Output Devices¶
Group Assignment¶
- Measure the power consumption of an output device
This week for the group assignment, we had to measure the power consumption of an output device. We decided to do two — an LED and a servo motor — just to see how different they are.
Measuring Power Consumption of an LED¶
Power consumption is basically how much energy a device uses over time. It’s usually measured in watts (W) or milliwatts (mW). To find it, you need two things: the voltage (V) across the device and the current (I) flowing through it. Then you just use:
P = V × I
To measure current, the multimeter has to be connected in series with the circuit — meaning the current actually flows through the multimeter. This part is really important: if you connect it in parallel by mistake, you’ll short-circuit your power source.
Equipment¶
- Multimeter (AstroAI DM6000AR) with ammeter function
- Power source: 4.7V from 3× AAA batteries
- Blue LED, 220Ω resistor, breadboard, and connecting wires
Setup¶
Here’s our workstation for the assignment:
We turned the multimeter dial to ammeter mode (A⎓). The red probe goes into the 10A port and the black probe into COM.
Circuit¶
We built a simple series circuit: battery → resistor (220Ω) → LED → back to battery. The ammeter is inserted in series so all the current passes through it.
Circuit diagram (without ammeter):
Circuit diagram (with ammeter in series):
Ammeter connection steps: 1. Connect the positive terminal of the battery to one end of the 220Ω resistor. 2. Connect the other end of the resistor to the anode (+, longer leg) of the LED. 3. Connect the cathode (−, shorter leg) of the LED to the red (positive) probe of the multimeter. 4. Connect the black (COM) probe of the multimeter back to the negative terminal of the battery — completing the series circuit through the multimeter.
Result¶
The ammeter read 0.005 A (5 mA).
Calculations¶
Voltage across the resistor (Ohm’s Law): $$V_R = I \times R = 0.005\text{ A} \times 220\text{ Ω} = 1.1\text{ V}$$
Voltage across the LED (Kirchhoff’s Voltage Law): $$V_{LED} = V_{supply} - V_{resistor} = 4.7\text{ V} - 1.1\text{ V} = 3.6\text{ V}$$
A typical blue LED has a forward voltage of around 3.0–3.6V, so our 3.6V reading is right at the top of the normal range — which makes sense.
Total power drawn from the supply: $$P_{total} = V_{supply} \times I = 4.7\text{ V} \times 0.005\text{ A} = 23.5\text{ mW}$$
Power consumed just by the LED: $$P_{LED} = V_{LED} \times I = 3.6\text{ V} \times 0.005\text{ A} = 18\text{ mW}$$
The remaining 5.5 mW is dissipated by the 220Ω resistor as heat. In our notebook we calculated 18mW using the supply voltage directly — both are useful numbers depending on whether you want total circuit power or just the LED’s share.
For more accurate results, repeat the measurement several times and average the readings.
Measuring Power Consumption of a Servo Motor¶
A servo motor is way more complex than an LED — it has an internal motor, a gearbox, and a control circuit all packed in. Because of that, how much current it draws depends a lot on whether it’s just spinning freely or actually pushing against something.
Equipment¶
- Multimeter (AstroAI DM6000AR) with ammeter function
- Power source: 4.7V from 3× AAA batteries
- SG90 (or equivalent) servo motor
- XIAO ESP32-C3 (to generate the PWM control signal)
Setup¶
We set the multimeter to ammeter mode, same as before.
Circuit¶
The servo has three wires: VCC (red), GND (brown/black), and Signal (orange/yellow). The XIAO ESP32-C3 sends the PWM signal from its D0 pin, and the power comes from the 3× AAA battery pack. We put the ammeter in series on the VCC line to measure how much current the servo was pulling.
Circuit diagram (without ammeter):
Circuit diagram (with ammeter in series):
Ammeter connection steps: 1. Connect the positive terminal of the battery to the A (10A) port of the multimeter (red probe). 2. Connect the COM port of the multimeter (black probe) to the VCC wire of the servo — completing the series path for current measurement. 3. Connect the servo’s GND wire to the battery’s negative terminal (and to the ESP32 GND). 4. Connect the servo’s Signal wire to the D0 pin of the XIAO ESP32-C3.
Result¶
We measured it in two conditions: - Without load (servo spinning freely): 0.04 A (40 mA) - With load (servo held against resistance): 0.725 A (725 mA)
Calculations¶
Without load: $$P = V \times I = 4.7\text{ V} \times 0.04\text{ A} = 0.188\text{ W} \ (188\text{ mW})$$
With load: $$P = V \times I = 4.7\text{ V} \times 0.725\text{ A} = 3.41\text{ W}$$
The difference here is pretty wild — 188 mW when free vs 3.41 W under load. That’s because when the motor is fighting against resistance, it has to pull a lot more current to keep turning. This is something you really have to think about when picking a power supply.
For more accurate results, repeat the measurement several times and average the readings.
Conclusion¶
Overall this assignment was really useful. We got to actually see the difference in power consumption between a simple passive component like an LED and an active electromechanical device like a servo.
The LED only drew 5 mA and consumed about 18 mW — pretty much nothing. Since it’s just a diode that emits light, its behavior is very predictable using Ohm’s Law and Kirchhoff’s Voltage Law.
The servo was a completely different story. At idle it was pulling 40 mA (188 mW), but the moment we applied load it shot up to 725 mA (3.41 W) — roughly 18× more current. That happens because the internal motor works harder against the resistance, drawing more current to maintain torque.
The biggest thing we learned is that you can’t just look at idle current when choosing a power supply. If a motor stalls or gets loaded, the current spike can be huge — enough to crash a microcontroller or damage components. You always need to design for the worst case.
We also got comfortable using the multimeter as an ammeter, and learned why the series connection and the correct probe port (10A fused input) actually matter.
See you Next time!