Balancing a Hays inductance bridge - UK Vintage Radio Repair and Restoration Discussion Forum

mpegjohn · 4th Aug 2019, 9:40 am

I have mocked up a Hays inductance bridge, to allow me to measure the inductance of some DC chokes.
I have a digital inductance meter, that is good, but only supplies 1V to the sample at 100Hz.
I wanted something like 10V at 100Hz, and be able to supply a DC current.

So I setup my bridge so I had 100Hz, 1V initially, the same as the digital meter, for comparison.

The Hay bridge gave me a reading of 11.2H and the meter says 10.1H

The best balance I can get, using a scope as the balance indicator, still leaves around 100mV of cruft.

Higher excitation leaves more 'cruft'. I was expecting a true Null, but I am unable to get rid of this completely.

Any ideas what I need to do to get a true null, or is it not possible?

Regards,
John

Herald1360 · 4th Aug 2019, 9:55 am

The trouble with dc chokes is that they're more like exotic RLC circuits than simple Ls. Not to mention all the core nonlinearities.......

Getting a decent bridge null involves balancing out the R and C components as well as the L. Different flavours of bridge do this in their own (probably originally patented) ways.

At low frequencies the series resistance is probably the most noticeable, but aside from power losses, it's not a problem for series choke use since it just makes the series impedance a little higher.

4th Aug 2019, 9:40 am	#1
mpegjohn Hexode Join Date: Aug 2013 Location: Chichester, West Sussex, UK. Posts: 270	Balancing a Hays inductance bridge I have mocked up a Hays inductance bridge, to allow me to measure the inductance of some DC chokes. I have a digital inductance meter, that is good, but only supplies 1V to the sample at 100Hz. I wanted something like 10V at 100Hz, and be able to supply a DC current. So I setup my bridge so I had 100Hz, 1V initially, the same as the digital meter, for comparison. The Hay bridge gave me a reading of 11.2H and the meter says 10.1H The best balance I can get, using a scope as the balance indicator, still leaves around 100mV of cruft. Higher excitation leaves more 'cruft'. I was expecting a true Null, but I am unable to get rid of this completely. Any ideas what I need to do to get a true null, or is it not possible? Regards, John

4th Aug 2019, 9:55 am	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Balancing a Hays inductance bridge The trouble with dc chokes is that they're more like exotic RLC circuits than simple Ls. Not to mention all the core nonlinearities....... Getting a decent bridge null involves balancing out the R and C components as well as the L. Different flavours of bridge do this in their own (probably originally patented) ways. At low frequencies the series resistance is probably the most noticeable, but aside from power losses, it's not a problem for series choke use since it just makes the series impedance a little higher. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O