9. Output Devices¶
Members: Hajime Itoh / Koji Yamada
Group Assignment Overview¶
This is the group assignment of the week.
 Measure the power consumption of an output device
Methodology¶
For this week’s group assignment, we have decided to use the same test board that the students of Fab Acadeny 2021 used for their group assignment. It has one LED, connected to two types of resistor(1kΩ and 100Ω), and shunt resistor (1Ω). One difference from FA2021 was that we used Seeed Xiao RP2040 for power supply and ground connection. Xiao RP2040 gave us two options on voltage: 3.3V and 5V.
Here is the schematic and PCB design of the board. The shunt resistor is a 1Ω resistor intentionally inserted to detect the current value of the circuit. There is a swicher between the shunt and the test resistors, which helps to measure the voltage, current and power consumption.
Understanding Resistor Codes¶
Before we proceeded to the measurement, we were briefed about the resistor codes. As for the typical throughhole resistors, we could read the resistance by the combination of the color bands printed around the resistor. DigiKey even offers online Resistor Color Code Calculator.
1Ω
1kΩ
100Ω
Note: Then one question came up: How could we read the SMD (surface mount) resistor codes? We searched in the DigiKey webpage and found the SMD Resistor Code Calculator.
"4991": 4.99kΩ
"4990": 0.499kΩ (= 499Ω)
"49R9": 49.9kΩ
Understanding Basic Formula for Calculation¶
In addition to resistor codes, we needed to understand the basic formula of the electricity: Ohm’s Law and power consumption.
Ohm’s Law: V(Voltage) = I (Intensity of electricity)x R(Resistance)
Power Consumption (W): P = I x V
Voltage: V is always maximum at the power output pin and zero at the GND.
Current: I always has the same value wherever we measure the current.
For measuring the voltage, current and resistance, we used a multimeter. Multimator is connected to two probes: red and black. Red should be connected to power source side while the black probe should be connected to the GND side.
5V x 1kΩ¶
Measurement¶
 PowerSupply Voltage: We first measured the powersupply voltage and found that it was 5.07V.

Shunt Resistor: 0.0022V (= 2.2mV)

1kΩ Resistor: 2.35V

LED: 2.72V
Calculation¶
Since the current (I) is always the same, the voltage at the shunt resistor (1Ω) could automatically mean that I has the same value, 2.2mA in this case. Therefore, we could calculate the power consumption:

1kΩ Resistor: 2.2mA x 2.35V = 5.17mW

LED: 2.2mA x 2.72V = 5.984mW

Whole Circuit: 2.2mA x 5.07V = 11.154mW
5V x 100Ω¶
Measurement¶

PowerSupply Voltage: 5.07V

Shunt Resistor: 0.019V (= 19mV)

100Ω Resistor: 1.93V

LED: 3.11V (Brighter than the case for 1kΩ)
Calculation¶
Since the current (I) is always the same, the voltage at the shunt resistor (1Ω) could automatically mean that I has the same value, 19mA in this case. Therefore, we could calculate the power consumption:

100Ω Resistor: 19mA x 1.93V = 36.67mW

LED: 19mA x 3.11V = 59.09mW

Whole Circuit: 19mA x 5.07V = 96.33mW
3.3V x 100Ω¶
Measurement¶
 PowerSupply Voltage: We first measured the powersupply voltage and found that it was 3.27V.
 Shunt Resistor: 0.0045V (= 4.5mV)
 100Ω Resistor: 0.453V
 LED: 2.81V
Calculation¶
Since the current (I) is always the same, the voltage at the shunt resistor (1Ω) could automatically mean that I has the same value, 4.5mA in this case. Therefore, we could calculate the power consumption:

100Ω Resistor: 4.5mA x 0.453V = 2.04mW

LED: 4.5mA x 2.81V = 12.6mW

Whole Circuit: 4.5mA x 3.27V = 14.715mW
3.3V x 1kΩ¶
Measurement¶

PowerSupply Voltage: 3.27V

Shunt Resistor: 0.0007V (= 0.7mV)
 1kΩ Resistor: 0.673V
 LED: 2.6V
Calculation¶
Since the current (I) is always the same, the voltage at the shunt resistor (1Ω) could automatically mean that I has the same value, 0.7mA in this case. Therefore, we could calculate the power consumption:

1kΩ Resistor: 0.7mA x 0.673V = 0.4711mW

LED: 0.7mA x 2.6V = 1.82mW

Whole Circuit: 0.7mA x 3.27V = 2.29mW
Conclusion¶
Here is the summary table of the power consumption:
Resistance  5V  3.3V 

1kΩ  11.154  2.29 
100Ω  96.33  14.715 
Unit: mW 
When we light up the LED, power consumption means the brightness of the LED. Therefore, as a matter of course, we could observe:

Power consumption is larger if powersupply voltage is bigger.

Power consumption is larger if resistance is smaller.