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Arduino Inductance measuring meter

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picclock:
I found myself needing some moderately accurate inductors for a project. If you have experience of electronics you will know that designers have a reluctance ( :zap:) to use inductors because of size, cost and the large range of values, operating frequencies and currents.

A quick search around the net shows a design using a mini(328) arduino an op amp and a handful of components. I figure that I can go one better and use just an arduino,  some resisters and 1 capacitor. So this thread will be a record of my attempt to do so.

To achieve these economies I will use the internal comparator amplifier and the tri state characteristics of one or more of the port pins.

Method of operation is to cause the IUT (inductor under test) to resonate with a known value of capacitor. Then measure the period of induced oscillation to ascertain the frequency and hence the inductor value. For a readout I will use the serial monitor from the arduino compiler, though I may add a simple oled display if I think its a keeper.

Attached is proposed circuit.

Best Regards

picclock





 

eskoilola:
It all depends on how accurate readings and what type of inductors and what range of inductances You are planning to measure.

Some suggestions:

Make the capacitor(s) switchabe to a few known values. this way You can extend the measure range. Mechanical Switch is far the best but one might use J-fet for switching as well. Analog Switches (4066) might be useful as well but they have some internal capacitances and are not suitable for higher frequencies aka lower inductance values.

Use a separate oscillator circuit and buffer the output before feeding to MCU. Using the MCU OpAmp as oscillator is of course tempting but that brings in quite a few limitations.

Another Approach is to inject known frequency to the IUT + known capacitor circuit and use it as a filter. Then just detect the Signal Level (J-fet) and You will get a frequency response of the resonant circuit. This allows You to measure the Q-value as well as the inductance of the coil.

picclock:
Hi eskoilola

Easy to extend the range by extending the time count. Should not need any range selection.

Problem with filter circuit is it adds to complexity without increasing accuracy. Finding resonant frequency can often result in multiple modes of resonance, again, problems with no benefit.

As it stands its a simple circuit with no switches or extras. Just connect an inductor and get the value. Highest frequency I reckon can be reliably measured is in the 100's of khz. Combine this with a large (non inductive  :palm:) capacitor and the jobs a good'un (well as long as your in the uH range and upward).

Best Regards

picclock

 

PK:
Rather that finding resonance (where your accuracy depends, in part, on Q) would you not just use an inductance bridge? 

picclock:
hi pk

For a bridge you would have to have much higher complexity and many more components, plus as a bridge is required to balance, different frequencies of operation and or different bridge components.

The circuit above once in resonance will have minimal loading so Q should not be a factor. The input pins of the processor only contribute pF, swamped by the external capacitor, and have high resistance in the order of MOhms, so once ringing frequency should be unaffected.

By enabling/disabling the counter on a positive going edges the interrupt generated delays will be canceled giving quite good accuracy for a minimum component device (I hope).

Best Regards

picclock

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