Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: How to make an 8mH inductor which can handle 13V peak square wave
 without saturation
Date: Wed, 13 Mar 2024 02:05:39 +1100
Organization: A noiseless patient Spider
Lines: 48
Message-ID: <uspr0d$blh5$1@dont-email.me>
References: <upb77l$1277s$1@dont-email.me>
 <psdqri5k6d1u4u4dls257jqa57qsb0h2v5@4ax.com> <uspjj8$91ne$2@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 12 Mar 2024 15:05:49 -0000 (UTC)
Injection-Info: dont-email.me; posting-host="100e3439807ba324b13ca9c776e62736";
	logging-data="382501"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+A10oUjMYx2Ay4UeLvSQ0HSDF8cmUsF60="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:hE6/chpZnT0u/keIA+X73M+CgXc=
Content-Language: en-US
In-Reply-To: <uspjj8$91ne$2@dont-email.me>
Bytes: 3069

On 12/03/2024 11:59 pm, Peter wrote:
> Just an update.
> 
> I tested that 8mH coil from Mouser. Verified at 8mH with an LCR meter
> and with the $500 micro tweezer thingy.
> 
> Doesn't work!! It behaves like it was 20mH or something like that. The
> attenuation is way too high. But 8mH was the perfect value with that
> funny milspec 0-10mH box I got off Ebay, which was heavy enough to
> contain pretty big inductors.
> 
> So what is happening?
> 
> Must be something in the waveform which is buggering up the way these
> inductors behave, in conjunction with the demodulation scheme used to
> "decode" the LVDT position.

An inductor in series with the excitation coils of an LVDT will shift 
the phase of the current going through the LVDT. If the output of the 
LVDT is being demodulated with a phase sensitive detector you'd need a 
matching shift in the phase of the drive to the demodulator to get a 
sensible (or useful) output.

A precision rectifier wouldn't have that problem, but they weren't all 
that precise decades ago, and not all that popular. They also don't 
reject noise and out-of-phase pick-up, so nobody sensible would have 
used one.
> Maybe it is saturating, but saturation has the opposite effect: it
> *lowers* the effective inductance.

Worse - it could be saturating on part of the cycle and giving you a 
very funny current waveform going through the LVDT - actually a rotary 
variable transformer, but we've been through that,

Looking at the current waveforms is always a good idea, but not always 
all that easy.


> So I bought a 2.5mH one from Mouser, again one which can do 1.5A or
> so, and will try that, after the potting compound has gone off :)

More looking a what's actually going on might be a good idea - buying 
random parts before you had worked out exactly why the last one didn't 
work isn't good policy.

-- 
Bill Sloman, Sydney