11. Output devices¶
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Group assignment:¶
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Measure the power consumption of an output device.¶
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Document your work on the group work page and reflect on your individual page what you learned.¶
Learn how to measure Currency¶
Preparation:
Multimeter
Development Board made by Daisuke Hanamido
Bread Board
Power Supply
Prepare Power Bench
LED light Red
LED Light Blue
Resister 50Ω, 150Ω, 10Ω
This week, in a group assignment, we learned about Ohm’s law, parallel circuits, series circuits, how to calculate the power consumed in those circuits, how to handle DC regulated power supplies, and how to use the current measurement function of a multimeter.
Broken terminal¶
Our multimeter could not measure properly at 400mA, so we measured using the A range on the 10A terminal.
what is Power (W)¶
Power P (W) is the amount of electricity needed at a given moment. Multiplying this by time (h) gives the energy (Wh), which tells us how much energy is needed per hour.
Power P (W) = Current I (A) * Voltage V (V).
If you know any two of the following: current, voltage, or resistance, you can calculate it using Ohm’s law:
Voltage V (V) = Current I (A)× Resistance R (Ω).
P = IV = RI^2 = V^2/R.
By adding up the power consumption of each device calculated using this method for each device, you can find the total power consumption. When selecting a power supply, choose one that can output more than the calculated total wattage. Also, for safety, leave a margin of about 20% so that the total wattage is used at 80% of the power supply’s rated power consumption. (When changing the AC adapter, it’s best to buy one with the same voltage and connector pin assignment, but one with a slightly higher current.)
How to Calculate LED Power Consumption¶
LEDs have different voltages and current consumptions depending on their color. The voltage required for an LED to emit light is called the forward voltage (Vf). - Red, orange, yellow, and yellow-green: approximately 1.8 ~ 2.2V;
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white, blue, green, and warm white: approximately 3.0 ~ 3.6V;
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ultraviolet: 4.5 ~ 6V.
vf red
vf green
The current required is usually around 20mA (1 ~ 5mA for low-current LEDs), and this allows us to calculate the required power.
1.8V × 0.02A = 0.036W
The above formula gives the power consumption of one red LED. Calculating the green LED in the same way, we get:
2V × 0.02A = 0.04W
Considering both LEDs being lit simultaneously, the total power consumption of the LEDs is 0.076W.
Furthermore, when the power supply voltage is 5V, the value of the current-limiting resistor required to light each LED individually is:
For a red LED: R = (5V - 1.8V) / 0.02A = 3.2 / 0.02 = 160Ω The power consumed by this resistor is: 3.2V × 0.02A = 0.064W
For a green LED: R = (5V - 2V) / 0.02A = 3 / 0.02 = 150Ω The power consumed by this resistor is: 3V × 0.02A = 0.06W
The total power consumption is 0.124W.
Measured current values red
Measured current values green
parallel¶
Measured values green
When lighting both LEDs in a parallel circuit, using a power supply voltage of 5V and the current-limiting resistors of the above values, the current flowing through each LED is 0.02A. Therefore, the total current flowing through the circuit is the sum of these currents, which is 0.04A.
The voltage across the entire hand warmer is 5V, so the power consumption is 0.2W. The total power consumption of each element is also 0.2W, so the calculation is correct.
series¶
When lighting them in a series circuit, if the power supply voltage is 10V, the above resistors can be connected in series as is. However, if the power supply voltage is not changed, the resistance value needs to be changed. In this case, the value of the current limiting resistor is: R = (5V - 2V - 1.8V) / 0.02A = 1.2V / 0.02A = 60Ω The power consumption is 1.2V × 0.02A = 0.024W.
The voltage across the series circuit is 5V and the current is 0.02A, so the power consumption is 0.1W. This is the same as the sum of the power consumption of the two LEDs (0.076W) and the power consumption of the 60Ω resistor (0.024W), so the calculation result is correct.
