Am thinking of rigging up a low-pass filter circuit like this, to be able to measure inductances of self-wound coils.
Is my thinking correct or have i missed some other fundamental knowledge ?
I'm not sure how the duty cycle would come into this - something less than 50% at least ?
Would the voltage measured be zero when the low pass filter is blocking the current ?
(Should i be measuring current instead ?)
Seems to me, you could measure inductance with a single rising edge using the equation:
V = L di/dt
You measurement would have to be a single capture relative to the initiation of the pulse.
If you were thinking of a more steady state measurement you would have to AC couple your excitation voltage.
BabyGeezer:
Am thinking of rigging up a low-pass filter circuit like this, to be able to measure inductances of self-wound coils.Would the voltage measured be zero when the low pass filter is blocking the current ?
This might not work, because these cutoff frequency formulae/formulas are based on sinusoidal input waveform, and sinusoidal output waveform - not square-wave inputs (such as PWM).
The low pass filter is not going to result in a sinusoidal waveform across the resistor. That is - applying a PWM waveform to a low pass filter won't give a sinusoidal waveform at the output.
Could search online for inductance measurement methods - like AC bridge method. This won't involve PWM though, and it probably won't involve arduino.
Voltage square wave into inductor gives a triangular current waveform. Add a resistor and that triangle becomes segments of decaying exponential.
The formula in the original post doesn't work as written because a square wave contains the fundamental PWM frequency and odd harmonics of that frequency.
Per post #1 one could apply a step change, and measure the voltage at a specific time after the step and solve the time domain response equation 
A better approach might be to put the inductor in an LC resonant circuit, ping it with a pulse, and measure the frequency of oscillation. e.g. Easily measuring inductance with Arduino | ReiBot.org
JohnRob:
Seems to me, you could measure inductance with a single rising edge using the equation:V = L di/dt
You measurement would have to be a single capture relative to the initiation of the pulse.
so then; L = V * dt/di ;
would dt have to be small, or could a longer time span be used ?
i don't have an oscilloscope, so i'd be measuring di rather crudely.
JohnRob:
If you were thinking of a more steady state measurement you would have to AC couple your excitation voltage.
i don't know what that means - does 'AC' here mean it has to have a negative voltage rail and not just GND ?
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Southpark:
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The low pass filter is not going to result in a sinusoidal waveform across the resistor. That is - applying a PWM waveform to a low pass filter won't give a sinusoidal waveform at the output.
i see.
Southpark:
Could search online for inductance measurement methods - like AC bridge method. This won't involve PWM though, and it probably won't involve arduino.
okay, thanks for the tip - will have to look that up.
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MarkT:
Voltage square wave into inductor gives a triangular current waveform. Add a resistor and that triangle becomes segments of decaying exponential.
like this ?
MrMark:
The formula in the original post doesn't work as written because a square wave contains the fundamental PWM frequency and odd harmonics of that frequency.Per post #1 one could apply a step change, and measure the voltage at a specific time after the step and solve the time domain response equation ...
okay - that's a lot more math to learn then.
MrMark:
A better approach might be to put the inductor in an LC resonant circuit, ping it with a pulse, and measure the frequency of oscillation. e.g. Easily measuring inductance with Arduino | ReiBot.org
thanks for the suggestion and link, will study it accordingly.