Operating instructions

University of Saskatchewan
Electrical Engineering Laboratory Equipment Manual
Power Supplies
The dc power supplies are the most often used pieces of equipment, yet they are
also the most misunderstood and misused. This short document will address these
misconceptions.
There are a wide variety of dc power supplies in the laboratories, yet they are all
very similar. All have three output connections: +, GND, and -. The greatest confusion
arises when people must consider what to connect where to use the supplies.
If you were to have a fresh 9 V battery in front of you, all that you could truthfully
say about that battery would be that there is 9 V between the + and – terminals, with the
+ terminal being at the higher potential. Nothing says that the – terminal is at ground
potential unless you actually connect it to ground—right? The same goes for the dc
power supplies. If you adjust a power supply to 9 V, then that means that the + terminal
is at + 9V with respect to the – terminal. Nothing says that the – terminal is at ground
potential unless you tie it there.
Thus, if you wanted to create a positive voltage with respect to ground, say +15
V, you would connect the – and GND terminals and adjust the supply to 15 V. The
voltage available from the + terminal would then be at +15 V with respect to ground. If
you wanted to create a negative voltage with respect to ground, say –12 V, then you
would connect the + and GND terminals, and adjust the supply to 12 V. The voltage
available at the – terminal would then be at –12 V with respect to ground. Keeping
these concepts in mind, never use a power supply unless you connect one terminal to
ground. Unexpected results will ensue if you don’t.
There are two broad classes of dc supplies – fixed and variable. The fixed
supplies, as their name suggest, have a fixed output voltage that is not variable. There
are two types of fixed supplies in the laboratories: 5 V and 15 V. The 5 V fixed
supplies are ideal for powering digital logic ICs, most of which require a +5 V supply
voltage. The 15 V supplies are ideal for powering op-amps and the various specialty
experimental modules available from the technicians in 2C94 for use in the labs.
The variable supplies have variable output voltages and some have variable
output currents as well. The adjustable output current feature is frequently a source of
great confusion. The current adjust knob controls the maximum current that the supply
will source. If the circuit that is connected to the supply attempts to draw more current
than allowed by this setpoint, the power supply will switch from CV (constant voltage) to
CC (constant current) mode. In other words, the supply will only source as much
current as is currently allowed by the setpoint, and the supply voltage will fall from its
expected value accordingly.
Some variable supplies are single supplies, while others are dual. The dual
supplies can be considered to act as two independent single power supplies. You
should be warned that three models of power supplies (the Instek PC-3030 and PC-