EE 312 Basic Electronics Instrumentation Laboratory

Experiment 4 Bridges

(Formerly Called Impedance Measurements)

Fall 2000

Reference: Lecture 4 on

PROCEDURE:

0. Function Generator Settings

Make the following connections on the Fluke 8000A DMM. Connect the black lead to common and the red lead to V-. Push in the ACV button. Set the RANGE to 20. Connect the Tektronix FG 502 Function Generator (FG) to the Fluke 8000A DMM. Set the Tektronix FG 502 Function Generator to obtain a sinusoidal waveform with no dc offset. Connect the Tektronix FG 502 Function Generator trigger output to the input connector of the Tektronix DC Counter. Note that one terminal is GROUND for the CRO, FG, & Counter. Set the frequency to obtain a 1 kHz display on the Counter. Adjust the amplitude of the sinusoid to obtain a maximum reading on the Fluke 8000A DMM. The maximum reading should be ~ 7 VAC. The corresponding peak-to-peak voltage should be ~ 20 V. During measurements the FG output should be set to a maximum value. Use the Fluke DMM to monitor the FG output. It is good practice to reduce the FG output to a minimum value when making connections to lessen the chance of damage to components. Thus now reduce the FG output to a minimum value. Change the scale to 2 VAC. The measured value should be less than 1 VAC.

Also connect the Tektronix FG 502 Function Generator trigger output to the trigger-input connector on the HP 54600 CRO. Use external triggering on the HP 54600 CRO during all parts of the experiment. When Autoset is pushed on the HP 54600 CRO, triggering may change to one of the signal inputs or even to line if a strong 60 Hz signal is present. It may be impossible to trigger on a signal input when a null is obtained. Thus you need to check frequently that the CRO is set to external triggering.

1. Resistance Measurements on a Wheatstone Bridge.

Assemble the Wheatstone Bridge. Connect the 200 F capacitor directly to the FG output. Connect a long red lead from the 200 F capacitor to the isolation transformer (xfmr). Connect a long black lead from the ground on the Tektronix CRO to the isolation transformer. Twist these leads. A good way to do this is to make the first connections to the isolation transformer, then twist the leads to form small loops, and connect to the ground & 200 F capacitor. Twisted pairs contain alternating loops. The magnetic fields created by adjacent loops tend to cancel. Similarly, the voltages induced by a time-varying magnetic field in adjacent loops also tend to cancel. Use the large precision resistance boxes for R1 & R2. The resistors R1 & R2 form the ratio arm of the bridge. Use the Heath decade resistance box for the standard resistor R3. Use the Heath resistance substitution box for the unknown resistor R4. Connect the secondary of the isolation transformer to four resistance boxes. A good way to do this is to use the small leads to connect the four resistance boxes. Then make connections to the isolation transformer. You will need to use the long red & black leads. Twist the leads to form small loops, and connect to the junction of R1 & R3 with a red lead and to the junction of R2 & R4 with a black lead. Note that no ground connection has yet been made to the bridge circuit. Connect a long red lead to the junction of R3 & R4. Connect a long black lead to the junction of R1 & R2. Twist the leads to form small loops, and connect the red lead to the CRO CH1 Input and the black lead to the CRO Ground. You cannot avoid having larger loops at the Wheatstone Bridge and at the CRO. If you have an even number of small loops, the induced voltages in the larger loops will tend to cancel. So twist to obtain an even number of small loops. It is more important to do this for the connections between the Wheatstone Bridge and the CRO than for the connections between the isolation transformer and the FG. The reason is that null voltages less than 10 mV occur. Noise voltage induced in the leads connected between the Wheatstone Bridge and the CRO may be larger than the null voltage. The smaller the noise voltage the easier it is to obtain a sharp null.

Set R1 = R2 = 1 kohm. Set R3 = R4 = 1 kohm. Increase the FG output to its maximum value. Record the DMM VAC reading. Adjust R3 to obtain a minimum voltage on the CRO. This is the null condition. Record the value of R3 and the peak to peak value of the null voltage. You may need to use averaging to make a good measurement. Now increase R3 by 1 ohm. Does the null voltage change? If not, increase R3 by 1 ohm again. Repeat until you see a change in null voltage. Record the value of R3 and the peak to peak value of the null voltage. Now decrease R3 in 1-ohm steps until the null voltage is observed to increase again. Record the value of R3 and the peak to peak value of the null voltage. What you are trying to do is to determine the range of R3 for which a null is obtained. That range determines the precision of the measurements. For example, assume that R3 =1050 ohms at null and that the null voltage was 11 mVpp. When R3 was increased to that R3 =1055 ohms, the null voltage increased to 13 mVpp. When R3 was decreased to that R3 =1045 ohms, the null voltage increased to 13 mVpp. Thus the range for the measured value of the unknown resistor R4 is 1046 to 1054 ohms. (R4 = R3 = 1050  4 ohms.) The precision for the measurement is 4/1050X100% = 0.38%. The accuracy of the measurement is less because there is a 1-% tolerance on the value for the resistors in the three resistance boxes R1, R2, & R3. The worst case accuracy would occur if R2 & R3 had a value 1% larger than their nominal value and R1 had a value 1% smaller than its nominal value or vice versa. Thus the worst case error would be  3%. The rms error is obtained by squaring the each of the three 1-% tolerances, summing them to obtain (3%)2, and taking the square root to obtain 1.732%. The value 1.732-% would be rounded off to 1.7%. Also record in your notebook the precision for the measured value for R4 in ohms and in percent.

Repeat the previous measurement for R4 equal to 10 kohms, 100 kohms, 100 ohms and 15 ohms. Note that the measurement precision could also be determined by varying the ratio arm resistors R1 & R2. For low values of R4 this will most likely be necessary.

2. Capacitance Measurements on a DeSaulty Bridge.

This part of the experiment is based upon the 1999 EE 312 Lab Practical Exam. Some of the information has been deleted or re-worded. The actual 1999 EE 312 Lab Practical Exam will be posted on the web.

Capacitors available

1. The standard capacitor C1 is a precision capacitance box with dials that can be adjusted to yield capacitance values in the range 0.001 to 1.11. The accuracy is believed to +/- 2%.

2. The unknown capacitor C2 is a Heathkit Capacitance Substitution Box. The nominal values are believed to be accurate +/- 20%.

3. The unknown capacitor C3 with a capacitance value less than 100 pF is mounted on a dual banana jack.

Resistors available

There will be two precision resistor boxes available for the ratio arm resistors R1 & R2. The accuracy is believed to +/- 1% or better.

Isolation Transformer

An isolation transformer will be available.

Equipment Available

The usual equipment is available and includes two DMM's, two CRO's, one FG,

one counter, & one dc power supply.

Lab Practical Exam Procedure

1. Investigate experimentally what resistance ratios can be used. I suggest

proceeding in the following manner.

A. Use the smallest value capacitor in the Heathkit Capacitance Substitution Box. I believe that is 0.0001 uF (0.1 mF or 100 pF).

B. Use the smallest possible standard capacitance & resistance ratio to

determine its precise value. The smallest possible standard capacitance should be 0.001 uF. The appropriate resistance ratio should be selected. It will be necessary to vary the resistance ratio to obtain a null. Determine the uncertainty in the measurement by the varying the resistance ratio. There will be a range of resistance ratios that yield essentially the same null. The precision of the measurement can be determined from the range of resistance ratios. Record the setting of the resistance boxes and the precision capacitor. Record the null voltages.

C. Increase the standard capacitance by a factor of 10, e. g. to 0.010 uF. Increase the resistance ratio appropriately. It will be necessary to vary the resistance ratio to obtain a null. Determine the uncertainty in the measurement by the varying the resistance ratio. There will be a range of resistance ratios that yield essentially the same null. The precision of the measurement can be determined from the range of resistance ratios. Record the setting of the resistance boxes and the precision capacitor. Record the null voltages.

D. Increase the standard capacitance by another factor of 10, e. g. 0.100 uF. Increase the resistance ratio appropriately. It will be necessary to vary the resistance ratio to obtain a null. Determine the uncertainty in the measurement by the varying the resistance ratio. There will be a range of resistance ratios that yield essentially the same null. The precision of the measurement can be determined from the range of resistance ratios. Record the setting of the resistance boxes and the precision capacitor. Record the null voltages.

E. Increase the standard capacitance by another factor of 10, e. g. to 1.000 uF. Increase the resistance ratio appropriately. It will be necessary to vary the resistance ratio to obtain a null. Determine the uncertainty in the measurement by the varying the resistance ratio. There will be a range of resistance ratios that yield essentially the same null. The precision of the measurement can be determined from the range of resistance ratios. Record the setting of the resistance boxes and the precision capacitor. Record the null voltages.

F. Determine the precision in +/- pF and in % as a function of resistance ratio. Write a brief discussion in your notebook.

2. Determine how to measure as accurately as possible an unknown capacitor with a capacitance less than 100 pF. "C" students may use any connection method and any resistance ratio. "B" students need to do better than that. Using the smallest possible resistance ratio is one step. Consider that a pair of twisted leads may contribute 1 to 3 pF/inch. Simply connecting the unknown capacitor with the shortest possible twisted leads would be a start. However, the measurement will be affected (notice that I use affected and not effected) by the capacitance of the leads. "A" students need to do better than that. They need to develop a technique were the effect (notice that now I use effect and not affect) of the leads is not a factor. This may require thinking.

During the 1999 EE 312 Lab Practical Exam only a few students thought

of the best way to measure the value of the unknown capacitor. You may ask

the SA’s who took the 1999 EE 312 Lab Practical Exam how they did it.

They did it well.

3. Write in your lab notebook the results and discussion for the capacitance measurements.

3. Inductance Measurements on a Maxwell Bridge.

Measure the dc resistance R3dc of the 1 mH inductor using a Fluke DMM. Record the value in your lab notebook.

Assemble a Maxwell bridge. Use the large precision resistance boxes for R1 & R2. The resistors R1 & R2 form the ratio arm of the bridge. Use the Heath decade resistance box for the standard resistor R4. Note that the resistor R3 is the internal resistance of the coil and not a separate resistor that has to be connected. The standard capacitor C2 is a precision capacitance box with dials that can be adjusted to yield capacitance values in the range 0.001 to 1.11. Use the Maxwell Bridge to measure the value of the inductance L3 and its ac resistance R3ac at a frequency of 1 kHz. Record the setting of all resistance and capacitance boxes in your notebook. Calculate values for the inductance L3 and its ac resistance R3ac at a frequency of 1 kHz. Record the calculations in your notebook.

Eliminate the isolation transformer & repeat Maxwell Bridge measurement using a CRO Differential Measurement Technique. Use a different resistance ratio R1/R2.