Understanding DC Power Supplies

Overview

Power supply is a reference to the source of electrical power. Most electronic circuits require a DC power supply.
The most common operating voltages for microcontrollers and digital processors are 5V and 3.3V.
There are many different kinds of DC power supplies but this one is most commonly used here at ITP:

– Click on any image for a larger view

   

Photo of a DC power supply. A rectangular block approximately 2 inches by 3 inches with plugs to plug into the wall. A wire extends from the plug to connect with your circuit.
DC Power Supply

 

Jameco 12V Regulated Switching Power Supply
Part# 170245 (12V, 1000mA)

Available at the NYU computer store.

DC power supply rating label. Output is 12 VDC 1000 mA. Center Positive.
DC supply rating label

Most power supplies have a rating label that looks something like the one above. Make sure you know the polarity of the plug so you don’t reverse polarity for your circuit and damage your components. The diagram below showing positive tip polarity is on the left and negative tip polarity is on the right. The center positive drawing on the left indicates that the center (tip) of the output plug is positive (+) and the barrel of the output plug is negative (-).

 

Symbol for a center-positive power supply.
Symbol for a center-positive power supply.

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Symbol for a center-positive power supply.
Symbol for a center-negative power supply.

 

Abbreviations

V : Volts
A : Amperes
W : Watts
mA : miliAmperes
VA : Volt Amperes
VAC : Volts AC
VDC : Volts DC
DC : Direct Current
AC : Alternating Current

Testing your power supply

It is always good practice to test a power supply before using it for the first time. The example below will show how to test a power supply with positive polarity. If you have a negative polarity power supply, then you will get a negative reading. You should then switch the position of the multimeter probes.

Testing a center positive power supply with a multimeter. Red probe goes into the tip Black probe touches the barrel
Red probe goes into the tip
Black probe touches the barrel
  1. Plug your power supply into an AC outlet. 
  2. Red probe goes into the tip 
    Black probe touches the barrel
  3. Turn on your multimeter and set it to read DC voltage.
  4. Take the red (positive) probe from your multimeter and stick it into the end of the power supply plug.
  5. Take the black (negative) probe from your multimeter and carefully touch it against the barrel of the plug without touching the tip or your red probe. If you make a connection, you will be creating a short circuit.
  6. On your multimeter you should see a reading of the voltage coming from your power supply. If you are checking a 12V power supply and your multimeter shows “12.56V” everything is fine and dandy. If you get a reading of “-12.56V” then your probes are attached in reverse. If this happens and you are positive you connected your probes correctly, double check the polarity on your power supply’s label and make sure the circuit you will be powering with this unit is designed to handle this polarity.
Multimeter displaying output DC voltage of 12.56 volts
Multimeter displaying output DC voltage

If the voltage showing on your multimeter is more than half a volt or a volt off its rating, then you most likely have what is called an unregulated power supply. The 12V Jameco power supply we used in this example is a regulated one, so that is why the voltage we received was so close to the voltage it was rated for.

Originally written on August 22, 2014 by Tom Igoe
Last modified on September 6, 2018 by Jim Schmitz