When Self Inductance breaks down?

[Dave the bass]

When you have an SE gapped transformer, be it output/Interstage/Input, there's usually a data sheet that specify's the max standing DC current or magnetising current that the TX can handle whilst maintaining its own (I believe titled) 'Self Inductance'.

Am I right in thinking that the DC bias we apply through the TX causes the Self Inductance to vary depending on the current we pull through the windings? In effect the TX is battling against our applied current? Am I also right in thinking that its the lines of flux collapsing that results in this loss of inductance if we exceed the manufacturer's max DC current spec?

Is this collapse fast or is it usually a gradual process? I.e. an interstage rated 15mA max DC bias maintains 100H but at (say) 20mA inductance has dropped to (say) 80H or 5H?

This question came up in conversation at the recent AGM of the Kent Massiff.

I'm having 'issues' with my Owston amp and the available current to drive the grid of 300B. It's sounded a trifle 'dark' sounding on MrT's ultra revealing Horning Speakers. Available drive current for the 300B grid was muted as a possible suspect, hence question about Inductance collapse.

DTB

[Mike H]

Can't find my picture of it (LTspice simulation of inductance versus DC)

But imagine a parabolic curve, max. peak is max. inductance at zero DC (AC only), slopes off either side as DC applied (say left side of curve is negative going DC, right side is positive going DC).

Curve of slope then dives steeper as more DC is applied, ending up as zero.

BTW doing this sim for a non-gapped core shows how very intolerant it is to any DC offset.

ANY DC applied starts to reduce the zero DC inductance value.

The way I understand it it's not lines collapsing, it's the core being magnetized in one direction until there's little or no 'spare' left to transfer any AC. Or, effective inductance is reduced from 'X' to 'Y'

HTH

[Mike H]

I.e. an interstage rated 15mA max DC bias maintains 100H but at (say) 20mA inductance has dropped to (say) 80H or 5H?

Certainly, but will need to test for the actual amount of loss at so-and-so DC.

Not sure how much it would affect sound quality though, apart from the most obvious lower bass region.

[pre65]

apart from the most obvious lower bass region.

We don't talk about that here.

[Mike H]

[Nick]

If you look at this graph, which shows how the flux density varies against the magnetising field.



The magnetising field is proportional to the current through the windings, as you can see for iron and steel, past a certain point, more field does not cause more flux, thats the point where the core is said to be saturated.

The AC inductance of a choke is a measure of its ability to convert the current into flux and back again.

As long as a increase in current will cause a increase in flux, and a decrease in current a decrease in flux, then a increase in current will be resisted by the existing flux in the core, and likewise a reducing current will be resisted by the flux. In that state where the flux density can very we see the choke has inductive reactance. or just Inductance.

But once the core is past the point where increasing fleld does not increase the field. there is no reaction to changes in current, as there is no possible change in flux, so the Inductance goes away.

If you look at the line for air, you see its a lot lower, you get less flux per field, but it linear, air does not saturate. Adding a air gap in a iron core gives a composite curve, so the total inductance is lower, but it takes a lot more current to saturate, and in fact the gap means the inductance will never drop to zero, as the air gap will never saturate.

[Dave the bass]

Aaaahhhh, so is that why Air cored inductors are HUGE compared to Steel/Iron items of the equiv Inductance?

IIRC our-Paul hand wound some gigantic air core inductor for his OB's at an EggFest.

Thanks Nick and Mike. Much 'preciated.

DTB

[Nick]

Yep, or conversely, why adding a iron core increases the inductance of a coil many times

[Paul Barker]

Aaaahhhh, so is that why Air cored inductors are HUGE compared to Steel/Iron items of the equiv Inductance?

IIRC our-Paul hand wound some gigantic air core inductor for his OB's at an EggFest.

Thanks Nick and Mike. Much 'preciated.

DTB

True.

Vry was always wanting to have some air cored HF output transformers wound. I shied away from the project. Not saying never. Just not ready for that yet. It is feasible at HF because little inductance required. Air would be the best sounding core material one would imagine.

James's Quasars also have air core LF inductors.

[IslandPink]

Are you sure about that ? - or was it just the weight of copper that caused me to need both hands and stagger across the room with each one , at Owston last time ?

[Mike H]

Just by the way, I remember seeing a very early battery powered radio with 2 I think air-cored audio anode chokes, like big tubes.

[Paul Barker]

Are you sure about that ? - or was it just the weight of copper that caused me to need both hands and stagger across the room with each one , at Owston last time ?

You'r right.