This article is part of a guided learning series to explore real-world applications of MOSFETS and microcontrollers.
Canonical Article: How to Interface a Microcontroller with a Relay Using a MOSFET
Learning Companion (Q&A): Explore All Questions
You are reading: Question 1
Why do the Arduino microcontroller and 12 VDC power supply share a common ground?
Hint
What are the requirements for KVL?
Answer
We know that a MOSFET is turned on via a positive voltage applied to its gate. However, we must be very clear that this voltage is measured with respect to the MOSFET’s source. For example, the N-channel MOSFET in the featured article requires about 5 VDC on the gate relative to the source as shown in Figure 1.
The Arduino microcontroller can provide this 5 VDC, but only if the microcontroller’s ground is connected to the MOSFET source ground. Without this critical ground connection, the MOSFET floats relative to the Arduino leading to unexpected operation or even damage due to overvoltage applied to the MOSFET’s sensitive gate region.
In terms of KVL, we recognize that current flows in loops, but we need to read the fine print. Technically, the summation of voltages in a closed loop system is zero. The closed loop stipulation implies that the microcontroller, MOSFET, and 12 VDC supply must share a common reference. The common ground ensures that the gate to source voltage will be correctly applied. Stated another way, we can’t measure the voltage on a battery with a single wire. We need both wires to close the loop.
Figure 1: Representative schematic for the Arduino microcontroller, MOSFET, and relay. The schematic was built using MultisimLive.
Alternative answer
The MOSFET and microcontroller do not necessarily need to share the same ground. In fact, there may be advantages to providing galvanic isolation using an optoisolator. An example is shown in this article which demonstrates how to optically isolate a MOSFET in a bootstrap circuit. Note that the N-channel MOSFET’s source toggles between 50 VDC and ground (or nearly so when we consider the voltage drop across D2).
Figure 2: This schematic presents a highly simplified high-side MOSFET gate drive circuit. Bootstrap capacitor C1 provides energy for the MOSFET gate drive.
Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey