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----
-title: MOSFETs as electronic switches
-date: 2025-06-22
-layout: post
----
-
-Recently, I needed a low-power circuit for one of my battery-operated projects.
-Much of the system's power savings depended on its ability to electronically
-switch off components, such as servos, that draw high levels of quiescent
-currents. My search for a solution led me to MOSFETs, transistors capable of
-controlling circuits operating at voltages far above their own.
-
-## Acknowledgments
-
-This article is a summary of what I learnt about using MOSFETs as switches.
-I'm not an electronics engineer, and this is not an authoritative guide. The
-circuits in this post must be considered within the narrow context in which
-I've used them. All credits for the schematics belong to <a
-href="https://electronics.stackexchange.com/users/292884/simon-fitch"
-class="external" target="_blank" rel="noopener noreferrer">Simon Fitch</a>. 
-
-## Preamble
-
-For a typical MOSFET-based switch, we can connect a GPIO pin of a
-microcontroller to the gate of a logic-level N-channel MOSFET placed on the low
-side of the load and tie the gate and the drain pins of the MOSFET with a
-pull-down resistor. This would work as long as the power supplies of the
-microcontroller and the load don't share a common ground. Things become more
-complicated when they do (e.g., controlling power to a component driven by the
-same microcontroller).
-
-In that scenario, the source potential visible to the load is the difference
-between the gate and the threshold potentials of the MOSFET. For example, when
-the gate and the threshold potentials are 3.3 V and 1.5 V, the potential the
-load sees is 1.8 V. So, to use a low-side N-channel MOSFET, we need the gate
-potential to be higher than the source potential, which may not always be
-practical. The alternative would be a hide-side switch.
-
-## P-channel high-side switch
-
-The following schematic shows how a high-side P-channel MOSFET (M1) could
-switch power to a 6 V servo driven by a 3.3 V MCU.
-
-![P-channel high-side switching circuit](p_high_side.png)
-
-When the microcontroller outputs low, the M2 N-channel MOSFET stops conducting.
-The R1 resistor pulls the gate of the M1 P-channel MOSFET up to +6 V, switching
-the servo off. When the microcontroller outputs high on the GPIO pin, M2's
-source-drain connection starts conducting, causing M1's gate potential to drop
-to 0 V, which switches on power to the servo. 
-
-## N-channel high-side switch
-
-The P-channel high-side switch would be the typical architecture for our use
-case. However, if we have access to a potential high enough to safely raise the
-gate potential above the threshold such that their difference outputs the source
-potential required to drive the load, we can switch on the high side using an
-N-channel MOSFET:
-
-![N-channel high-side switching circuit](n_high_side.png)
-
-In the schematic, both M1 and M2 are N-channel MOSFETs. When the
-microcontroller output is low, M2 stops conducting. This causes the M1's gate
-potential to rise above the threshold, turning the servo on. Conversely, a high
-output on the GPIO line switches M2 on, which lowers M1's gate potential. This
-switches the servo off. The R2 pull-up resistor prevents the high impedance of
-the output pins at power-up from switching the servo on.
-
-Both topologies require M2 to act as a level converter between circuits
-containing the microcontroller and the servo, converting between 0 V and +6 V
-or +9 V. M2 is a low-power signal converter carrying less than a milliamp of
-current. The gate-source threshold voltage of M2 must be lower than the MCU's
-supply voltage. 2N7000, 2N7002, and BSS138 are popular choices for M2.
-
-The D1 flyback diodes used in the two topologies safeguard the MOSFET from
-voltage spikes caused by inductive loads such as servos.
-
-## A BJT alternative
-
-A Bipolar Junction Transistor (BJT) is a simpler, cheaper, and more widely
-available type of transistor that can be used as a switch.
-
-![BJT architecture](bjt.png)
-
-In the schematic, when the MCU outputs high, Q2 starts conducting. Q2 amplifies
-Q1's base current. Unlike MOSFETs, which are voltage-driven, BJTs are driven by
-base current. Resistors R3 and R4 must be chosen carefully to output the
-desired base currents. <a
-href="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/"
-class="external" target="_blank" rel="noopener noreferrer">"How to choose a
-transistor as a switch"</a> is an excellent guide on using BJTs as electronic 
-switches.
-
-## Which topology to choose?
-
-The professional community appears to prefer MOSFETs over BJTs. MOSFETs are
-more efficient when the switch is on. However, they are more challenging to
-drive, especially with a 3.3 V MCU, due to the V<sub>GS</sub> potentials
-required to achieve specified R<sub>DS(on)</sub> values (i.e., to turn them on
-fully).
-
-N-channel MOSFETs have lower on-resistance values, making them more efficient
-than P-channel ones. They are also cheaper. However, they are harder to drive
-on the high side as their gate potential must be higher than the source
-potential. This often requires extra circuitry such as MOSFET drivers.
-
-## Further reading
-
- - <a href="https://www.embeddedrelated.com/showarticle/98.php"
-   class="external" target="_blank" rel="noopener noreferrer">Different MOSFET
-   topologies</a>
- - <a href="https://www.embeddedrelated.com/showarticle/809.php"
-   class="external" target="_blank" rel="noopener noreferrer">How to read 
-   MOSFET datasheets</a>
- - <a src="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/"
-   class="external" target="_blank" rel="noopener noreferrer">How to use a 
-   transistor as a switch</a>
- - <a src="https://forum.digikey.com/t/guide-to-selecting-and-controlling-a-mosfet-for-3-3-vdc-logic-applications/42606"
-   class="external" target="_blank" rel="noopener noreferrer">Guide to 
-   selecting and controlling a MOSFET for 3.3 VDC logic applications</a>
- - <a src="https://forum.digikey.com/t/driving-a-large-relay-from-a-3-3-vdc-microcontroller-using-an-npn-darlington-transistor/41751"
-   class="external" target="_blank" rel="noopener noreferrer">Driving a large 
-   relay from a 3.3 VDC microcontroller using an NPN Darlington transistor</a>