# Bridge circuit impedance

Discussion in 'Power Electronics' started by Boure, Nov 6, 2020.

1. ### Boure

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Nov 5, 2020
Impedance of a Bridge circuit
It is usually easier to analyse a circuit once you reduce it entirely to Thevenin equivalent circuit; a source connected to an impedance in series.
Impedance in series and parallel are easily reduced to a single impedance value, however a bridge circuit is not readily reduced to a single impedance, but it is not impossible to do so.
Alternative methods exist for reducing bridge networks to a single impedance to a single impedance, such as;
• conversion of sections of a bridge into equivalent star and delta networks
• reducing a complex network into a Norton equivalent circuit.
Both methods require memorizing some formula but I am a firm believer in derivation from first principles. Finding the arbitrary current flow in the bridging impedance then working out current distribution using Kirchhoff current law. Ultimately, the impedance is then found by the ratio of voltage applied to current drawn from the source.
Any thoughts concerning this approach.

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2. ### Kabelsalat

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Jul 5, 2011
I think the most apropriate method is to convert b-y-z from a delta connection into an y connection.

It is some years ago since I did calculation on this, but I may be able to find the papers.

3. ### Boure

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Nov 5, 2020
You're right.... sections can be converted into equivalent star networks and you can find the total current drawn into the bridge.
In my opinion....trying to remove the hassle of trying to memorize the conversion formula for star to delta and vice versa.(which many people usually get mixed up especially in my case).

4. ### Ratch

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Mar 10, 2013

What impedance are you talking about? The impedance across points 2-3? Or 1-4, 1-2, what? What is Zbridge? Wouldn't the diagram be more clearer if block "z' were replace with "b" and block "b" replaced with some letter from the middle of the alphabet like maybe "m"? All the words of "flow" can be dropped because current already means "charge flow". Once I know what you want, I can help you.

Ratch

5. ### Harald KappModeratorModerator

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Nov 17, 2011
The assignment calls for the impedance V/I which would be between points 1 and 3.
Not if that is how the assignment was originally phrased.

Please, please, do not start this discussion again. You've made your point clear quite a few times and while you are totally correct with regard to the physical background, "current flow" is a very common expression. Disagree as you like, but don't bring up this discussion time and again. It will confuse less experienced users more than it helps to clarify matters.

6. ### Ratch

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Mar 10, 2013
I think you want to read the attached file from my "Old Gold" collection. It sure is old, but its information is golden.

Ratch

Last edited by a moderator: Nov 12, 2020
7. ### Ratch

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Mar 10, 2013
Thanks moderator for the edit job. I wanted to upload the whole 1947 Weston file, but the site squawked because it was too big. Lots of good info in the rest of the Weston Engineering Notes which I could not upload. Ratch

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10. ### The Electrician

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Jul 6, 2012
Ratch, I think the final result in the image attached to post #1 is incorrect. Have you worked out the impedance seen at terminals 1 and 3, and compared your result with the result in the image?

11. ### Ratch

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Mar 10, 2013
Electrician,
I did not work out any formulas. Are you confusing me with someone else? I only submitted Weston's Lab Notes.
Ratch

12. ### The Electrician

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Jul 6, 2012
There is an image attached to post #1 with the problem worked out. You, like me, tend to check results, Have you worked out the solution to the impedance seen at terminals 1 and 3, and compared to the result in the image attached to post #1?

13. ### Ratch

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Mar 10, 2013
Electrician,
No, I was lazy this time. However, I would put my money on Weston Labs as being correct. That is ground that has been plowed over and over throughout many generations of electrical engineers. Those Weston Lab Notes were published in 1947 before Mathcad, Mathematica or other computer programs that could do the gritty algebra . Hats off to alll those pencil pushers who did all that work the hard way. Ratch

14. ### The Electrician

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Jul 6, 2012
I know you have Mathematica, so I figured you would have checked the post #1 result. I solved it with Mathematica, and the post #1 result appears incorrect.

15. ### The Electrician

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Jul 6, 2012
Now that I look again at the final result in the image attached to post #1, I see immediately that it's incorrect. If the resistance "b" is not there its value becomes infinite. The final expression given says that if "b" is missing (becomes infinite), the bridge resistance becomes infinite, which is obviously wrong.

16. ### Ratch

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Mar 10, 2013
It no doubt is. I would compare your calculations with the Weston Lab Notes. That would be the clincher. Ratch