Week 10 - 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.
Overview
For this week’s Group Assignment, we explore and measure the power consumption for the following output devices:
• NeoPixel
• DC Motor
NeoPixel
A good place to start for the NeoPixel is to understand what it is, and how it is different from "normal" LEDs. We use the following prompt in Chat GPT to get an answer:
What is a NeoPixel and how does it differ from normal LEDs?
Chat GPT Reply:
A NeoPixel is a brand name (by Adafruit) for individually addressable RGB LEDs that are controlled via a single-wire communication protocol. They are based on WS2812, WS2811, or SK6812 LED driver chips.
Differences
| Feature | NeoPixel (WS2812, WS2811, etc.) | Normal LED (e.g., single-color or RGB) |
|---|---|---|
| Control | Controlled via a digital data signal | Controlled via direct voltage or PWM |
| Wiring | Uses a single data line (+ power & ground) | Each LED needs its own connection or transistor circuit |
| Colour | Full RGB (or RGBW) control per LED | Fixed single color or manually mixed RGB |
| Addressability | Each LED can be independently set to any color | All LEDs in a circuit usually share the same color unless controlled separately |
| Power | Typically 5V (sometimes 3.3V or 12V versions) | Varies (e.g., 1.8V–3.3V for basic LEDs) |
| Complexity | Requires a microcontroller to send data | Simple on/off with a resistor or transistor |
| Chaining | Can be daisy-chained in a strip or matrix with only one data pin | Requires multiple control lines or multiplexing |
Knowing this, we proceed onto our power consumption measurement for the NeoPixel.
Setup
To measure the power consumption for the NeoPixel, we set it up as shown in the picture above.
1. 5V NeoPixel light strip with 6 LEDs
2. XIAO RP2040 board as a microcontroller
3. USB power cable
4. USB power meter – We use this to measure the power
We use the following program provided by our instructor, Steven to control the NeoPixel.
To get the NeoPixel to display certain colours, we adjust the RGB values as follows:
Red: 255, 0, 0
Green: 0, 255, 0
White: 255, 255, 255
We then run the program and observe what happens.
Power Consumption Measurement
We observed that the USB meter was showing some power consumption (0.203W) even when the NeoPixel had not lit up yet. We note that this is the power consumption of the board itself.
We then observe the values displayed by the USB meter when the NeoPixel lights power on. Below are the videos we recorded.
The following values were observed for the different NeoPixel colours:
| Colour | Current (A) | Voltage (V) | Power (W) RAW Reading (Including board) |
Power (W) Actual Power Consumption (Just NeoPixel) |
|---|---|---|---|---|
| Red | 0.011 | 5.08 | 0.558 | 0.355 |
| Green | 0.011 | 5.08 | 0.558 | 0.355 |
| White | 0.025 | 5.06 | 1.265 | 1.062 |
From the results in the table above, we can conclude that power consumed by the NeoPixel when displaying white colour is a lot higher than when displaying the red and green colours.
DC Motor
For our next output device, we measure the power consumption of a DC motor. The setup is simple – we connect it to a bench power supply to run the DC motor and then observe what happens.
Power Consumption Measurement
The Current and Voltage values observed are as follows:
| State of DC Motor | Current (A) | Voltage (V) | Power (W) (P = I x V) |
|---|---|---|---|
| Running | 0.46 | 4.0 | 1.84 |
| Stalled (By hand) | 0.46 | 0.9 | 0.414 |
To interpret our observations, I asked ChatGPT:
What happens to the power of a small DC motor when you stall it using your hand?
ChatGPT reply:
When you stall a small DC motor using your hand (preventing it from rotating while still supplying power), several things happen:
1. Current Increases – The motor's resistance is relatively low, so when it stalls, the back EMF (which normally opposes the supply voltage) drops to nearly zero. This causes a surge in current, which can be much higher than the normal running current.
2. Power Dissipation as Heat – Since electrical power is given by P=V×I, the power drawn from the supply increases due to the high current. However, instead of being converted into mechanical work (rotation), almost all of this power turns into heat in the windings.
3. Overheating Risk – If the stall condition is maintained for too long, the motor can overheat, potentially damaging the winding insulation or even burning out the motor.
4. Torque at Maximum – At stall, the motor is generating its maximum possible torque, which is why you feel resistance when holding it.
5. Potential Power Supply Stress – The high current draw might stress your power supply, potentially causing voltage drops or triggering overcurrent protection.
End of ChatGPT reply.
This is actually not consistent with our observation. From what we observed, it seems like the current had remained stable but the voltage dropped. So, we asked ChatGPT again…
But from what I observed on my bench power supply, the current remains stable, but the voltage drops when I stall the DC motor with my hand. What is happening here?
ChatGPT reply:
What you're observing—a stable current but a voltage drop—suggests that your bench power supply has a current limit enabled (often called "current limiting" or "constant current mode").
Here's what's happening step by step:
1. Normal Operation (Motor Spinning)
When the motor is running freely, it generates a back EMF that opposes the supply voltage. This limits the current draw, keeping it at a moderate level.
2. Stalling the Motor
When you stall the motor, the back EMF disappears. Normally, this would cause a huge surge in current, limited only by the winding resistance and internal motor losses. However, your bench power supply is limiting the current to a set maximum value.
3. Power Supply Response (Voltage Drops)
Since the power supply is in constant current mode, it lowers the voltage to prevent the current from exceeding its set limit. Essentially, instead of allowing unlimited current to flow (which could damage the motor or power supply), it reduces the voltage to maintain a safe current level.
4. Effect on Power
Electrical power is given by P=V×I. Since voltage drops but current stays constant, the overall power output decreases compared to what would happen if the power supply didn't limit the current.
Conclusion
Your power supply is protecting your motor and itself by preventing excessive current flow during stall conditions. If you want to verify this, check if your power supply has an indicator for current limiting mode (often labeled as "CC" for constant current mode).
End of ChatGPT reply.
That clears things up for us. We knew that our bench power supply has a current limiting mode, which explains the voltage drop observed when we stall the motor. If the current limiting was absent, the power will be dissipated as heat instead of motion.
Power Consumption Measurement (Retest)
After submitting the weekly assignment, our instructor Steven pointed out that we had performed our test with the DC Motor incorrectly.
Here is what we should have done:
1. Maintain the Voltage level – We were supposed to set the current limit to a higher level and maintain the voltage level when performing our measurements, but we did not do this.
2. Use a different DC motor – The DC motor we used previously had an RPM that was too high so when we tried to stall it, it was not able maintain the voltage level. As such we performed our tests again, this time setting a higher current limit while maintaining the voltage level.
| State of DC Motor | Current (A) | Voltage (V) | Power (W) (P = I x V) |
|---|---|---|---|
| Running | 0.07 | 5.0 | 0.35 |
| Stalled (By hand) | 0.31 | 5.0 | 1.55 |
Below is a video of our retest.
This concludes our Group Assignment for this week.