[retailer]

I have more or less finished the inductance bridge as described by Steve Bench not that much to do just the front panel, paint the cover and find matching knobs in my parts box, I swapped the role of the 2 meters - the analog meter is now for the DC bias when it is used and AC 200mV DPM with a hi/lo sensitivity switch is for null. Finding null is not always quick, it is a matter of going back and forward between switches and gradually getting the null voltage down towards zero, I find it is never zero usually in the range of 4mV to 10 or so mV. After reading quite a few posts in the forum on use of inductance bridges I came to the conclusion that not everyone likes them, must be an acquired taste like red wine. For me it was not complicated or costly to build - I had all of the hardware switches etc in my spare parts drawers, I did have to buy some parts for the variable DC current supply - all up cost so far around $75, I wouldn't want to spend any more, used older units that my need repairs can be had from around $150 up.

I spent some more time with it and for the most part it is giving good enough predictable results - a power supply choke by Rola Aust marked 14H at 60mA measures 15.5Hy with no DC bias and 13.6Hy with 60mA DC bias.

As it stands it isn't really possible to measure inductances much less than 100 millihenrys with any great accuracy - there are 3 switches used for null, the finest is in tenths, this combined with the lowest range of x 0.1 gives 0.01Hy or 10 millihenrys, I checked a 54mHy inductor, with no setting between 50 and 60 I had to settle on 50mHy - not very accurate for small inductors, I may if I find the need investigate the possibility of switching in a smaller standard reference capacitor - presently 1uF, a 0.1uF would divide the smallest range by 10.

I went through the output transformers I have and small single ended mantle radio style transformer, primary inductance measured 12Hy with no DC bias and drops to less than 7Hy with 50mA DC bias, I guess that is about what one would expect, they are made to a price. A decent looking good sized power supply choke that came from an organ amplifier measured 7.5Hy with no DC bias and 7.2Hy with 80mA bias

One output transformer however has proved a problem, it was salvaged from a damaged signal generator running a pair of EL84's in the output, secondary has 2 x 4ohm windings which can be series or parallel. On my project inductance bridge with this transformer I only get close to null with all of the switches set to max which is 320Hy, while it is possible that it really is around 320Hy or just over I somehow doubt it, I would have thought possibly up to 100 or 120Hy max.

Still not convinced that the OT could be over 300Hy I referred to Eric Lowdon's 'Practical Transformer Design Handbook' - on pg 367 he gives a method using a scaled vector diagram where the reactance can be determined and then the inductance calculated using XL = 2piL.

The procedure uses a known value resistor connected in series with the primary winding, an AC voltage is applied across the two parts and the voltage measured across the resistor, the primary and also across both in series, the resistor value and measured voltages are used to draw a scaled vector diagram.
I tried this with a 45Vac supply and a 1% 12K series resistor, voltage across the resistor was 3.753v, across primary was 44.4v and across both 46.9v. I went through the steps and drew the vector diagram with Cad at 10:1 scale the final computed inductance was 346Hy, I am however still a bit skeptical so I'm going to do some internet searching for alternate inductance checking method.