{"id":1602,"date":"2014-08-22T10:37:02","date_gmt":"2014-08-22T14:37:02","guid":{"rendered":"https:\/\/itp.nyu.edu\/physicalcomputing\/?page_id=1602"},"modified":"2025-09-12T07:28:00","modified_gmt":"2025-09-12T11:28:00","slug":"understanding-dc-power-supplies","status":"publish","type":"page","link":"https:\/\/itp.nyu.edu\/physcomp\/lessons\/electronics\/understanding-dc-power-supplies\/","title":{"rendered":"Understanding DC Power Supplies"},"content":{"rendered":"\n

<\/span>Overview<\/span><\/h2>\n\n\n\n

Power supply is a reference to the source of electrical power. Most electronic circuits require a DC power supply. Chances are you have one at home already, and can use it for physical computing projects. <\/p>\n\n\n\n

The most common operating voltages for microcontrollers and digital processors are 5V and 3.3V. You can find power supplies in many voltages, but 5V and 12V are common. To convert 12V to 5V or 3.3V, you’d need a voltage regulator. The Breadboard Lab<\/a> covers how to set that up.<\/p>\n\n\n\n

There are many different kinds of DC power supplies but this one shown in Figure 1 is most commonly used at ITP:<\/p>\n\n\n\n

– Click on any image for a larger view<\/p>\n\n\n\n

\"Photo<\/a>
Figure 1. DC Power Supply<\/figcaption><\/figure>\n\n\n\n

 <\/span><\/p>\n\n\n\n

Jameco 12V Regulated Switching Power Supply<\/a>
Part# 170245 (12V, 1000mA)<\/strong><\/p>\n\n\n\n

\"DC<\/a>
Figure 2. DC supply rating label. This is the back of the supply in Figure 1. <\/figcaption><\/figure>\n\n\n\n

Most power supplies have a rating label that looks something like the one in Figure 2. Make sure you know the polarity of the plug so you don’t reverse polarity for your circuit and damage your components. The diagram in Figure 3 and Figure 4 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 (-).<\/p>\n\n\n\n

\"Symbol<\/a>
Figure 3. Symbol for a center-positive power supply.<\/figcaption><\/figure>\n\n\n\n

,<\/p>\n\n\n\n

\"Symbol
Figure 4. Symbol for a center-negative power supply.<\/figcaption><\/figure>\n\n\n\n

<\/span>Abbreviations<\/span><\/h2>\n\n\n\n

V<\/strong> : Volts
A<\/strong> : Amperes
W<\/strong> : Watts
mA<\/strong> : miliAmperes
VA<\/strong> : Volt Amperes
VAC<\/strong> : Volts AC
VDC<\/strong> : Volts DC
DC<\/strong> : Direct Current
AC<\/strong> : Alternating Current<\/p>\n\n\n\n

<\/span>Testing your power supply<\/span><\/h2>\n\n\n\n

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.<\/p>\n\n\n\n

\"Testing<\/a>
Figure 5. Red probe goes into the tip
Black probe touches the barrel<\/figcaption><\/figure>\n\n\n\n
    \n
  1. Plug your power supply into an AC outlet.\u00a0<\/span><\/li>\n\n\n\n
  2. Turn on your multimeter and set it to read DC voltage.<\/li>\n\n\n\n
  3. Take the red (positive) probe from your multimeter and stick it into the end of the power supply plug as shown in Figure 5.<\/span><\/li>\n\n\n\n
  4. 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.<\/li>\n\n\n\n
  5. 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 as shown in Figure 6. 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.<\/li>\n<\/ol>\n\n\n\n
    \"Multimeter<\/a>
    Figure 6. Multimeter displaying output DC voltage<\/figcaption><\/figure>\n\n\n\n

    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\u00a0unregulated<\/strong>\u00a0power 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. Some\u00a012V DC power supplies will actually\u00a0read at 15-16V. <\/p>\n\n\n\n

    Unregulated power supplies run high on voltage when there is no load. When the supply starts to take a load, the current consumed\u00a0will go up, and the voltage will drop.\u00a0 There are (more expensive) constant current supplies and constant voltage supplies, which will adjust their current or voltage when there is a change in the voltage or current, respectively. For more on this,\u00a0here’s a decent explanation<\/a>.\u00a0<\/p>\n\n\n\n

    This is a reason why we use voltage regulators, and why most microcontroller modules have them built in. They’re designed to take a range of voltage and output a constant, lower voltage. In a sense, a voltage regulator is a very minimal constant voltage supply. <\/p>\n\n\n\n

    <\/span>Powering an Arduino Project from a Mobile Phone Charger<\/span><\/h2>\n\n\n\n

    Many people have old mobile phone chargers around the house, and wonder, “Can I use this for powering an Arduino project?” Generally, you can. Just get a USB cable with the appropriate connectors to connect the phone charger to your Arduino. Most phone chargers output 5V and a few hundred milliamps, which will power an Arduino, some sensors, and some LEDs. If you try to program your Arduino from the phone charger’s cable, though, you may be out of luck. Many phone chargers come with USB cables that contain only the power connections, not the data connections.<\/p>\n\n\n\n

    <\/span>Matching A Power Supply to an Electronics Device<\/span><\/h2>\n\n\n\n

    To determine whether a power supply is right for your project, you need to note the voltages that each component operates at, and the current they consume, and make sure your power supply can provide the right amount of power. <\/p>\n\n\n\n

    Here are a few examples:<\/p>\n\n\n\n

    <\/span>Arduino, Pushbuttons, Potentiometers, LEDs, Speaker<\/span><\/h3>\n\n\n\n

    Imagine you’re making a project that includes an Arduino, a few LEDs, some pushbuttons, some potentiometers or other variable resistors, and perhaps a speaker. The Digital In and Out lab<\/a> and the Analog In lab<\/a>, and the Tone Output<\/a> labs all describe projects that meet this description. All of the components other than the Arduino in this project are powered from the Arduino voltage output. None of the external components consume more than a few milliamps each. The whole circuit, Arduino included, will likely consume less than 200 milliamps of current. Here’s a breakdown, measured using an LED and a potentiometer:<\/p>\n\n\n\n