Wien bridge oscillator with diodes in feedback loops

Have you simulated it? If so, please give me the thermistor's resistance that results in balance.

John

That was one of many the things I meant by "Remembering the usual precautions of depending on simulators".

I think you agreed with my philosophy on that. Thanks.

John

Nice Wheatstone bridge there. The bases for strain gauge and Kelvin bridge measurements.

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Dang, I hardly ever simulate circuits. It's a lot more fun to build them!

You want me to give you the balance condition of a bridge? ??

It's a resistor ratio on each 'side' of the bridge. If you put 'say'

10k resistors every where then it's balanced when the thermistor R is 10k ohm... whatever temperature that is.

Bridges are really nice circuit elements. Have you ever used one?

George H.

I understand and have used bridges. I think you have not looked deeply into the schematic. I believe the bridge is permanently unbalanced *with respect to the sensing op-amp* by the zero crossing circuit.

If you do not simulate, I will accept a mathematical analysis. But you must show all components attached to the bridge in the analysis.

I am not trying to be argumentative. It will be educational which ever way this goes.

John

BTW, George, do you have LTSpice?

John

I think the capacitor in the zero-crossing circuit puts a permanent imbalance on the bridge. Just as in an SWR bridge, there is no way to balance without a balancing reactance somewhere. Note that the zero-cross circuti forces current flow through the upper left hand bridge resistor.

Does this make sense?

John

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Hi John, Sorry.

I looked at the 'stuff' below the bridge and then just ignored it. Does it change the balance? (That sucks.) I'd do synchronous detection differerntly, but I got the idea. I do have LTspice. I've not tried any new circuits lately, too much ...else... to do. What's an SWR bridge? Standing wave ratio? Ahh.. don't get me started on nights spent keeping a low temp ESR spectrometer in balance... (the liquid helium level would slowly move down the waveguide)

George H.

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I was going to offer you an LTSpice ascii file for you to play with and see the problem with the circuit. However, it sounds as if you are very busy and I don't want to impose on that. It isn't that important.

Cheers, John

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Thanks for the offer John. Aren't you always busy? But still it's nice to take a little time to do something fun and educational. That bridge circuit just didn't do it for me.

George H.