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Old 15th May 2021, 7:53 am   #1
retailer
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Default DIY Inductance bridge

I'm working on an Owens inductance bridge and have reached the preliminary test stage, not having used a bridge type of measuring device before I'm a little unsure what to expect, I'm planning a builtin AC digital voltmeter as the null detector but for now am using my Fluke DMM. I'm getting varying results for the null voltage - it is never zero, with some chokes I can get it as low as 30mV, with commercial good quality output transfomers I can only get it down to 90mV or so, does this seem right.
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Old 15th May 2021, 8:31 am   #2
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Default Re: DIY Inductance bridge

We'd probably need to know the other component values and the amplitude(s) and frequencies of the applied voltage(s) to judge whether the residual voltages are reasonable or not. In my experience inductors are the most imperfect of the three types of passive component. As well as their 'stray' resistance, which the Owens bridge does take into account, they can easily have stray capacitances too, which it doesn't.

Cheers,

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Old 15th May 2021, 9:19 am   #3
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Default Re: DIY Inductance bridge

... and of course the contruction of the bridge itself can result in residual balance voltage.

I do not think I have ever seen a bridge balance a real-world inductor with totally zero balance.

By the way, one advantage of using magic eyes as the balance indicator is that they do seem to show a bit more than a meter would do e.g. you kind of see any waveform distortion issues because it changes the look of the display (OK, depends on the circuit).
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Old 15th May 2021, 9:22 am   #4
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Default Re: DIY Inductance bridge

AAHHH GMB!!
Vacuum versus digital again?

My take

Joe
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Old 15th May 2021, 10:53 am   #5
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Default Re: DIY Inductance bridge

I have a bridge that is fully working.
It has an EM34 magic eye that is in mint condition.
It uses an internal resistance chain and has external connections for a decade box for capacitance or inductance.
I rarely use it as I have another unit with a meter and internal decade components.
If I sold it the magic eye would put the price up a fair bit.
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Old 15th May 2021, 11:04 am   #6
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Default Re: DIY Inductance bridge

Have you had a look at the lab-grade bridges made by Wayne-Kerr?

These measure conductance and susceptance separetely, at teh same time. You could use the same system without their ultra-precision decade transformers and just use 10-turn pots for balancing the bridge.

The clever bit is that their AC voltmeters (two of them) don't simply rectify the unbalance signal, they use synchronous detectors, essentially diode bridges commutated by the same oscillator signal which drives the bridge. The output of these rectifiers will be zero on balance, but will show negative one side of balance, and positive the other side.

Voila! a bridge which when off-balance shows you which direction to turn the knob!.

Wayne-Kerr even put scales on their meter to tell you how far to change the balance controls to get closer to balance.

The next trick is a biggie. They duplicated the synchronous detecrtor, amplifier, meter sections and commutated the second synchronous detector with the oscillator signal phase shifted by 90 degrees. This meter tells you the degree of inbalance for the susceptance balance. It's like having two bridges running in parallel, one balancing the real part, and the other the imaginary part of the admittance of the thing being measured. You can just put the values of the real and imaginary controls into a spreadsheet and get R/L/C/G results as you wish.

Once you've driven one of these bridges, you don't want to go back. And yes, by balancing up both their G and C controls, they do go all the way down to a complete null in both real and imaginary terms.

On the full Wayne Kerr instruments, the balance controls are a set of decadic transformers (10 taps) which are nested to give 2 or 3 digits of precision controlled by push-buttons. The meter is rigged to show which button to press next. Once you press a button for that decade, the meter amplifier gain goes up x10 and it shows you which button to press for the next decade!

This is all duplicated for the real and imaginary sides.

In use, you select a range to get both meters on-scale with all the transformer tap buttons out. You read G meter, press the appropriate button of the top decade of G. Read the C meter and press the appropriate button for the top decade of C. the meters now get 10x more sensitive, so read G again and press the indicated button on the next decade of G, then do the same for C. You work your way donw the decades alternating from G to C to G to C. When you've run out of transformer decades, the meters have gone up in gain x10 again, and read the remainders.

This is all done with analogue circuitry and transformer tappings. No digital logic, Joe!

A lower precision inatrument could be done with potentiometers in place of the ratio transformers, but the synchronous rectifiers in quadrature trick is amazing, and you really can measure the real and imaginary terms of any impedance.

If it sounds tedious, you connect a component, select a range and press 6 buttons, following the guidance of the meters and then you have the unknown thing measured to 4+digits resolution in both real and imaginary components.

https://www.vintage-radio.net/forum/...d.php?t=133081

They are quite a looker, and in terms of lab-cred, score you almost as many points as a Jacob's ladder running in the corner, certainly equal to the score achieved by a scope left running a Lissajous figure.

David
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Old 15th May 2021, 4:18 pm   #7
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Default Re: DIY Inductance bridge

The bridge is the Steve Bench implementation, it would be nice to have a commercial unit by Wayne-Kerr or in fact any reasonable commercial unit but for occasional hobby use a home brew version will have to do, I've run some factory made inductors and chokes through it and it does produce results that agree reasonably closely with the factory made inductors etc. A small14Hy marked power supply choke came up as 15Hy with no DC and 13Hy with 50mA DC, and a 54mHy ferrite cored inductor came up 50mHy, the null voltage was in the region of 30-40 mV. I checked a couple of output transformer primary windings and it is with these that the null voltage is in the region of 70-90mV, I'll spend a bit more time tomorrow, I found it does take a bit of a 'knack' to use, it is not like a multimeter where one just sticks on the probes and reads the value.

While I tested it using my DMM as the null indicator I made the mistake of building it with an analog AC voltmeter 0-30v as the null indicator - not really knowing I would be looking for millivolts rather than volts, I tried the analog meter but found it was totally useless. I have a spare 200mV AC DPM and will need to make a new front panel so it can be used, it suffers from non linearity once the measured voltage gets below 20mV but that won't matter as I'm only looking for a null point.
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Old 15th May 2021, 7:56 pm   #8
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Default Re: DIY Inductance bridge

Using unfiltered mains as the excitation waveform relies on the mains wave being pure. Your bridge is unlikely to be balanced at all the harmonic frequencies present at once. This can prevent you getting a complete null on a broadband AC meter and leave you with some doubt and uncertainty. The indicated inductance value can be slewed away from the real value as you tune the bridge for a compromise trying to get the best overall null at all frequencies present. It isn't obvious that this is what you're doing even while you're doing it!

You may want to filter either the incoming mains to clean up the excitiation, or you could tune the meter to read only 50Hz.

David
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Old 15th May 2021, 9:22 pm   #9
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Default Re: DIY Inductance bridge

Quote:
Originally Posted by Radio Wrangler View Post
Have you had a look at the lab-grade bridges made by Wayne-Kerr?

These measure conductance and susceptance separetely, at teh same time. You could use the same system without their ultra-precision decade transformers and just use 10-turn pots for balancing the bridge.

The clever bit is that their AC voltmeters (two of them) don't simply rectify the unbalance signal, they use synchronous detectors, essentially diode bridges commutated by the same oscillator signal which drives the bridge. The output of these rectifiers will be zero on balance, but will show negative one side of balance, and positive the other side.

[snip]...

https://www.vintage-radio.net/forum/...d.php?t=133081

They are quite a looker, and in terms of lab-cred, score you almost as many points as a Jacob's ladder running in the corner, certainly equal to the score achieved by a scope left running a Lissajous figure.

David
Recently acquired a slighty newer Wayne-Kerr bridge, it's currently in the repair queue as it neither DVM seems to be working other then being stuck showing zeros.
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The advert has a bit of artist exaggeration for the symbol display, they are in reality only small sections with filament lamps behind, guessing they didn't make a readout-bulb* with all the correct symbols required.
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I'm guessing this is the precision transformer you were referring too.
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Also need to find some triax connectors & cable to make up proper test leads for it.

*don't want to use the correct name, as I don't want to help people that are only interested in removing them.

David
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Old 15th May 2021, 11:33 pm   #10
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Default Re: DIY Inductance bridge

You may have noticed I hadn't mentioned them either, and the earlier bridges are full of them too.

One trick with the older bridges is the the source and detectors will run off of a quite small battery, but without mains for the high voltage the indicator bulbs remain dark.

David
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Old 17th May 2021, 1:55 am   #11
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Default Re: DIY Inductance bridge

I've been doing some more testing and results are mixed, iron cored inductors/chokes seem to be ok, output transformer primaries are measuring at 200Hy where I would have expected something like 30-60Hy.
I can't achieve a null with very small ferrite inductors marked 2.6mHy, the meter showing around 700mV regardless of the switch settings which would indicate some unwanted oscillations. I made up a temporary passive 50Hz bandpass filter, (2 resistors and 2 capacitors this can be seen poking out the top on the left) it did not seem to help.
With my scope across the meter connections I could - if I used my imagination - see a sort of sine wave with some of the larger inductors and nothing like a sine wave with the smaller inductors, the indicated frequency on the scope was jumping around from tens of Hz to a few Khz.
I'm not quite sure if it's my layout possibly causing capacitive coupling between parts of the bridge circuit or that I may just need a better 50Hz bandpass filter on the output, as it stands the instrument is not really useable, I'm going make a breadboard circuit on the benchtop and see if i can get meaningful results on the output transformers - hopefully it may give me an idea if my layout is the problem, the case I made is quite small and things are a bit packed in.
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Old 17th May 2021, 1:46 pm   #12
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Default Re: DIY Inductance bridge

I think using 50Hz is a mistake as it means your circuit must be totally screened from any mains influences. It is also not the best frequency for measuring inductance as typical impedances will be a bit on the low side. Most bridges would use a higher frequency for mH ranges.

I would dump the mains and have a battery powered oscillator to energise it.

The ideal null detector is an amplifier and magic eye. Second choice is amplifier with AGC and a meter - giving a very non-linear response. If you can add a phase detector to show which way to go then that is even better.
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Old 17th May 2021, 2:13 pm   #13
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Default Re: DIY Inductance bridge

Many bridges use an oscillator on 10^4 radians/second frequency, which makes conversions between reactance and L or C more convenient in the age before calculators.

It also raises the reactance of small value inductances and makes them easier to measure.

David
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Old 17th May 2021, 3:14 pm   #14
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Default Re: DIY Inductance bridge

Hi Folks, the bible on bridges is "Alternating Current Bridge Methods" by B Hague. up to the 5th edition by 1945. 1st edn was 1923

Lots of good techniques in there as well as some obscure bridge types.
Plenty of tips on screening an the use of guard potentials.

Try bringing out the bridge leads and the det leads separately to front panel terminals then strap them together

NPL in the UK published some monographs on bridge methods in the 50's/ 60's


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Old 17th May 2021, 3:46 pm   #15
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Default Re: DIY Inductance bridge

The few induction bridges that I have manuals for seem to use 1KHz and it probably makes sense, I found an error in my passive filter, wrong capacitor value, should have worn my glasses when I rummaged through the box looking for a close enough value, with that fixed the filter is doing a reasonable job, sinewave looks a bit better, it is not perfect but doesn't need an imagination to see it is a sinewave.
I did try a benchtop breadboard setup that duplicated the circuit, a standard Owen's bridge, aligator clip leds and potentiometers in place of switched 1% resistors, not calibrated of course so I had to set null, measure the resistance values and then calculate the inductance, I got very similar results to my project so I don't think layout has much bearing.
An output transformer I wound for a guitar amp project a few years measures 18Hy on the primary, it is nothing fancy, I recall I aimed for 70hz-15Khz so 18Hy seems about right. I plan to hook up my signal generator as an AC source in place of the mains and see if that makes any difference at 1KHz, I may even settle for a couple of sockets on the back and use my signal gen as the AC source, battery power makes sense but I know this is something that will be a long time between uses and batteries tend to go flat just when you need to use it.
The only thing that drew me to this project is the ability to put DC up to 200mA through the test inductor/transformer, I'd like to wind some SE transformers for a 2A3 SE amp project and I though it may help determining the optimum lamination gap - enough so the core doesn't saturate but not too much so the bottom end performance is affected.

Jeez - radians/second, making me think back a few years there, without googling it I can only recall 2pi radians in a circle which is 360deg or 1 cycle ?
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Old 18th May 2021, 3:35 am   #16
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Default Re: DIY Inductance bridge

The one I showed in post#5 has 1khz or mains derived 50hz.
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Old 20th May 2021, 2:49 pm   #17
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Default Re: DIY Inductance bridge

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.
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Old 20th May 2021, 4:02 pm   #18
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Default Re: DIY Inductance bridge

You have checked the "bleeding obvious" I presume? I.e. that it isn't open circuit?

When something doesn't make sense you have to look for the hidden assumption. You keep assuming it is an inductance.

So a bridge "heading for a null" when you run out of range is not balanced. It may be heading there for ever!
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Old 20th May 2021, 11:13 pm   #19
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Default Re: DIY Inductance bridge

What was the frequency range of the signal generator the transformer was salvaged from?
This might give us some idea about the value to be expected.
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Old 21st May 2021, 12:23 am   #20
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Default Re: DIY Inductance bridge

The transformer is not open circuit, I ran AC through it to determine the impedance ratios of the primary and secondary, primary has ultra linear taps - the signal gen was made by Dawe Instruments UK, an Audio Signal Generator model 440B, I been unable to uncover any info about it on the internet but being 'audio' it is more than likely it covered 20Hz to 20KHz - the transformer was made by Albion, there is an Albion part number on it but I couldn't find an Albion catalog anywhere in my internet searching - my Fluke 87 acts in an odd way when I try to measure the DC resistance of the primary, the display just flashes as the Fluke is unable to settle on a range, I have to switch to a range manually before I can measure the DC resistance, this doesn't happen if I measure each primary half separately.
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