Specifications

Procedure
1. Turn on the power supply
and set it to its rated
output voltage. Ensure
that the current adjust
knob, if present, is set to
its maximum value.
2. Write down the measured
voltage of the power
supply. Call this value V
0
.
3. Press I-set on the DC load
to set to power supply's
rated value. Turn on the
input of the load by
pressing On/Off.
4. Write down the measured
voltage of the power
supply. Call this value V.
5. Turn off the DC load's input
by pressing the On-Off
button.
6. Turn off the power supply.
Calculate the load regulation
(in percent) of the power
supply by
100
V
0
−V
V
0
(1)
You can also calculate the
output resistance of the power
supply as
V
0
−V 
i
where
i
is the current drawn in this
test.
High quality power supplies
have low output resistances in
the order of 1 m or less.
This means measuring V
0
and
V with enough digits is
necessary to avoid loss of
significance in the subtraction.
As an example, for a 30 V
supply rated at 3 A with an
output resistance of 1 m ,
measuring 30 V with 100 V
resolution is required to have
two significant figures in the
load regulation number. This
measurement would require a
6 digit voltmeter.
Current Limiting
Power supplies in constant
voltage mode generally have a
preset limit to the maximum
current output. Current limit
testing consists of
measurements that define the
behavior of a power supply
and its current regulation.
These measurements can be
characterized by a voltage vs.
current curve, which portrays
how and when the power
supply enters from CV to CC
mode. Ideally, precise current
regulation reflects a voltage
vs. current curve similar to
Figure 4.
For CV/CC mode power
supplies, typical current limit
characteristics look like Figure
4, with small or very minimal
bending near the crossover
point. It is good practice to
test the current limits of a
power supply because it will
aid in protecting the
equipment for its application.
Without a certain degree of
current regulation, the supply
may even over power or
damage certain devices. The
following describes a test
setup for testing current limit
of a B&K Precision 9120A
power supply.
Objective
Determine the current limit of
a power supply under test.
Approach
Use a power supply, DC load,
and a computer to analyze
voltage versus current
characteristics.
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Figure 4. Voltage vs. Current Curve