How to measure inductance indirectly?

DaveC · Dec 3, 2003

I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

Mike · Dec 3, 2003

On Tue, 2 Dec 2003 21:28:45 -0800, DaveC wrote:

I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,

It can be, but your results might be better if you add a series resistance
before the tank circuit (and possibly a terminating resistance to the
transmission line from the signal generator).

If your tank circuit is placed in parallel with a low resistance (like 50
ohms from your signal generator, for example), the effective Q of the
circuit will be reduced, and it will be more difficult to distinguish the
amplitude peak when you sweep the frequency. On the other hand, increasing
the series resistance before the tank will reduce the peak amplitude that
you see at resonance, but the frequency peak will be sharper. Infinite
resistance will give the sharpest peak, but also zero voltage - somewhere
between infinite resistance and zero resistance is the 'best' measurement
point.

-- Mike --

Tony Williams · Dec 3, 2003

In article <0001HW.BBF2B48D0044934CF0305600@news.individual.net>,
DaveC <me@privacy.net> wrote:

I've got a scope and signal generator. Is it as simple as hooking
a known value of C in series with the L and sweeping the
frequency range looking for a peak in V (accross the inductor)
and a drop in I?
/|\

peak in the feed-I for a series-tuned LC.

More or less, yes. Although I go for a parallel L-C
and high value resistive connection to the generator.
Using different values of tuning-C then allows an easy
guesstimate of the inductor's own stray-C.

--
Tony Williams.

Fred Bloggs · Dec 3, 2003

I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Not really- it depends on what the application is. If for example the
inductors are for a switching power supply application, then you would
want to measure the current response to a step voltage at various levels
of DC bias- inductance is inferred from rise time L/R. If the inductors
are to be used for RF or some other linear application where Q might be
of interest then a simple technique is to use the Hay bridge at the
application frequency:

Please view in a fixed-width font such as Courier.

Hay Bridge

+-----------+-----------------------------------+
| | |
| \ _ \
| R1 / /| R3 /
| \ \
| // /
| | _ |
| | /| |
| C1 === +------------------------------+
--- /| | +-----+ |
/ \ | )|( | |
| ~ | sig | )|( | |
\ / gen | )|( === |
--- | | | | |
| +----+ | sd | +------+
| | +-|>|-+-/\/\-+-> VOM | |
| \ | | ) \
| R2 / | === Lx ) Rx /
| \ | detector | ) \
| / | | ) /
| | | | | |
| | | | | |
+-----------+------+------------+---------------+------+
|
|
---
///

Adjust R1 C1 for null at operating w=2pif where:

(R1-j/(wC1))(Rx*jwLx)=R2R3(Rx+jwLx)

Lx
=> Rx* -- = R2R3Rx or Lx=C1*R2*R3
C1

and

R2*R3
=> R1RxwLx=R2R3wLx or Rx= -----
R1

Rx
Q= ---
wLx

Winfield Hill · Dec 3, 2003

Fred Bloggs wrote...

If the inductors are to be used for RF or some other linear
application where Q might be of interest then a simple technique
is to use the Hay bridge at the application frequency:

Hay Bridge

+-----------+--------------------------------+
| | |
| \ _ \
| R1 / /| R3 /
| \ \
| // /
| | _ |
| | /| |
| C1 === +---------------------------+
--- /| | +-----+ |
/ \ | )|( | |
| ~ | sig | )|( | |
\ / gen | )|( === |
--- | | | | |
| +----+ | sd | +------+
| | +-|>|-+-/\/\-+-> VOM | |
| \ | | ) \
| R2 / | === Lx ) Rx /
| \ | detector | ) \
| / | | ) /
| | | | | |
+-----------+------+------------+------------+------+
|
---
///

Adjust R1 C1 for null at operating w = 2pi f, where:

(R1-j/(wC1)) (Rx*jwLx) = R2R3 (Rx+jwLx)

Lx
=> Rx* -- = R2R3Rx or Lx = C1*R2*R3
C1

and
R2*R3
=> R1RxwLx = R2R3wLx or Rx = -----
R1

Rx
Q = ---
wLx

Very nice. Tell us about the requirements on the transformer.

Thanks,
- Win

whill_at_picovolt-dot-com

Fred Bloggs · Dec 3, 2003

Winfield Hill wrote:

Fred Bloggs wrote...

If the inductors are to be used for RF or some other linear
application where Q might be of interest then a simple technique
is to use the Hay bridge at the application frequency:

Hay Bridge

+-----------+--------------------------------+
| | |
| \ _ \
| R1 / /| R3 /
| \ \
| // /
| | _ |
| | /| |
| C1 === +---------------------------+
--- /| | +-----+ |
/ \ | )|( | |
| ~ | sig | )|( | |
\ / gen | )|( === |
--- | | | | |
| +----+ | sd | +------+
| | +-|>|-+-/\/\-+-> VOM | |
| \ | | ) \
| R2 / | === Lx ) Rx /
| \ | detector | ) \
| / | | ) /
| | | | | |
+-----------+------+------------+------------+------+
|
---
///

Adjust R1 C1 for null at operating w = 2pi f, where:

(R1-j/(wC1)) (Rx*jwLx) = R2R3 (Rx+jwLx)

Lx
=> Rx* -- = R2R3Rx or Lx = C1*R2*R3
C1

and
R2*R3
=> R1RxwLx = R2R3wLx or Rx = -----
R1

Rx
Q = ---
wLx

Very nice. Tell us about the requirements on the transformer.

Thanks,
- Win

whill_at_picovolt-dot-com

The transformer can be a MiniCiruits RF type- out to several 100MHz on
the high end and maybe down to 10KHz on the low end- it is the detector
and its minimum detectable signal level that might require more work
than I've shown.

Jerry G. · Dec 3, 2003

The simplest way, is to get a meter that is designed to read inductance. A
good inductance meter is fairly expensive. Leader has a very good laboratory
type at a reasonable cost, compared to many of the others.

The more complex way is to make a resonant circuit with the inductor, and
use your generator and scope, with some math to figure it out, as per the
suggestions you received.

I've used a lot of Leader equipment, and found it to be very good.

Site for inductance meters
http://www.tequipment.net/LeaderLCR-745G.html

http://www.testequipmentconnection.com/manu-search.asp?smanufacturer=leader

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BBF2B48D0044934CF0305600@news.individual.net...
I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

DaveC · Dec 3, 2003

On Wed, 3 Dec 2003 9:25:54 -0800, Jerry G. wrote
(in message <bql6qq$nuk$1@news.eusc.inter.net&gt

:

The simplest way, is to get a meter that is designed to read inductance. A
good inductance meter is fairly expensive. Leader has a very good laboratory
type at a reasonable cost, compared to many of the others.

I looked around and found Wavetek's Meterman series LCR55 which retails for
about US$180. Cheapest solution for a meter.

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

Paul Burridge · Dec 3, 2003

On Wed, 3 Dec 2003 12:25:54 -0500, "Jerry G." <jerryg50@hotmail.com>
wrote:

The simplest way, is to get a meter that is designed to read inductance. A
good inductance meter is fairly expensive. Leader has a very good laboratory
type at a reasonable cost, compared to many of the others.

The more complex way is to make a resonant circuit with the inductor, and
use your generator and scope, with some math to figure it out, as per the
suggestions you received.

I've used a lot of Leader equipment, and found it to be very good.

Site for inductance meters
http://www.tequipment.net/LeaderLCR-745G.html

http://www.testequipmentconnection.com/manu-search.asp?smanufacturer=leader

Alternatively, get a grid dip meter! Made for the job...

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

John Popelish · Dec 3, 2003

DaveC wrote:

I looked around and found Wavetek's Meterman series LCR55 which retails for
about US$180. Cheapest solution for a meter.

I made one of these from a kit and it has worked quite well for me.

http://www.aade.com/lcmeter.htm

--
John Popelish

Robert Monsen · Dec 4, 2003

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BBF371EE005D3008F0305600@news.individual.net...

On Wed, 3 Dec 2003 9:25:54 -0800, Jerry G. wrote
(in message <bql6qq$nuk$1@news.eusc.inter.net&gt:

The simplest way, is to get a meter that is designed to read inductance.
A
good inductance meter is fairly expensive. Leader has a very good
laboratory
type at a reasonable cost, compared to many of the others.

I looked around and found Wavetek's Meterman series LCR55 which retails
for
about US$180. Cheapest solution for a meter.

Various folks have mentioned this one as a cheap meter that does a fairly
good job:

http://www.aade.com/lcm2binst/LC2Binst.htm

Its $100 in kit form, or $130 assembled and tested. That's USD.

Terry Given · Dec 4, 2003

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BBF2B48D0044934CF0305600@news.individual.net...

I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking
for
a peak in V (accross the inductor) and a drop in I?

Thanks,
--
DaveC

If you are mesuring smps type inductors (or, for that matter, any form of
power inductor/transformer) then V=LdI/dt is the simplest way. All you need
is a scope and a couple of bits. Charge up a large cap to some voltage V,
then short your inductor across the cap while measuring current (a hall
effect DCCT is useful here, but so is a resistor. beware peak pulse power
and resistor ESL). If the energy stored in the cap 0.5CV^2 >> energy in L at
current of interest 0.5LI^2, then V remains pretty much constant, and from
the slope of the current waveform, L=V/(dI/dt). If you dont have a
storage/digital scope, repetitively pulse the choke using a big FET. I
typically charge the caps with a current limited PSU & a series R. This
approach costs very little, takes a few seconds and gives a lot of
information - saturation current, and inductance-vs-current - iron powder &
step-gap chokes have very non-linear inductance, and the inductance at very
low current can be order(s) of magnitude higher than that at rated current,
leading to rubbish results from most inductance meters. Its easy to test a
choke at 5,000A this way too.

Terry

Klaus Vestergaard Kragelu · Dec 4, 2003

"Fred Bloggs" <nospam@nospam.com> wrote in message
news:3FCDF645.8060804@nospam.com...

I have several inductors I'd like to know the values of. I have no way
to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a
known
value of C in series with the L and sweeping the frequency range looking
for
a peak in V (accross the inductor) and a drop in I?

Not really- it depends on what the application is. If for example the
inductors are for a switching power supply application, then you would
want to measure the current response to a step voltage at various levels
of DC bias- inductance is inferred from rise time L/R. If the inductors
are to be used for RF or some other linear application where Q might be
of interest then a simple technique is to use the Hay bridge at the
application frequency:

Also for SMPS or RFI filters one major thing to look out for is the
saturation current (core saturation) versus the core temperature. At
elevated temperatures, 100 degrees or so, the saturation current may be
significantly reduced.....

Cheers

Klaus

Martin Riddle · Dec 5, 2003

"Fred Bloggs" <nospam@nospam.com> wrote in message news:3FCDF645.8060804@nospam.com...

I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Not really- it depends on what the application is. If for example the
inductors are for a switching power supply application, then you would
want to measure the current response to a step voltage at various levels
of DC bias- inductance is inferred from rise time L/R. If the inductors
are to be used for RF or some other linear application where Q might be
of interest then a simple technique is to use the Hay bridge at the
application frequency:

Please view in a fixed-width font such as Courier.

Hay Bridge

+-----------+-----------------------------------+
| | |
| \ _ \
| R1 / /| R3 /
| \ \
| // /
| | _ |
| | /| |
| C1 === +------------------------------+
--- /| | +-----+ |
/ \ | )|( | |
| ~ | sig | )|( | |
\ / gen | )|( === |
--- | | | | |
| +----+ | sd | +------+
| | +-|>|-+-/\/\-+-> VOM | |
| \ | | ) \
| R2 / | === Lx ) Rx /
| \ | detector | ) \
| / | | ) /
| | | | | |
| | | | | |
+-----------+------+------------+---------------+------+
|
|
---
///

Adjust R1 C1 for null at operating w=2pif where:

(R1-j/(wC1))(Rx*jwLx)=R2R3(Rx+jwLx)

Lx
=> Rx* -- = R2R3Rx or Lx=C1*R2*R3
C1

and

R2*R3
=> R1RxwLx=R2R3wLx or Rx= -----
R1

Rx
Q= ---
wLx

Is this a classic Grid Dipper?

Cheers

Eric · Dec 5, 2003

Mike <mike@nospam.com> wrote in message news:<il1kr8aj5kbf$.1vlqrxtfz8gpe.dlg@40tude.net>...

On Tue, 2 Dec 2003 21:28:45 -0800, DaveC wrote:

I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,

It can be, but your results might be better if you add a series resistance
before the tank circuit (and possibly a terminating resistance to the
transmission line from the signal generator).

If your tank circuit is placed in parallel with a low resistance (like 50
ohms from your signal generator, for example), the effective Q of the
circuit will be reduced, and it will be more difficult to distinguish the
amplitude peak when you sweep the frequency. On the other hand, increasing
the series resistance before the tank will reduce the peak amplitude that
you see at resonance, but the frequency peak will be sharper. Infinite
resistance will give the sharpest peak, but also zero voltage - somewhere
between infinite resistance and zero resistance is the 'best' measurement
point.

-- Mike --

can anyone be more clear on how to get the current drop (I) ? and for
the voltage, is it just simply measure the highest peak value ? ( I
only have a pulse generator)

thank you..

eric

Eric · Dec 5, 2003

Mike <mike@nospam.com> wrote in message news:<il1kr8aj5kbf$.1vlqrxtfz8gpe.dlg@40tude.net>...

On Tue, 2 Dec 2003 21:28:45 -0800, DaveC wrote:

I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,

It can be, but your results might be better if you add a series resistance
before the tank circuit (and possibly a terminating resistance to the
transmission line from the signal generator).

If your tank circuit is placed in parallel with a low resistance (like 50
ohms from your signal generator, for example), the effective Q of the
circuit will be reduced, and it will be more difficult to distinguish the
amplitude peak when you sweep the frequency. On the other hand, increasing
the series resistance before the tank will reduce the peak amplitude that
you see at resonance, but the frequency peak will be sharper. Infinite
resistance will give the sharpest peak, but also zero voltage - somewhere
between infinite resistance and zero resistance is the 'best' measurement
point.

-- Mike --

can anyone be more clear on how to get the current drop (I) ? and for
the voltage, is it just simply measure the highest peak value ? ( I
only have a pulse generator)

thank you..

eric

How to measure inductance indirectly?

DaveC

Guest

Mike

Guest

Tony Williams

Guest

Fred Bloggs

Guest

Winfield Hill

Guest

Fred Bloggs

Guest

Jerry G.

Guest

DaveC

Guest

Paul Burridge

Guest

John Popelish

Guest

Robert Monsen

Guest

Terry Given

Guest

Klaus Vestergaard Kragelu

Guest

Martin Riddle

Guest

Eric

Guest

Eric

Guest

Welcome to EDABoard.com

Sponsor

Online statistics

Forum statistics

How to measure inductance indirectly?

DaveC

Guest

Mike

Guest

Tony Williams

Guest

Fred Bloggs

Guest

Winfield Hill

Guest

Fred Bloggs

Guest

Jerry G.

Guest

DaveC

Guest

Paul Burridge

Guest

John Popelish

Guest

Robert Monsen

Guest

Terry Given

Guest

Klaus Vestergaard Kragelu

Guest

Martin Riddle

Guest

Eric

Guest

Eric

Guest

Log in

Welcome to EDABoard.com

Sponsor