# Bridge Circuit

#### paul_alan

Joined Nov 5, 2011
43
I am building a bridge circuit in class using 3 1kΩ resistors and a 5kΩ potentiometer. (There are 2 resistor in series and the other resistor and potentiometer are in series, parallel to the other 2 resistors). I have two questions: 1.) What, in laymans terms, constitutes a bridge circuit? It seems to me that this is built exactly like a series-parallel circuit and I'm not really getting the difference. We skipped over the Complex Ciruit Analysis Chapter, so I'm having trouble with it. And 2.) How does the bridge circuit really differ from a simple series-parallel circuit? Is it just the use of the potentiometer to control current flow?
Also, what's the use for making changes in the potentiometers resistance to control the current flow? I measured the current between the two branches and when the potentiometer is set to a low resistance, current moves from B2 to B1. And when I adjust the potentiometer to a higher resistance (say 5k Ohms) I get a negative current measurement because the current is moving from B1 to B2. Why would you ever need to make adjustments to a rheostat to change the current like that? Thanks for any help anyone can give me.

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#### studiot

Joined Nov 9, 2007
4,998
Hello Paul,

You didn't mention why you are building a bridge circuit in class?

Every class practical I have met has stated aims and objectives. What bearing do these have on your construction?

#### ke5nnt

Joined Mar 1, 2009
384

#### paul_alan

Joined Nov 5, 2011
43
We are just doing a Lab called "Balanced Resistive Bridge Circuits". I guess just learning how they work. We are changing resistor values and changing the value of the potentiometer, then recording data on different tables. We then have to compare current values and understand why, when the potentiometer is either increased or decreased, the current measurement between the two branches changes direction.

#### russ_hensel

Joined Jan 11, 2009
825
Resistive bridges are basically 2 voltage dividers connected together ( often with a meter ) at the voltage dividing point ( the other points are common voltage source and common ground ) When both voltage divideres are set to the same ratio the voltages are equal and no current flows across the bridge.

Joined Dec 26, 2010
2,148
The word "balancing" is key to understanding the importance of these bridge circuits. Essentially, they are about comparison of conditions in two halves. If you consider the bridge as two potential dividers, one fixed, the other adjustable, you should be able to see why the current can reverse. At the point where the current between the two sides changes sign, (that is, goes to zero), the resistances on the two sides of the bridge are in equal proportion. If one side of the bridge is made from a calibrated potentiometer, the position giving balance can give an indication of the proportions of the resistances of the resistors on the other side.

One of the oldest uses of bridges is in measuring unknown components, by comparison with standard ones. In this case the bridge is adjusted for balance, when the unknown can be calculated from the setting which gives the balance condition. It should be noted that in this use the voltage input to the bridge does not need to be accurately regulated, as this does not affect the balance condition.

In another use of bridges, one or more of the bridge elements is a sensing element, for instance a resistance which varies with temperature, mechanical stress, light intensity. The bridge can be arranged to go to balance at some standard condition, say 0°C, zero stress, or darkness. The bridge then gives an output voltage or current indication approximately in proportion to whatever is being sensed. In this case, a stable supply input is required for accurate results.

#### paul_alan

Joined Nov 5, 2011
43
So, how does a Bridge really compare to a simple series parallel circuit? And what is the difference? Is it simply to find an unknown?

Joined Dec 26, 2010
2,148
So, how does a Bridge really compare to a simple series parallel circuit? And what is the difference? Is it simply to find an unknown?
A bridge is a bridge. There are lots of different kinds. Once the central element is added (in a measuring bridge, this is usually the balance detector), it is more complex than a simple series / parallel circuit.

If the middle element is absent or can be ignored, as it may at the point of perfect balance, a simple Wheatstone bridge can be considered to be a parallel combination of two branches, each of two resistors in series.

Some bridges are used to find an unknown. There are DC bridges, and AC bridges of various kinds, such as the Wien bridge, once used to measure capacitors but now more familiar as a feedback network for oscillators.

Other bridges are used to standardise the output from a sensor. Yet more circuits of bridge form but containing semiconductors are used for rectification ((Graetz) bridge rectifier), or to obtain a bidirectional drive voltage from a single DC input (H-bridge).

http://en.wikipedia.org/wiki/Bridge_circuit