Major inrush surge problem with Toroid, HELP!

[Elbert]

OK, I'm a bit desperate here right now..

I have a 600 VA Toroid (Antek) wired for 220V on the primaries (I'm in Europe).

I have buildt a soft-start circuit that's supposed to be good for up to 1kVA.

I've decreased the shunt resistance from 40 Ohm to 33 Ohm and beefed up the wattage rating quite well. (from 5 to 17W resistors, four off)

Now, this is what happens:

I throw the switch and when the relay closes and bypasses the resistors after approx 1 second, the 10A glass fuses go with a spectacular green flash.

The resistors get quite nice and hot, so current is being drawn before the relay closes.

To the best of my understanding, this shouldn't rerally happen. What is wrong??

One thing though, I have wrapped the outer diameter of the toroid with a strip of permalloy, or mu-metal in the hope of reducing any stray fields further.

Is it a possibility that this could somehow mess up the magnetic circuit of the transformer and thus cause problems on start-up?

Any inteligent advice would be much appreciated, I'm really at loose ends here!!

Desperate regards.

[richie00boy]

When you mean wrapped, do you mean all the way round? If so that is your problem as you have created a shorted turn.

Also 1 second is too long for a soft start, 0.25 seconds is about ideal. You might also like to consider increasing the resistance back again, or use an NTC thermistor.

[Elbert]

Wrapped.. well, how to clarify this better... I'll take a picture.. just to make sure everybody is on the same page.

It is not as if the entire toroid is wrapped up completely, I just made a flat strip which I then looped round the outer diameter of the Toroid.

Would this cause a shorted turn?? If so, would I brake the turn if the loop is not making a closed electric/ mechanical contact??

Yes, from the frantic reading up I've done, it seems that 1 second is more than needed, but I figured that this was OK after all provided the resistors didn't thermally overload. This is a design i found in an ELEKTOR magazine, and I guess their stuff is mostly sound..

[Elbert]

As I did some snooping around on Toroids and shielding, I came across this on the Plitron home page:

Magnetic Shield
Toroidal transformers radiate about 1/10 the magnetic field of standard EI transformers, but to further attenuate stray magnetic fields in very sensitive applications, such as in close proximity to a CRT, an additional magnetic shield may be called for. Magnetic shields consist of multiple layers of grain oriented silicon steel, or a high nickel content alloy, such as MU-Metal, Magnifier 79, etc.., laminated between insulation. Wrapped around circumference of transformer. Disks are also available for use within potted assemblies.

As far as I can see, this is what I've atempted, but perhaps there was some critical detail I got wrong??

[Nisbeth]

That looks ok.


/U.

[Elbert]

Well,

The area of uncertainty here is wether or not I've actually made a shorted turn and the effects thereof.. could be that this sort of shielding requires that the ends of the metal strip which is wrapped arround the toroid circumference, are insulated..

I could of course verify this by removing the permalloy from my Transformer, but it would be a bit messy, not to mention getting it back in place if no difference was made, hence some second opinions are welcome!

[janneman]

Elbert, it could be that the fuse blows because you have a (near) short on the secondary. With the series resistor, the current is limited so a secondary short isn't 'seen'.
Try with the secondary disconnected. If the fuse holds, try with only the rectifier and caps, without the amp. At some point it will be clear where the problem is. It's very unlikely that the inrush current circuit is the cause.

jd

[Redshift187]

A few things. First, Janneman is correct, never test everything at once, do it in stages. Second, you should be using a bulb tester and then you won't be blowing fuses as often, and risking your equipment, etc. Lastly, the toroid in the photo is not mounted. Is that how you are testing? The reason I ask is mounting mistakes can lead to a shorted turn. A shorted turn is not when you wrap a conductor around the outside of the transformer, it's when you wrap a conductor through the middle and around the outside, connecting the ends to make a loop.

[Elbert]

Janneman,

I've allready worked through that route, so unfortunately, I am running out of options here, hence my slight desperation.

I have allso ran and checked the soft-start circuit and it doesnt do anything it is not supposed to, ie cause a short circuit in stead of bypassing the resistors etc..

Redshift,

The picture doesn't show the transformer as it is installed, but close to, the only thing effectively missing, is the mounting bolt and the steel plate holding it in place.

That was allso my understanding, that a shorted turn would have to be something going through the centre of the Toroid, i.e. I've reads that the bolt must not contact the chassis in both ends, etc.. There arehowever no such conditions like that present in my set-up..

[janneman]

Originally Posted by Elbert

Janneman,

I've allready worked through that route, so unfortunately, I am running out of options here, hence my slight desperation.

I have allso ran and checked the soft-start circuit and it doesnt do anything it is not supposed to, ie cause a short circuit in stead of bypassing the resistors etc..

Redshift,

The picture doesn't show the transformer as it is installed, but close to, the only thing effectively missing, is the mounting bolt and the steel plate holding it in place.

That was allso my understanding, that a shorted turn would have to be something going through the centre of the Toroid, i.e. I've reads that the bolt must not contact the chassis in both ends, etc.. There arehowever no such conditions like that present in my set-up..

Do you mean that the fuse blows without anything connected to the secondary?

BTW Does the mounting plate touch the screen winding & the screen winding touch the chassis bottom...?

jd