Electronic – Measuring small inductances

inductanceinductionmeasurement

I'm working on a small hobby project – a small (230V wall plugged) coreless induction furnace. I generally know what I'm doing, chosen a topology, calculated inverter and coil parameters, desired resonance frequency etc. Now my problem is – how do you, in reality, measure the very small inductance that the coil would have, on the order of 1-5 microhenrys? Is banging the coil with a voltage step and measuring the time it takes to reach certain current a good idea, and if not – how?

Or am I trying to reinvent the wheel and should I just shop for a LCR meter? I've seen some very reasonably priced models (for examplpe Voltcraft products, ~100 EUR) that are advertised as capable of measuring from 0.1 uH. Are they any good?

Best Answer

Arguably, the easiest way is to just measure it:

      +--------+          
      |        | 
      |      [50R]
      |        |
      |       [C]
    [GEN]      |
      |        +------+
      |        |      |
      |       [L]  [SCOPE]
      |        |      |
      +--------+------+ 

With \$C\$ known, adjust the generator's output frequency for a peak across the scope, then determine the reactance of the capacitor with:

$$X_c=\frac1{2\pi fC}$$

Next, since the reactance of the capacitor and the inductor will be equal at resonance, (the frequency at the maximum amplitude of the peak on the scope) solve for the inductance with:

$$L=\frac{X_c}{2\pi f}$$

Both equations can be combined:

$$L=\frac{1}{(2\pi f)² C}$$

You may need to adjust the value of the \$R\$ in order to get a nice peak.