10. Output Devices¶
Group assignment:
- Measure the power consumption of an output device.
Gyumri lab¶
Overview¶
For this group assignment, we needed to measure the power consumption of an output device. To understand why this matters and where it applies in practice, we decided to look at a real example — the final project of Ani Petrosyan, which requires the use of LED strips.
Context: Ani’s Project¶
Since LED strips operate on 12V DC, Ani’s board is powered from a 12V DC supply. The board includes a voltage regulator that steps 12V DC down to 5V DC to power the microcontroller and sensors.
The 12V LED strips are controlled via MOSFET transistors (50N03, TO-252 package). According to the KIA 50N03 datasheet, the absolute maximum continuous drain current is 50A — but this assumes ideal thermal conditions with a heatsink.
In Ani’s project, no heatsink is used. The KIA 50N03 datasheet specifies a junction-to-ambient thermal resistance (RθJA) of 50°C/W and a maximum power dissipation of 60W at 25°C. Without a heatsink, heat dissipation is significantly limited by the TO-252 package alone. Based on these thermal constraints, we adopt a conservative practical limit of 5A for safe continuous operation without a heatsink.
Setup & Initial Measurement¶
In our lab, we had an LED strip with 5 sections, each section measuring 25 mm in length.
We started by powering the strip from the lab’s bench power supply (YIHUA 3010D III), setting the output to: - Voltage: 12V - Current limit: 1A
After turning it on, the power supply displayed a current draw of approximately 0.13A.
However, the power supply readout has limited precision, so we used a multimeter for a more accurate measurement.
Measuring with a Multimeter¶
When using a multimeter, probe connections differ depending on what is being measured:
- Voltage or resistance: probes are connected in parallel across the contacts.
- Current: probes are connected in series, breaking the circuit.
- The black probe (negative) always goes into the COM port; the red probe goes into the port that matches the measurement type.
Since we were measuring current, the red probe was connected to the 600mA input port.
The multimeter gave us the following result:
Calculations¶
Current per LED Strip Section¶
To find the current draw of a single LED section, we divided the total measured current by the number of sections:
Safety Derating for the MOSFET¶
To ensure reliable operation, we applied a standard 30% safety derating to the practical maximum current. This accounts for real-world factors such as ambient temperature, PCB heating, and component tolerances — it is common practice to never run a component at its rated limit.
Given the practical limit of 5A = 5000 mA without a heatsink:
Maximum Number of Sections¶
With 28.42 mA per section and a 3500 mA safe limit:
Maximum LED Strip Length¶
Since each section is 25 mm long:
Conclusion¶
Based on the measurements and thermal constraints of the 50N03 MOSFET without a heatsink, Ani can safely use up to 3.08 meters of this LED strip in her final project. This ensures stable and reliable operation within the designed circuit constraints.