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diff --git a/_log/mosfet-switches.md b/_log/mosfet-switches.md index 4fc1d31..bfe471a 100644 --- a/_log/mosfet-switches.md +++ b/_log/mosfet-switches.md @@ -4,9 +4,9 @@ date: 2025-06-22 layout: post --- -Needed low-power switching for the [fingerprint door -lock](../fpm-door-lock-lp/). Servo and FPM draw high quiescent current--had to -cut power electronically during sleep. MOSFETs can do this. +Needed low-power switching for [fingerprint door lock](../fpm-door-lock-lp/). +Servo and FPM draw high quiescent currents--had to cut power electronically +during sleep. MOSFETs can do this. Schematics belong to <a href="https://electronics.stackexchange.com/users/292884/simon-fitch" @@ -40,7 +40,7 @@ suitable through-hole MOSFET I could find.  -Less common but works if you have voltage high enough to drive the gate. Both +Less common but works if you have a voltage high enough to drive the gate. Both M1 and M2 are N-channel. MCU low → M2 off → M1 gate rises above threshold → servo on. MCU high → M2 on → M1 gate drops → servo off. R2 prevents high-impedance power-up from switching servo on. @@ -49,7 +49,7 @@ M2 needed in both topologies for level conversion (0V ↔ +6V or +9V). Carries <1mA. Gate-source threshold must be lower than MCU supply. Common choices: 2N7000, 2N7002, BSS138. -Note: D1 flyback diodes protect MOSFETs from voltage spikes caused by inductive +Note: D1 flyback diode protects MOSFET from voltage spikes caused by inductive loads (servos, relays). ## A BJT alternative @@ -58,14 +58,14 @@ loads (servos, relays). Simpler, cheaper, more available. Q2 conducts when MCU outputs high. Q2 amplifies Q1's base current. Unlike MOSFETs (voltage-driven), BJTs are -current-driven. R3 and R4 must be calculated for desired base currents. <a +current-driven. R3 and R4 must be calculated for desired base currents. <a href="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/" class="external" target="_blank" rel="noopener noreferrer">Guide on BJT switches</a>. ## Which topology? -MOSFETs preferred in professional work—more efficient when on. Harder to drive +MOSFETs preferred in professional work--more efficient when on. Harder to drive at 3.3V due to V<sub>GS</sub> requirements for full saturation (low R<sub>DS(on)</sub>). @@ -73,7 +73,7 @@ N-channel: Lower on-resistance, cheaper, more efficient than P-channel. Harder to drive high-side (gate must be above source—requires extra circuitry like MOSFET drivers). -Used P-channel high-side for the door lock redesign. Simpler to drive from 3.3V +Used P-channel high-side for door lock redesign. Simpler to drive from 3.3V MCU, no driver needed. ## Further reading |