really sceptical about how dual toroids can sound "much" better, since bigger toroids tend to have better regulation. Moreover, I don't think either parameter would be audible at all, unless if there is a serious manufacturing defect. Well, at least I don't think I could hear it.

Secondly, it seems you are implying that use of two 250VA produces much lower inrush current than 1x500VA? I believe this is only true if you switch 2x250VA manually one at the time, don't think that having them in parallel on the same switch could make a huge difference.
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Old 4th March 2008, 10:16 AM   #19
ratza is offline ratza  Romania
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Quote:
Originally posted by zdr


Akhm, I am really sceptical about how dual toroids can sound "much" better, since bigger toroids tend to have better regulation. Moreover, I don't think either parameter would be audible at all, unless if there is a serious manufacturing defect. Well, at least I don't think I could hear it.

Oh, yes you would. Do this experiment: on a single toroid PS, keep one speaker connected and on the other channel mount an 8 ohm resistor instead. Now put some music or a signal on this channel and hit the volume. Do you hear anything in the loudspeaker? Yup, I thought so. With dual toroids, this will never happen.
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Old 4th March 2008, 10:25 AM   #20
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Default Re: Toroid inrush current testing

Quote:
Originally posted by zdr
I decided to play a bit with inrush current. I implemented earlier blindly softstart circuit for two 230VA toroids with 4x10mF behind, thinking that they must need it. I have 16A automatic circuit breaker on the outlet where amp is connected, so I went ahead and bypassed softstart. Bottom line, I could not make breaker jump. I then placed another breaker of 10A in series and still could not jump anything at all. I presume that 2x230VA is not exactly equivalent to 1x460VA in terms of inrush current, but they are probably well over 1x300VA.

Anyone has some inrush current data measured for various trafo sizes?
My amp with 600 VA /230 VAC toroid generates 77 A, worst case.

Measure the primary winding, should be 3-4 ohms. Take (230*1.4)/3 = maximum theoretical current

Over 300 VA you will need a softstarter, to spare the mains switch and to let you have a fuse which protects better.
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