Output Devices: Power Consumption Measurement
Group Assignment Documentation — Fab Lab Rwanda
Overview
This week as a group, we focused on characterization and measuring the power consumption of different output devices (specifically an SG90 Servo Motor and an I2C LCD Display). Understanding electrical limits is critical to prevent MCU board resets and chip damage from current overdraw.
Measurement Setup & Tools
To record power draw parameters, we utilized the following apparatus:
- Digital Multimeter (DMM): Wired inline to log active current variables.
- Regulated DC Power Supply: Used to maintain separate, isolated 5.0V voltage lanes for heavy loads.
- Test Output Loads: TowerPro SG90 Micro Servo and a 16x2 I2C Character LCD Module.
Mathematical Power Modeling
Electrical power dissipation within a continuous DC environment is modeled by Joule's Law, calculating the work conversion rate as a function of potential difference across a path and electron velocity:
• P = Power Output in Watts (W)
• V = Operating Voltage Potential in Volts (V)
• I = Measured Electrical Current in Amperes (A)
1. SG90 Micro Servo Analysis Calculations
Since an inductive actuator pulls variable current depending on internal friction, mechanical resistance, and movement acceleration profiles, we performed calculations across three core operating thresholds:
P_idle = 5.0 V * 0.01 A = 0.05 W
P_active = 5.0 V * 0.22 A = 1.10 W
P_stall = 5.0 V * 0.55 A = 2.75 W
2. 16x2 I2C Display Performance Calculations
The screen's baseline consumption scales according to state control over the embedded background illumination LED network array:
P_backlight_on = 5.0 V * 0.03 A = 0.15 W
P_backlight_off = 5.0 V * 0.02 A = 0.10 W
Hardware Wiring Implementation Methodology
To safely capture current values without causing short circuits, the digital multimeter was integrated directly in series with the positive voltage line (VCC). This setup creates an electrical bridge, forcing all electrons entering the load to loop through the measurement instrument first.
The Series Measurement Setup
To measure current (I), the multimeter must become a "bridge" in the circuit so that all the electricity flowing into the output device passes through the meter.
Step-by-Step Hardware Wiring Connections
- Power Supply to Multimeter connection: We routed the positive (+) 5V output line originating from the external power source (or ESP32 Vin tracking rail) directly into the 10A high-current measurement terminal port of our Digital Multimeter via the Red Probe wire lead.
- Multimeter to Servo VCC connection: We ran the common COM terminal port matching lead from the multimeter directly through the Black Probe wire line to plug securely onto the VCC power input pin (Red line) of the SG90 Micro Servo Motor.
- Control Logic & Common Ground Loop:
- The PWM Logic Control Pin (Orange/Yellow line) exiting the servo was hooked directly into the digital input/output pin D2 routed on our custom XIAO RP2040 micro-development board.
- The Ground Pin (Brown/Black wire) from the servo was unified into a common junction tying directly back into the primary GND of our custom board AND the GND terminal of the external power supply module.
Collected Power Data Matrix
| Output Device | Operating Voltage (V) | Idle Current (mA) | Active/Peak Current (mA) | Calculated Max Power (W) |
|---|---|---|---|---|
| SG90 Servo Motor | 5.0 V | 10 mA | 550 mA (Stall/Movement spike) | 2.75 W |
| 16x2 I2C LCD | 5.0 V | 20 mA (Backlight OFF) | 30 mA (Backlight ON) | 0.15 W |
Analysis & Group Reflections
- Servo Inrush Spikes: The servo pulls high transient currents when initiating motion. This structural characteristic explains why powering them straight from an MCU regulator (like the XIAO RP2040) drops processing voltage levels, inducing immediate loop restarts.
- Common Ground Importance: When separating power tracking rails (e.g., using an external board line or battery source), linking the ground reference back to the primary MCU board ground is vital to preserve clean PWM signal reading loops.