Does anyone know what a Dielectric loss angle bridge is?

  • Thread starter tim9000
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  • #1
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All I can find on google is about what the dielectric loss angle is.
I imagine the bridge is something like a Whitstone bridge but for this....

Thanks
 

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  • #2
Bystander
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Oliver & Cage, AC Bridges (?).
 
  • #3
The Electrician
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  • #4
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Oliver & Cage, AC Bridges (?).
I was looking for a simplistic response but I just downloaded the ebook so I'll look at the AC bridges chapter, thanks.

Here's a thorough app note on the topic:

http://cp.literature.agilent.com/litweb/pdf/5989-2589EN.pdf [Broken]

Here are links to some of the equipment needed:

http://www.keysight.com/en/pd-1000000508:epsg:pro-pn-16453A/dielectric-material-test-fixture?cc=US&lc=eng

http://www.etesters.com/see/Dielectric_Constant/Fixtures
I'm not sure what the significance of the .png 's were but the 'keysight' link didn't work. I'll look at the:
http://cp.literature.agilent.com/litweb/pdf/5989-2589EN.pdf [Broken]
tomorrow. Thanks
 
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  • #5
The Electrician
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I don't know how those png's got there!

The simplistic answer is that nobody uses bridges anymore to measure dielectric properties of materials. Nowadays, an impedance analyzer takes the place of a bridge. The idea is to place a sample of the material in a suitable fixture which is usually a pair of metal plates with the dielectric material between them, making a capacitor. Then the real part of the impedance of the fixture gives a measure of the losses of the material being measured.
 
  • #6
Baluncore
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The dielectric loss of a capacitor is a resistive component that combines with the reactance of the capacitor to give a complex impedance. Now consider an AC bridge used to compare capacitor ratios.

With a simple amplitude detector the null will not be deep due to the fact that it is comparing a reactive C reference with a complex C lossy, that is with some resistive component. The single bridge balance adjustment will not give an accurate result. The detected signal can be minimised, but it will not be zero and the exact value of C will be indeterminate because of the wider null region.

With a synchronous detector the ratio of capacitance only will be accurately determined, there will be no influence from the resistive dielectric losses.

If an adjustable phase shift network is introduced before a simple differential amplitude detector then both the magnitude of the capacitance and the phase angle of the dielectric loss can be balanced accurately by searching for the deepest null. That is a “dielectric loss angle bridge”. Any failure to null at zero will be due to harmonic distortion of the AC test signal used.
 
  • #7
864
17
The simplistic answer is that nobody uses bridges anymore to measure dielectric properties of materials. Nowadays, an impedance analyzer takes the place of a bridge. The idea is to place a sample of the material in a suitable fixture which is usually a pair of metal plates with the dielectric material between them, making a capacitor. Then the real part of the impedance of the fixture gives a measure of the losses of the material being measured.
Ah ok, interesting to know; thanks.

The dielectric loss of a capacitor is a resistive component that combines with the reactance of the capacitor to give a complex impedance. Now consider an AC bridge used to compare capacitor ratios.

With a simple amplitude detector the null will not be deep due to the fact that it is comparing a reactive C reference with a complex C lossy, that is with some resistive component. The single bridge balance adjustment will not give an accurate result. The detected signal can be minimised, but it will not be zero and the exact value of C will be indeterminate because of the wider null region.

With a synchronous detector the ratio of capacitance only will be accurately determined, there will be no influence from the resistive dielectric losses.

If an adjustable phase shift network is introduced before a simple differential amplitude detector then both the magnitude of the capacitance and the phase angle of the dielectric loss can be balanced accurately by searching for the deepest null. That is a “dielectric loss angle bridge”. Any failure to null at zero will be due to harmonic distortion of the AC test signal used.

I'll admit I had a bit of trouble following all of what you said (my fault) but was the guist of it that:
You have two signals that should be in phase, but one has a phase angle. You know the magnitude of both of them and the phase angle is caused by some complex impedance that should be real-resistive, the observed phase angle gives you an idea of what this complex impedance is. How close Is this to what you meant?

Thanks
 
  • #9
Baluncore
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Now that is a nice bridge.
I use an old GR-1608A Impedance bridge to measure C and C loss. It gives accurate results and so can be used to test the disposably priced “Chinese Junk” brand impedance meters like the DT-9935 found on eBay.
 

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