EXPERIMENT 2-1 COMPONENT TESTING

In this experiment, you will measure some power-supply components that will help you recognize normal operation.

EQUIPMENT

(4) silicon rectifier diodes, type 1N4001 or equivalent

(1) transformer,VPri = 120 V, Vs = 12.6 V

(1) Black Breadboard w/Lamp module

Modified AC extension cords

VOM

LCR meter

AC Power Supply

DIODE CHECKS

Select any one of the rectifier diodes. Using an ohmmeter on the R x 1 scale, connect the negative lead of the meter to the cathode and the positive lead to the anode. Record the measured resistance in Table E2-1, in the column labeled Forward Resistance and the row for diode 1. Next, reverse the meter leads and record the measured resistance in the column labeled Back Resistance. Now divide the high-resistance value by the low-resistance value and record this ratio in the space provided. Repeat this procedure for the remaining three diodes. Using a DMM (aka DVOM) for forward resistance don’t use the one identified for PN junctions and for the reverse use a scale that doesn’t flash its out of range indication.

Table E-2-1

Diode / Forward
Resistance / Back
Resistance / High/Low
Ratio
1
2
3
4

TRANSFORMER CHECKS

1. DC resistance checks.

a. With no power applied, measure the d-c resistance of the primary winding.

Rprimary =

b. Measure the resistance from the center tap to one end of the secondary.

RAC =

c. Measure the resistance from the center tap to the other end of the secondary.

RBC =

d. Measure the resistance across the entire secondary.

RAC=

E2-1 Circuits for transformer tests in experiment 2-1.

(A) Measuring VAc / (B) Lamp Circuit

C.Lamp Circuit image

2.Next, refer to the circuit of Fig. E2-l.. Set the AC Power supply to 11VAC using the meter. Plug in the line cord to the AC Power Supply and

a. measure the secondary voltage from the center tap to side A.

VAC=

b. measure the voltage from the center tap to side B.

VBC =

c. finally, measure the voltage from B to A.

VAB =

3.Now unplug the line cord and connect a lamp in series with the primary, as shown in Fig. E2-1B. Plug the line cord in again.

a. Does the lamp light?

b. Measure the line voltage to two decimal places. VXZ = ______

b. Measure the voltage across the lamp. VXY = ______

c. Now measure the voltage across the transformer primary. VYZ= ______

4.Next, connect a clip lead across half of the secondary winding, from point A to point C. This represents a short across some of the secondary winding.

a. Does the lamp light?

b. Measure the line voltage to two decimal places. VXZ = ______

b. Measure the voltage across the lamp. VXY = ______

c. Now measure the voltage across the transformer primary. VYZ= ______

QUIZ

1.When a diode is forward biased, its resistance is relatively (a)high. (b)low.

2.When a diode is back biased, its resistance is relatively (a)high. (b)low.

3.In the center-tapped transformer, the resistance of the entire secondary(a)is, (b)is not—equal to twice the resistance from center tap to one end.

4.In the center-tapped transformer, the voltage across the entire secondary(a)is, (b)is not—equal to twice the voltage from the center tap to one end.

5.With no jumper across the secondary, the lamp____(a)did, (b)did not—light
because (c)there was excessive primary current. (d)there was high primary inductive reactance.

6.When a short was placed across part of the transformer secondary, the lamp(a)did, (b)did not—light because (c)the excessive secondary current drastically reduced the reactance of the primary. (d)the lamp would only light if the short were across the primary.

7.If the short were place across the entire secondary from point A to point B, (a)the lamp would light with the same brightness. (b)the lamp would burn brighter.

Based upon Experiment 2-1 from Electronic Troubleshooting, Jerome Oleksy, 2nd edition - 1990, Glencoe. Modifications for CSN’s ET 289B class.