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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.

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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.

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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.

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1Ω

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1kΩ

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100Ω

Note: Then one question came up: How could we read the SMD (surface mount) resistor codes? We searched in the DigiKey web-page and found the SMD Resistor Code Calculator.

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"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

  • Power-Supply Voltage: We first measured the power-supply voltage and found that it was 5.07V.

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  • Shunt Resistor: 0.0022V (= 2.2mV)

  • 1kΩ Resistor: 2.35V

  • LED: 2.72V

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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

  • Power-Supply Voltage: 5.07V

  • Shunt Resistor: 0.019V (= 19mV)

  • 100Ω Resistor: 1.93V

  • LED: 3.11V (Brighter than the case for 1kΩ)

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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

  • Power-Supply Voltage: We first measured the power-supply voltage and found that it was 3.27V.

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  • Shunt Resistor: 0.0045V (= 4.5mV)

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  • 100Ω Resistor: 0.453V

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  • LED: 2.81V

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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

  • Power-Supply Voltage: 3.27V

  • Shunt Resistor: 0.0007V (= 0.7mV)

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  • 1kΩ Resistor: 0.673V

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  • LED: 2.6V

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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 power-supply voltage is bigger.

  • Power consumption is larger if resistance is smaller.

Last update: June 18, 2024