11. Output devices¶

Group assignment:¶

Measure the power consumption of an output device.
Document your work on the group work page and reflect on your individual page what you learned.

Group Work¶

Power consumption¶

It is important to track power consumption of electronics. This can be informative when choosing power source (via line, a power brick, or a battery) to ensure that the device is sufficiently powered. Understanding power consumption can also understand the financial cost of using electricity, which is particularly true of large electronics and appliances.

Power is measured in Watts (W), where a Watt is equal to the Voltage (in volts/V) times the Amperage (in amps/A). Therefore, the formula can be expressed as W = V*A.

For billing, prices are often expressed in kilowatt hours (kWh). 1 kWh would represent the cost for using 1000 Watts for an hours. In Iceland the price for a kWh is approximately 0.18 Euros.

One can use a multimeter to measure power, which would require measuring the Voltage and Amps separately. The formula noted above can be used to calcutate Watts from the Voltage and Amperage. Measuring volts is relatively simple with the multimeter, as that can be measured in a parallel circuit. For that, in DC the one probe can be touched to the power source and the other to the ground. However, measuring amperage requires measurement in series. Therefore, the measurement would need then to be placed within the circuit. For a full circuit board, that might be difficult or impossible to achieve.

Another mechanism to measure power for small electronics is with a USB power meter. There are inexpensive meters, which have a small screen and USB power IN and OUT. To use such a meter, this is put inline with the USB connection, and therefore will be measuring in series.

Our instructor has such a meter, which we tested on a board running an output device.

Testing power consumption¶

The board used for the testing came from Albert’s final project. This board can control two stepper motors, a servo motor, and has an I2C connection. For the testing, we used the board while controlling two stepper motors.

As the stepper motors require 12V, this power source is coming from a QC power hack board. The design has a potentiometer. This feature was used to adjst the voltage output from the QC hack power board.

To start the QC power board was connected to the USB power meter, which was connected to a USB power brick (one that delivers up to 20V via USB QC negotiation). The potentiometer was adjusted with a screw driver to tune the voltage.

When maximizing the voltage via the potentiometer, nearly 20V was observed.

The voltage could also be set much lower, here showing it tuned to about 9V.

The target for the motors is 12V, so QC power board was tuned to 12V.

Then, this was connected to the motor control board. In series, Power brick => USB meter => QC power hack (at 12V) => motor control board => stepper motors. Then, a program was run to make the motors rotate. The specific program didn’t run the motors continuously, as they are part of a project to deal playing cards. When running the motors, variable amperage could be detected. From the following two pictures, one can see different amperage at times. This varied by the motor load at times.

The meter can make a graph on screen to show the amperage over time. While that is somewhat informative, it could be useful to have a text output over time, but that would need a USB power meter with greater functionality. While that might be nice to have, this simple meter gives all the essential information.

Power cost¶

At peak, about 0.11 amps were used in this setup. With the voltage measured at 12.8V, that corresponds to 1.4 W (12.8 V * 0.11 A). If that were to run for one hour, that would be 0.0014 kWh. At the price noted above, that would be approximately 0.025 Euro cents in power consumption cost. While this is not very costly, it also shows that the microprocessors and circuits being used are power efficient.