Analog circuit for arduino question

Hi Folks,

I have a question regarding an analog circuit front end that I am trying to get working for an arduino project. I have part of it working but can't see to get everything working. My basic circuit is a bridge where all the resistors but one are fixed, the one resistor varies a little so this changes the bridge voltage output, I currently feed this through an op-amp to add some gain, I can vary the gain and I usually get around 2 volts out.

However I don't want to measure this output voltage, what I want to do is adjust the bridge voltage so this op-amp output voltage remains constant, when the opamp voltage output drops I need to increase the bridge voltage back up to keep it constant. I've tried doing this two ways, one is to feed the opamp voltage into the sense pin of a voltage regulator (about 1.25 volts) and use the regulator to control the bridge voltage up and down, but this didn't work..? not sure why, could have been a bad component so I might try again.

The other way I tried doing it was to add a difference amplifier where I have a preset reference voltage going to into the + opamp input and the opamp output from the bridge going into the - (inverting) input. This way as the bridge output drops there is less voltage on the inverting input so the voltage output from the difference amplifier goes up. This didn't work either...?

Does anyone know how to make this work? I'd prefer doing it all with opamps as I can get a quad opamp package and do it all with one chip.

Many thanks!

8-)

P.S. I have a diagram but I can't figure out how to attach it to this message??

My gut tells me you're solving the wrong problem.

What is the application? What are you really trying to do?

And what is the nominal bridge supply voltage? Any other supply voltages in the circuit or is it all 5V?

http://www.ruggedcircuits.com

However I don't want to measure this output voltage, what I want to do is adjust the bridge voltage so this op-amp output voltage remains constant, when the opamp voltage output drops I need to increase the bridge voltage back up to keep it constant.

Well I think a review of how a DC bridge circuit work might help. A typical DC bridge has four resistance elements, normally 2 fixed, 1 adjustable and one element that changes via some physical property like say temperature. Usually one adjusts the initial balance of the bridge by setting the adjustable 'fixed' element such that the bridge output voltage is a some desired value, typically 0vdc at whatever the physical property the sensor element is exposed to at that time. This is sometimes called the inital calibration point and can be any desired output voltage rather then just 0vdc. Then as the sensing element changes resistance the bridge output will raise either positive or negative from the initial calibration output value, depending on the amount and direction of resistance change of the 'sensing' element .

Via measuring the bridge output voltage and then changing the bridge excitation voltage will not bring the bridge back to zero balance, but rather just increase or decrease the sensitivity of the bridge. That is to say that once a DC bridge is unbalanced, it doesn't matter what the amount of the bridge driving excitation voltage is, it will always be unbalance by some amount. This is because a balanced condition is when and only when the ratio of the resistance legs are equal and is not dependent on the amount of DC voltage driving the bridge. So changing the driving voltage to the bridge will only result in more or less voltage on the bridge output, but cannot rebalance the bridge.

To have some kind of automatic DC bridge balancing action your 'error' voltage output would have to be able to manipulate the resistance of the adjustable 'fixed' resistance element. Typically this would be some kind of transistor acting like the 'adjustable' resistance element.

That make sense?

Lefty

My bridge voltage is about 5.5 volts currently.

I have done everything manually and it all works, I can manually increase or decrease the bridge supply to get my output back to what I initially set it to. So it works.

I'd rather not go into detail on my application, it would be long winded but this is the only way my system can work, it would seem a trivial thing to do for an EE but I have mid-level circuit design capability and for wahtever reason I can't get a working feedback circuit to work as yet.

I have done everything manually and it all works, I can manually increase or decrease the bridge supply to get my output back to what I initially set it to. So it works.

Not doubting that you can't get a desired bridge output voltage by changing the excitation voltage, however that is not using a DC Wheatstone bridge as it's designed to work. Normally a bridge is used to measure a physical property of a resistance sensor element (temp, strain gauge, etc) but you seem to have a different application requirement and without more details it's hard to offer you a specific solution or reccomentdation. Would it not be possible to somehow accomplish your task in software rather then manipulating the bridge, that is just input the bridge voltage directly into the Arduino an then work with the value obtained? That is let software be your feedback.

Lefty

OK…I’ll bite. Here’s a simple approach:

The ‘diff’ node is fairly well controlled as R2 varies around 900 ohms. I’d be particularly concerned about oscillation in this circuit.

Again, if you tell us what you’re really trying to achieve we might be able to save you some time fighting to get this circuit actually working :wink:

Thanks rugged, that makes sense to me and I will breadboard it out. My resistor values in the bridge are a little different from what you have but should be close enough. I can perhaps add some capacitance to prevent oscillation.

Sorry if I'm being vague but I am doing this whole arduino project as a favor for a friend (I almost wish I hadn't as its taken more of my time than I thought it would take), I'll have to ask him if he's ok with me telling people more about this. If he is I'll get back to the forum and tell you more and let you know if this worked out ok.

thanks again for all the help everyone!

:)