Lab #3: Bridge Circuits

OBJECTIVES:After completing this experiment, you should be able to: Identify, build, and balance a bridge circuit. Become familiar with the application of a potentiometer.

MATERIALS REQUIRED:1 4.7 k, W, 5% resistor1 12 k, W, 5% resistor1 18 k, W, 5% resistor1 100 k, W, 5% resistor1 Leader Power Supply1 DMM1 breadboard (proto-board)1 10K, W potentiometer Misc cables and wires

PRE-LAB:1. The basic bridge circuit is shown in Figure-A. It is typically drawn in the diamond shape to clearly identity it as a bridge circuit. Note that the bridge circuit is basically two voltage dividers connected across the voltage source. The equivalent bridge circuit in Figure-B clearly shows this. Note that the output (load) is taken from between the two voltage divider outputs (between the voltage at node A and the voltage at node B), and not with reference to ground. This kind of output is referred to as a balanced output.

The basic neutral condition in a bridge circuit is called valance. A balanced bridge is one in which the output voltage is zero. By making one or more of the resistive elements in the bridge variable (Figure 3-2), it is possible to adjust the bridge for this balanced state. Prove that balance occurs when the following relationship exists:

R1 / R2 = R3 / R4

2. The bridge is unbalanced when one voltage divider output is higher or lower than the other. In Figure 3-1, if the voltage at point A is greater than the voltage at point B, then (circle the correct answer):

a. The current will flow from A to Bb. The current will flow from B to A

3. If the voltage at point B is greater than the voltage at point A, then (circle the correct answer):

a. The current will flow from A to Bb. The current will flow from B to A

PROCEDURE:1. Using the DMM, carefully measure the values of each of the fixed resistors in Figure 3-2 and record their values in Table 1.

Table 3-1 Resistor ValuesResistorsR1R2R3RL

Ideal Values12 k18 k4.7 k100 k

Measured Values

2. Using the formula given for a balanced bridge, compute the value to which the potentiometer R4 must be set in order to valance the bridge:

R4=________________________ (calculated)

3. Construct only the resistive part of the circuit in Figure 3-2 using the measured resistors from step 1. Adjust the potentionmeter (pot) R4 for the middle of its range as a starting point. Do not connect the power supply at this time. It is a good idea to make the actually physical arrangement of resistors on the breadboard as close to the schematic diagrams positioning as possible. This will also help you to keep track of each of the resistors, and will make the current and voltage measurements easier too. Once you have completed the wiring, go back and double check it for errors before proceeding.

4. With the power supply not connected to the circuit, turn on the power supply and adjust the voltage to 9.0 volts. Set the current limit control knob to 12:00 (mid-range).

5. In this step, you will supply power to the circuit. If the power supply voltage has changed significantly, turn it off IMMEDIATELY and notify the instructor.

6. Using the DMM, measure the voltage between terminals A and B. Adjust the range on the DMM for maximum resolution.

VAB= VA VB= _________________________ (measured)

7. Now adjust the pot R4 until the voltage between A and B is zero. When you obtain a zero reading, switch the DMM voltage scale to the next lowest setting. You may read a nonzero value. In this case, continue to fine tune the adjustment of the pot until a zero reading is obtained. This is the balanced setting of the bridge.

8. With the bridge in the balanced condition, measure the voltage at point A with respect to ground (the negative terminal of the power supply) and at point B with respect to ground:

VA=__________________ (measured)

VB=__________________ (measured)

9. Carefully remove the potentiometer R4 from the breadboard, to avoid changing its setting, and measure the resistance. Compute the % error from the calculated value.

R4=__________________ (measured) % error=______________________

10. Reconnect the pot in the bridge circuit. Measure the voltage between A and B to be sure that it is still zero. If it is not, readjust R4 as necessary to bring the bridge into a balanced condition.

11. While monitoring the voltage between terminals A and B, vary the potentiometer in both the clockwise and counterclockwise positions form the point where the bridge is balanced. Note the amplitude and polarity of the voltage at the maximum clockwise and maximum counterclockwise positions of the pot R4.

VAB= VA VB=_________________ (max. CW)

VAB= VA VB=_________________ (max. CCW)

12. Turn off the power supply; disconnect the circuit.

POST-LAB QUESTIONS:1. When a bridge is balanced, the output voltage is:

a. The supply voltageb. Not possible to determinec. Infinited. Zero

2. A bridge circuit is made up of two simpler circuits of what type?

a. Current dividersb. Voltage dividersc. Seriesd. Parallel

3. In the bridge circuit of Figure 3-1, the voltage between A and ground is 7.5V. The voltage from B to ground is 4.6V. Current will flow in the load in what direction?

a. Left to rightb. Not possible to determinec. Right to leftd. No current will flow

4. In a bridge circuit like Figure 4-1, R1 is unknown, R2 = 1 k, R3 = 3 k, R4 = 8 k. What is the value of R1 to balance the bridge?

a. 3.75 kb. 375 kc. 1.25 kd. 2.67 ke. Other (specify)

5. One output terminal of a bridge is connected to ground.

a. Trueb. False Figure 3-1A Bridge CircuitFigure 3-1B Re-Drawn Bridge Circuit

Figure 3-2 Bridge Circuit with Potentiometer