How to make an 8mH inductor which can handle 13V peak square wave without saturation

John Larkin snipped-for-privacy@997PotHill.com wrote

It is to reduce the output from an LVDT, by putting this in series with the input. The reason for reducing the output is a bit obscure...

We do a lot of LVDT measurement and simulation, usually at higher frequency but occasionally at 400 Hz.

You could use a step-down transformer instead of a series inductor, for less phase shift and better linearity and a predictable ratio. Some little audio transformer.

Ir maybe a resistive voltage divider.

Don't be surprised. It's common to have to brush up on basics, if you don't use them regularly.

Your AC source is seen by the sim to have the polarity of the first

• pulse. The -pulse that follows can only return mag current to zero, hence a net +DC can show up in early portions of the sim. Ad a load to get AC-only current, eventually.

RM10 DC saturation current will depend on turns count and gap. Saturation flux in ferrite is ~0.33T.

B = u N I / lm

B = flux density in Teslas u = 4.pi.E-7 N = turns count I = current in Amps lm = magnetic path length (or gap width, if present) in meters.

For pulsed DC

deltsB = V T / N Ae

deltaB = flux density change in Teslas V = applied volts in Volts T = time in seconds N = turns count Ae = cross-sectional area of flux path im meters^2

inductance of gapped structure

L = 4.pi.E-7 N^2 Ae / lg

L = inductance in Henries N = turns count Ae = cross-sectional area of flux path im meters^2 lg = gap width in meters

Solutions will depend on whether or not you intend to use the part in AC-only, small signal situations, or whether the part is intended to do some work in a resonant power or filter circuit.

Check what's on offer in distributors' catalogs. This will give you a ball park idea of the size and shape of stuff you'll be aiming for and may offer a simple solution off the shelf.

RL

It would be easier to help if you stated the application and known limits/requirements of size, weight, mounting method, environment etc.

'carry the current' really doesn't tell us anything.

If there are 'found' materials you'd prefer to use . . . and results from your attempts to do so.

etc.

RL

I see no reference to an LVDT application in the Peter's posting.

RL

It is an instrument which contains an LVDT bent into a half circle, to return an angle.

It is excited with 500Hz, and it is pretty well a square wave. I didn't design that bit :) And the output is too high, for reasons too complex to explain. So I need to reduce the output somehow.

Not on this reader. I have only:

Tue, 30 Jan 2024 16:13:41

RL

So you've got 500Hz @ 13V (pk?). and you can't adjust the source (why not?)

AC or DC?

You could reduce the output phase width with a saturable reactor.

For pulsating DC, it might be a regulated 'set' reactor.

For AC, it could be a bit more complicated, unless you were satisfied with just a fixed PW blocker. That might drift a bit with load or temperature. Gets physically bigger the more voltseconds you want to block, for the same current.

The 'complicated' saturable reactor for AC begins to look like a mag amp.

RL

Anyhow, he said he wanted to reduce the drive level into an LVDT.

I think he could use a simple LC or RLC to make a nice lower-amplitude sine from his big square wave.

It would make more sense to excite it with a lower amplitude sine wave. The angle sensing differential transformer will run cooler - those higher harmonics generate extra heat in the core material.

It's a bigger change than your boss had in mind, but would give you an appreciably better system.

About LVDTs, I know nada.

RL

...but nowhere near enough current. Ferroxcube have (or had) a free downloadable calculator.

As you have already some cores, you could stack several into a tube shape. I've done this where the available space made it the only game in town. Tricky to wind but fun, FSVO 'fun'.

Depends how the demodulation works, it's often synched to the drive signal and there may be no phase adjustment.

Yeah, there are a host of things I could fiddle with, but I like to keep my distractions at least vaguely on track.

RL

If you can scope a current waveform, you might get an idea of what the most troublesome harmonics are, then see if a simple LC filter will help. You'll need the C for any L to work.

RL

Probably, but the OP didn't specify the application. LVDTs can be

I'm assuming the OP has tried an 8mH inductor, is happy with the result, and doesn't want any more complication.

It's not a good idea to excite an LVDT with a square wave. The OP should set up a circuit to provide a stable sine wave of the right amplitude, rather than waste time trying to cut down the amplitude of the square wave with temperature dependent voltage drops through diodes.

The next tme that Russia or France or Germany wants to invade your tiny damp country, we'll sit it out.

That is exactly what I did.

I bought a 0-10mH (in 1mH steps) variable inductor off Ebay and found