A lot of makers don't know how important it is to know the current draw of your project, or why you need to know this. In this tutorial I will explain to you how to measure the current draw of your project, and why it is so important to know this.
I often get asked the question of what type of power supply to use for projects. Most of us know the voltage required, but how much current it draws and why you need to know this is a mystery to many beginner makers. To start with what is this current thing? "In comes the analogy that uses the flow of water to explain these things".
Let’s say that a battery that we use to power our project is a bucket of water with a hose attached to it. The water pressure at the end of the hose is what we consider the voltage. The speed the water runs through the hose is the current and that is measured in Amps.
Yes this never made sense to me either. It comes down to this if your project consumes more current then your power supply has to offer it won’t run, and can even damage your power supply. We measure current in Amperes or Amps for short (Symbol used is A).
One way to roughly figure out what power supply to use is by reading the documentation of all the components used in your project, and see their current draw (how much Amp(A) or milliamp(mA)). Add these numbers up and you roughly know what the amperage your power supply needs to be.
Note of caution: If you use your laptop USB to power your project you could damage your USB port if the current draw is too high. This is why it is a good practice to have an external power supply to power your project, even when you have a USB cable connected to your Arduino.
MATERIALS NEEDED IN THIS TUTORIAL
The other way to find out the current draw is by using a multimeter. Follow the these steps to setup your multimeter for this exercise:
Step 1) Look at the bottom of your multimeter you will notice 3 or 4 ports where you plug your probes in to. The black probe plugs into the COM port (COM stands for common ground). The red probe for this exercise should be plugged into the port with the A symbol (or 10A or something similar).
Note of caution: Multimeters have a limit on how much Amperage a port they can handle. Most likely your multimeter also has a port that measures the current in mA. I normally don’t use this one as it is so easy to damage if you don’t watch it, and you still get enough precision through the A port.
Step 2) Turn the dial of your multimeter to A and use the DC mode. Some multimeters will have multiple A options on the dial. We are using DC (direct current), look for an A with this symbol behind it .
Step 3) Next using the adapter we connect the red probe from your multimeter to the plus (+) terminal of your power supply. Connect the black probe to the Vin on your Arduino Uno. Finally connect the GND on your Arduino Uno to your power supply minus (-) terminal. This is called putting your multimeter in in series, or in line with your power supply.
Your setup should look a little like figure 1. You should now see how much current your project requires.
Note: Some components require a start-up current (DC motors are bad for this). Your multimeter might not be fast enough to register this, and might not notice this if the power supply can handle this. This happened to me when I started using my desktop power supply. I had a DC motor that needed 3A current to start, after it was running the current use dropped back to under 1A.
Another good reason for knowing how much current your project uses is when you try to run it of a battery pack. Most USB battery packs are graded (the Capacity ) in milliamp hours (mAh). To figure out how long it will last all you need to know is its capacity and how much current your project draws. All you have to do nexts is following this simple formula:
Divide the battery pack capacity in mAh by the projects current draw in mA. If you have a battery back that provides 16750mAh and your project consumes 32 mA just divide 16750mAh by 32mA
With a battery pack with a capacity of 16750mAh and a project consuming 32mA has a runtime of about 523 hours. Just a disclaimer; this is not precise in anyway. Environmental factors like temperature have a big impact on the actual run time. Another factor is the quality of the power cells used in your battery pack. All battery packs will start to discharge quicker as it loses charge, some might also have voltage drop. So the moral of this story is that you have to run a test to see the actual run time, the calculation will give you a proximate value only.
Knowing your projects current consumption can make sure you get the right power supply for your project, reducing the chance of power supply failure or unexplained freeze ups of your project. It can also help you figure out what battery pack you need for your project.
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