The Basics of Resistance, Voltage and Current Measurements using the HP34401A Multimeter
The objectives of this lab are to familiarize you with the principles
of test instruments for measuring DC voltage, current and resistance. You
will learn the use of the digital multimeter (HP34401A) and the effects
of the internal resistance of this meter. You will also use BenchLink® to acquire and display data graphically on the PC's monitor. In addition,
you will learn the meaning of the resistor color code and power rating.
One of the most important functions of the undergraduate Electrical
Engineering lab is to provide a working understanding of the functions
and uses of instruments commonly found in all electrical laboratories.
To the Electrical Engineering student, this knowledge is essential to efficiently
conduct laboratory assignments and in developing independent design projects.
Currents and voltages are the basic circuit variables of interest. In
this lab we are mainly concerned with accurately measuring resistance,
DC voltage and currents using a digital multimeter (DMM). We will use
the HP34401A digital multimeter which is a high performance instrument
capable of measuring resistance, DC and AC voltage and current, as well
as frequency. The HP34401A has a built-in microprocessor, memory and other
electronics components that give it numerous features such as built-in
math functions, recording and storing up to 512 readings, giving the maximum,
minimum and average of the readings. In addition, it can be remotely programmed
using the SCPI (Standard Commands for Programmable Instruments) language
and read by computer via a General Purpose Interface Board (GPIB) port.
The use of the instrument for measurement of resistance, current and
voltage is in principle very simple. Figure 1 shows the set up for resistance,
voltage and current measurements.
Figure 1: Measurement of resistance, voltage and current with a multimeter
For resistance measurements, one connects the DMM over the resistor.
Notice that for voltage measurements one puts the multimeter in parallel with the circuit element so that one measures the voltage across the element. In case of a current measurement, one must put the DMM in series with the element in order to measure the current through the element.
That involves breaking the circuit in order to insert the multimeter in
the circuit loop. In the next section we will discuss the use of the DMM
in more detail for resistance, voltage and current measurements.
Resistance measurements with
the HP34401A DMM
Principle of 2 wire measurement
The DMM measures a resistance by applying a known DC voltage over unknown
resistance in series with a small resistance Rm . It measures
the voltage over the resistance Rm as shown in Figure 2a. The
DMM (remember the DMM has a built-in microprocessor) can then calculate
the unknown resistance R.
Figure 2: (a) Two-wire resistance measurement; (b) four-wire measurement.
To use the DMM for resistance measurements, connect the resistor to
the terminals labeled HI (V W)and LO,
select the resistance measurement function by pushing the [W] button (one of the function keys) on the front panel as shown below in
Figure 3. Notice that the selection keys are annotated in black and blue.
To select the function in blue, you must first select the blue SHIFT key.
Figure 3: Function buttons to select resistance, voltage, current or
Measurement errors and NULLing function.
When one measures the value of a resistor one connects the resistor
to the DMM input terminals using cables. If the resistor one measures in
very small, it is possible that the resistance of the cables themselves
are comparable or even larger than the resistance of interest. The HP34401A
DMM has a handy way to overcome this problem by using the NULL feature.
The front panel of the DMM (see Figure 3) has a button labeled NULL. To
null the wire resistance, one shorts the ends of the test wires together
and then presses the NULL button. You can disable the NULL function by
pushing the button again.
For really accurate measurements of
small resistances, there is a clever method one can use: i.e. the 4-wire
method, as shown in Figure 2b. The DMM supplies a test current through
the resistor, as in the 2-wire method, but measures the voltage over the
resistance with two other terminals. The two leads used for the voltage
do not conduct any current, so that the lead and contact resistances
do not influence the measurement. The four terminals for the 4-wire
method are shown on the front panel in Figure 3.
CAUTION: When doing a resistance measurements, it is safest to
disconnect all voltage sources before connecting the DMM to the circuit.
Putting a large voltage over the input terminals of the DMM may damage
The multimeter automatically selects the range using the auto-ranging
feature. However, you can also manually select a fixed range (e.g. 1KOhm
or 1MOhm) using the Auto/Man button on the front panel (under Range/Digits)
buttons (Figure 4). The 'down' arrow selects the lower range and the 'up'
arrow the higher range.
Figure 4: Function, Math, Range and Menu keys
Additional features of the HP34401A: average, max
and min value
One often needs to take a series of data points to find the average
value of the measured variable. Instead of doing this by hand, the HP34401A
has a built-in feature that does this for us. Also, you can ask for the
maximum and minimum values during the measurement interval. To enable this
feature, push the Min/Max button (one of the Math buttons) on the
front panel. You will see the Math annunciator lit on the front
display. Also, the DMM will make short beeps indicating it is taking readings
and storing the MAX, MIN, the Average value, and the total COUNT. Push
the Min/Max button again to stop the readings.
To access these stored numbers, you have to turn the Menu on by pressing
the On/Off key (SHIFT <) on the front
panel (see Figure 4). Then, use the > or < keys untill
you are in the MATH (B) menu. You can now go down to the "parameter
level" of the selected MIN-MAX menu by pressing the "down" button until
you see the desired parameter menu (1:MIN_MAX) displayed. Push once more
the "down" button. Once you are in the MIN-MAX menu you can use the > or < buttons to scroll through the menu and read the values.
The menu is organized in a top-down tree structure with three levels, as
schematically shown in Fig. 5.
Figure 5: The front panel menu organization.
The MIN-MAX feature can be used for resistance measurements as well
as for voltage, current, and frequency measurements.
There are several other interesting features available. They are described
in the HP 34401A User's Guide which is available at each workbench in the
The resistance color
code and power rating
Resistors are usually color coded using color banding as shown in Figure
6. Two digits and a power 10 multiplier determine the resistance value.
The color band on the most left is the 1st significant digit, followed
by the 2nd digit and the multiplier. The fourth band is usually present
as well and indicates the tolerance of the specified resistance. The meaning
of the color code is given in Table I. There is a handy resistor
color code calculator available on the EE Undergraduate Lab homepage.
code is also available on the lab home page
TABLE I : Resistor Color Code
Figure 6: The 5% Resistor Color Code.
Power rating of resistors
In adition to the value and tolerance of a resistor, the power rating
is another important characteristic. It tells how much power the resistor
can dissipate before being damaged by overheating. Resistors come in different
power ratings: 1/8, 1/4, 1/2, 1 and 2 Watts are typical values. Lets look
at an example. Suppose you are using a 1kW resistor
with a 0.25W power rating. the maximum DC voltage and current the resistor
can tolerate is than Vmax=sqrt(P.R)=15.8V and Imax=sqrt(P/R)=15.6mA.
Exceeding the power rating will result in poor reliability and early break-down
of the circuit. The power rating depends on the physical size of the resistor,
the larger the size, the larger the power rating will be.
Principle of measurement
A DC voltage is measured by using a voltage amplifier and an analog-to-digital
converter as schematically shown in Figure 7. A microprocessor further
manipulates the data before displaying the results.
Figure 7: Schematic of the DMM as a DC voltage meter.
To measure a voltage, connect the nodes over which one wants to measure
the voltage between the HI and LO input terminals of the DMM (Figure 3).
In order to activate the DMM for DC measurements you have to select the
DC Voltage function by pushing the DC V button on the front panel
(see Figure 3). The Math functions, such as Max/Min and average, can be
activated in a similar fashion as was done for the resistance measurements.
Also, the range can be selected manually by pushing the Man/Auto key in
the Range menu.
Errors due to the internal resistance
An ideal voltmeter has an infinite input resistance so that it will
not draw any current from the circuit under testing. However, in reality,
there is always a finite input resistance Ri, as shown in Figure
7. As a result, one has a voltage divider that will cause the voltage Vm one sees at the input of the voltmeter to be slightly different from the
actual voltage VS one wants to measure. The HP34401A has a relatively
large input resistance of at least 10Mohm (depending on the selected voltage
range) so that the error will be small as long as Rs <<
CAUTION: Do not exceed the maximum allowable voltage input (1000V
DC). Also, never apply a voltage over the current input terminal (I) of
Principle of the measurement
An ammeter senses the current flowing through its input terminals.
The ammeter (or DMM) must be connected in series with the circuit
such the same current flows through the DMM and the test circuit. The principle
of the current measurement is quite simple. The ammeter has a small resistance
at its input terminals and measures the voltage that the test current
generates over this resistance (Figure 8). The microprocessor then calculates
the current, I=V/ri, according to Ohm's law.
Figure 8: Principle of DC current measurement.
To use the DMM as an ammeter, one connects the leads in which the current
flows to the current (I) and LO terminals (see the front
panel in Figure 3). To activate the ammeter, one must also select the DC
I key by pushing SHIFT and DC V button as shown in Figures 3 or 4.
Error due to the non-zero input resistance
An ideal ammeter has a zero input resistance so that it does not disturb
the current under test. The small input resistance will cause a small voltage
drop which gives a small error. Fortunately, the input resistance of the
HP34401A is pretty small (ri = 0.1W for 1 and 3 A range, and 5 W for the 10mA and
100mA ranges) and can, in most cases, be ignored as long as RS >> ri.
CAUTION: Do not exceed the maximum allowable current input (3A
DC). Also, never apply a voltage over the current input terminal (I) of the DMM. This will cause a large current to flow through the small input
resistor ri and can damage the DMM.
Specifications (HP 34401A)
- DC Characteristics:
- DC Voltage range and input resistance:
- 0.1V, 1V, 10V: input resistance selectable 10MW or > 10GW
- 100V and 1000V: Rin = 10MW
- DC Current range and shunt resistance:
- 10mA, 100mA: Rshunt= 5 W
- 1A and 3A: 0.1 W
- Resistance range: 2-wire and 4-wire method
- 100 W, 1 kW, 10 kW, 100 kW, 1 MW and
- Input protection: 1000V
- AC Characteristics: true RMS
- AC Voltage: from 3 Hz to 300 kHz (for accuracy specs consult the manual)
- AC Current from 3 Hz to 5 kHz
- Frequency and Period measurement:
- Frequency range: 3 Hz - 300 kHz
- Input voltage range: 100 mV to 750 V
1. HP 34401A Multimeter, User's Guide, Hewlett Packard.
2. Electronic Instrument Handbook, 2nd edition, C.F. Coombs,
Editor, McGraw-Hill, Inc., New York, 1995
Last updated by Sid Deliwala, Sep 19 2011