Lab-2, Electronic Basics II
Physics PHYS 2371/2372, Electronics for Scientists
Don Heiman, Northeastern University, 9/16/2016
For this Lab, and most other Labs, you will need the following:
 Variable DC power supply (PS) (12 or 15 V, 0.5 A)
 2 DMMs
 Resistors (kΩ range and 10 Ω, 1 W), capacitors (10-100 μF)
 Jumper wires
I. Voltage Divider – Here, you will investigate series resistors.
Note: you will see voltage dividers over and over again in circuits. Most of those will not contain two
resistors, but instead may contain a resistor and another component (capacitor, transistor, etc.).
1. Choose two resistors with different values in the kΩ range and arrange them in series with the power
supply. Next, apply a voltage and measure the voltage across each resistor using the first DMM.
2. Compare the ratio VR1/VR2 with the ratio of resistances, R1/R2.
3. Comment on any discrepancies or agreements.
4. Discuss how these two resistors in series make a Voltage Divider.
II. Capacitors – Connect two different capacitors in the 10-100 μF range in series and to the power
supply. Make sure you use the correct polarity on the electrolytic capacitors (longer lead is positive).
Set up the two DVMs to measure the voltages across each capacitor. Turn on the power supply and
measure the voltages quickly after the voltage is applied.
1. Compare your value for VC1/VC2 to C2/C1.
2. Do the voltages change with time after turning the power supply on? How do they change?
III. Voltmeter Input Impedance – All voltmeters have an effective resistance, referred to as the input
impedance. When placed in a circuit, the added resistance of the meter will draw a small amount of
current, in effect, changing the circuit. Determine the input impedance of the DVM by using a voltage
divider network with the DVM in place of one resistor and use Ro~1 MΩ for the other resistor. Use the
second DVM to alternately measure the voltage across the resistor (Ro) and the supply voltage (Vo).
1.
2.
3.
4.
5.
Compute the value of RDVM using the voltage divider formula?
Does the value of Ro~1 MΩ affect your value for RDVM?
Switch the range setting of the DVM to see if it affects the input impedance?
Does the information here help you to answer the question II-2?
Discuss why a DVM should have a large input impedance.
IV. Power Supply Internal Impedance – All power supplies have a maximum voltage and current which
they are capable of producing. For example, leaving the output terminals open produces the maximum
voltage, while shorting (R~0) the output terminals produces the maximum current. In effect, power
supplies have an internal resistance that limits the current. Determine the internal impedance RPS of the
power supply (PS) using a voltage divider network as before, where the PS impedance is one resistor and
the other is an R=10 Ω (P=1 W) resistor. Hint: measure the supply voltage (Vo) by disconnecting the 10 Ω
resistor.
Measure the internal impedance of the older unregulated Fisher power supply, not the GW supply.
1. Compute the maximum allowable voltage for the P=1 W resistor.
Do not exceed the power ratings of the resistor.
2. What is the computed value for RPS?
3. Explain why disconnecting the R=10 Ω resistor allows you to measure Vo.
4. Discuss the effect of a small internal impedance of a power supply.
V. Battery – Measure the internal resistance of a battery using a 1 kΩ resistor.
Optional - Explain how a liquid crystal battery tester works (one that is integrated on the outside case of
a battery).