# Find Large Inductance Without Meter

#### Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

•  Subject
• Author
• Posted on
I have an earth sensing magnetic coil with a resistance of 20,000KHz. It
has a mu metal core and the inductance is way over what my meter will read.

Is there a procedure for determining the inductance value by other
means? I have a good DMM, scope and function generator.

I need to know so I can select an appropriate tuning cap to resonate it
at around 10Hz.

Kevin Foster

Re: Find Large Inductance Without Meter

sonnect it in series with a known resistor and a known (or measurable)
AC source. measure voltages, frequency etc.

--
This email has not been checked by half-arsed antivirus software

Re: Find Large Inductance Without Meter
Kevin Foster wrote...

I would use a time-domain approach.  Apply
a pulsed voltage and measure di/dt.

--
Thanks,
- Win

Re: Find Large Inductance Without Meter
On 11/24/2016 08:39 AM, Winfield Hill wrote:

Here's a wacky idea: what about using a "negative power inductor"? Make
a grounded negative inductance circuit using an op amp and an audio
power amp, with appropriate local and global feedback (see figure 9
here: http://www.ti.com/lit/an/snoa600b/snoa600b.pdf)

Connect the unknown inductance in series, sweep the function gen and
watch for the current spike.

Re: Find Large Inductance Without Meter
On 11/24/2016 7:39 AM, Winfield Hill wrote:

Yes. In fact, you can sample the current using a scope, and pulse
generator and measure the time where the current reaches the 63% point
then use t = L/R. Knowing, of course, the coil plus sample resistors'
values.

Re: Find Large Inductance Without Meter
John S wrote:

Dream on....if inductor's internal resistance is too big, there will
be *NO* L/R curve; one will see a nice resistive divider.

Been there.

Re: Find Large Inductance Without Meter
On 11/24/2016 10:31 PM, Robert Baer wrote:

If the internal resistance is too big *compared to its inductance*, you
are correct. How much inductance must it have to show a L/R curve if it
has 20k resistance?

Re: Find Large Inductance Without Meter
On 11/24/2016 10:31 PM, Robert Baer wrote:

If the inductance is 2000Hy, it will take about .1 sec to reach .63mA if
20V is applied. You might argue that 2000Hy is unrealistically large,
but that is what we are trying to learn, isn't it?

Re: Find Large Inductance Without Meter
On Friday, November 25, 2016 at 7:53:40 AM UTC-8, John S wrote:

This reminds me of a story, about an oral quiz of a physics degree candidate,
who was asked how he would measure an inductor.

He replied, 'Connect a battery, switch it through an ammeter, and note the current and time'

"Oh, but wouldn't you use a function generator..." inquired the examiner.

"No, just the battery and a switch.   I've done it many times.   You take about
four turns around a battleship..."

Re: Find Large Inductance Without Meter
On 11/25/2016 6:23 PM, whit3rd wrote:

Winfield's suggestion is best. No need to look for the time constant.
di/dt does it all.

Re: Find Large Inductance Without Meter

Since the OP apparently wants to measure the inductance in order to
calculate the size of capacitor necessary to have the LC tank resonate at
10 Hz, why doesn't he just throw random capacitors in parallel until it
resonates at 10 Hz?  Skip the middleman and all that...

Re: Find Large Inductance Without Meter

capacitors are easily had with capacitance over an range of 12 orders of
magnitude, where to start?

--
This email has not been checked by half-arsed antivirus software

Re: Find Large Inductance Without Meter

I don't think it would be a good idea to start at 1pF and work up.  It
shouldn't be too hard to eliminate a few of those orders without
testing.

Re: Find Large Inductance Without Meter
On 11/25/2016 7:30 PM, Ralph Barone wrote:

What happens that no resonance is found due to low Q? A lot of time
wasted there.

Re: Find Large Inductance Without Meter
wrote:

Then the inductor isn't going to be of much use to him, is it?

Re: Find Large Inductance Without Meter
On 11/26/2016 9:46 AM, krw wrote:

It might be if he has a moving magnetic field nearby, if you can think
that reasonably.

Re: Find Large Inductance Without Meter
wrote:

He wants to resonate the circuit at 10Hz.  If he can't resonate it at
all, it's not going to resonate at 10Hz is it?

Re: Find Large Inductance Without Meter
On 11/26/2016 6:10 PM, krw wrote:

Correct. But that does not mean that it is useless. Just because it does
not resonate at 10Hz, does not mean it will not sense 10Hz. Turn in your
degree now.

Re: Find Large Inductance Without Meter
wrote:

It's obvious that you can't read.  He want's to *resonate* it at 10Hz,
dope!

Re: Find Large Inductance Without Meter
On 11/26/2016 8:40 PM, krw wrote:

It's obvious that you can't read. How stupid can you be? What's wrong
with you? If it won't resonate, it won't resonate. Damn, you're more
stupid than stupid! I'll bet your family moved away while you were out