Where LCR testers and DMM's with an LCR feature can't do it (show weird results / can't cope with the high inductance etc., or you don't possess such a device in the first place).
What you need:
1. A 5V heater mains transformer (or could use 6.3V).
2. A multimeter or DMM capable of measuring AC microamps & milliamps.
What you do:
All secondaries of transformer under test must be left open-circuit!
Set meter to AC microamps range. Or to be safe start with millamps range first, just in case.
Switch on the heater transformer (connect to the mains to make it work [test output])
Connect test transformer primary and meter in series across the 5V terminals of heater transformer.
Set meter to AC microamps range. If transformer is large, should show next to nothing on milliamps range.
Note if a split bobbin mains type, there may be losses i.e. a 'magnetization current', showing a higher reading than would otherwise be expected.
But we're assuming we're testing an OPT of some description, so that won't apply.
Write down the meter reading, as an example, 160 uA.
You know the Voltage, 5V.
Ohms law next: 5V / 0.000160 (A) = 31,250 Ohms
You know the frequency, 50 Hz.
Using a reactance chart, or the formula: R / F / pi / 2
... you can get the inductance. A rough 'pi' is 22/7
31,250 / 50 / pi / 2 = 99.4318 Henries
This is the incremental inductance, the primary only with no load on the secondary(ies).
You can then go onto loading the secondary(ies) with dummy load resistor(s) to find the equivalent primary impedance(s), the same procedure but needs a correspondingly higher range on the meter obviously.
This is 5V / current reading, same as Ohms law test above.
Seemples!
From Wireless World article
PS: a 10 Henries choke would measure as 1.56 mA AC
