Measuring variation in inductance

[grubertm@gmail.com]

I was wondering what precautions I should take for measuring a coil's
inductance. Following some capacitive sensing approaches my idea was
to use a MCU pin for pulling a pin connected to the coil L1 HIGH and
then measuring the voltage drop across a resistor R1 in series to the
coil.

WRITEPIN+-----------o----UUUU---o--------o---------R1--------| GND
\----|<|----/ \--READPIN

By measuring the time it takes for READPIN to reach HIGH it should be
possible to calculate L1 given a known R1 value..right?
My concern is about the coil's kickback: is the parallel diode
sufficient to protect the readpin from excessive voltages ?

 

[Jasen Betts]

On 2009-01-24, grubertm@gmail.com <grubertm@gmail.com> wrote:

I was wondering what precautions I should take for measuring a coil's
inductance. Following some capacitive sensing approaches my idea was
to use a MCU pin for pulling a pin connected to the coil L1 HIGH and
then measuring the voltage drop across a resistor R1 in series to the
coil.

WRITEPIN+-----------o----UUUU---o--------o---------R1--------| GND
\----|<|----/ \--READPIN

By measuring the time it takes for READPIN to reach HIGH it should be
possible to calculate L1 given a known R1 value..right?
My concern is about the coil's kickback: is the parallel diode
sufficient to protect the readpin from excessive voltages ?

you've got the coil's parallel capacitance to consider too.
and the threshold that the readpin responds at may be unpredictable,

 

[Chris Jones]

grubertm@gmail.com wrote:

I was wondering what precautions I should take for measuring a coil's
inductance. Following some capacitive sensing approaches my idea was
to use a MCU pin for pulling a pin connected to the coil L1 HIGH and
then measuring the voltage drop across a resistor R1 in series to the
coil.

WRITEPIN+-----------o----UUUU---o--------o---------R1--------| GND
\----|<|----/ \--READPIN

By measuring the time it takes for READPIN to reach HIGH it should be
possible to calculate L1 given a known R1 value..right?
My concern is about the coil's kickback: is the parallel diode
sufficient to protect the readpin from excessive voltages ?

The easiest way is to build an oscillator.

http://ironbark.bendigo.latrobe.edu.au/~rice/lc/

Chris

 

[grubertm@gmail.com]

The easiest way is to build an oscillator.

http://ironbark.bendigo.latrobe.edu.au/~rice/lc/

Chris

Thanks for the link Chris !

I built my circuit as above and got some decent results using a
somewhat largish inductor. Smaller ones (.03uH) were charging too
quickly to measure with my microcontroller- that's where the above
approach might come in handy.