"Paul Burridge" wrote in message
...
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?


"splat" test

V=L*dI/dt

charge up a large cap to some voltage V

short the cap thru the inductor, and measure the current with a scope
(digital is great, as long as the single-shot sample rate is high enough - I
did this once with an HP 56000 series scope, and got 1 data point - very
easy to fit a line to, very hard to fit the right line to!)

knowing dI/dt and V, calculate L

then, you can see saturation from where dI/dt increases rapidly (invaluable
for power electronics)

also, ensure that 0.5CV^2 0.5LI^2 at the current of interest - this
ensures that V is approximately constant

I have a couple of 33mF 35V caps in parallel, with 10 x 1R MRS25 resistors
paralleled as a current sensor. It takes about 45s to set up for a
measurement. I often use the shaft of a screwdriver to make the "splat" - a
nice hard material is good.

I have used this technique successfully to measure inductors ranging from
the tiny ( 500nH) to huge (3mH 2,000A chokes). Actually, big chokes tend
not to read true on LCR meters, as the magnetic material permeability is
often much higher at very low currents - especially true of iron powder.

if you dont have a digital scope, make a tiny, LF oscillator with a 555.
Drive a grunty FET, with a small (I use 0.1R - 10R) source resistor. Then,
you'll get a repetitive waveform, which works nicely on any analogue scope.
If Rsense is nice and low, use a 50R BNC cable to connect to scope, with a
50R terminator at the scope - lovely clean waveforms.

I built a little tester like this to test air gaps in planar cores for a 55W
smps that lives inside an LED video screen (actually, thousands of them live
inside). works a treat, and cost $5.

One of my techs once built a splat tester for big chokes - he had several
1200V 600A IGBT's in parallel running as a series pass regulator, with about
100mF of capacitance (charged to 700Vdc thru several lightbulbs) to control
V, even at currents on the order of 5,000A. A honking great LEM DCCT
measured the current, and the "switch" was purely mechanical - he used the
pole faces from an HV contactor, as the material is astonishingly arc
resistant, so no pitting/welding