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pcb-low-side-mosfet-switch/README.md

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Low side MOSFET switch

  • Fits a AO3400, 2301V or similar SOT23-3 N-MOS transistor
  • Lets you control a 5A load from a 3.3V logic pin
  • Designed at ELAB, bare PCBs and matching transistors can be found in the SMD drawers

Basic configuration

Basic configuration

  • Only the transistor needs to be populated, all other components are optional
  • This configuration is suitable for RESISTIVE loads for example:
    • LEDs with suitable resistors
    • Heating elements
    • Incandescent bulbs
  • small inductive loads should ok OK like a tiny < 0.5A DC motor

Notes and tips

  • This is a LOW side switch. OUT+ is always connected to VCC, only OUT- is switched. Because of this, this board is not recommended for switching modules that have more connections than just VCC and GND
  • At 5A the transistor will get hot. Ventilation might be necessary
  • Use wires thick enough for the current. AWG24 or better for 5A
  • Remember that max 5A 30V does not mean it's ok to do 30A 5V! But you knew that, right?

To use with inductive loads (DC motors, coils, speakers, etc)

  • Populate the FLYBACK DIODE
  • It protects the transistor from flyback voltage, caused by switching inductive loads
  • Recommended diode: SS34 in SOD-123 package
  • Note the direction of the diode - stripe towards VCC

Proportional "analog" control

  • To control the power of the load (instead of just ON/OFF operation), use PWM
  • The "analogWrite()" of an arduino produces a suitable PWM output
  • Do not use a potentiometer at the gate! The FET will not enjoy that at all

Other optional components

Full configuration

Gate pull-down resistor

  • Strongly recommended
  • Ensures safe operation when a controlling MCU is booting / sleeping / disconnected
  • Allows use with simple pushbutton as source of control signal
  • Can be anything between 1k and 100k, in 0603 package

LED and its resistor

  • LED shows status of control line - shines when switch is ON
  • Recommended resistor: 330 - 2700Ohms, depending on MCU voltage and LED
  • Both the resistor and LED are in 0603 package

Bypass capacitor

  • Only useful when intending to PWM the load at higher frequencies (>10kHz)
  • Reduces electromagnetic interference emitted by the input cables
  • Might somewhat improve performance of switched device
  • 1-100uF, must be rated for at least VIN * 1.5
  • Footprint for 0603 package, but an 0805 might fit as well.

Other options

  • For bi-directional motor control, use a motor driver or H-bridge, like this one
  • For digital modules, like an ADC or OLED module, use a HIGH-SIDE switch. A high side switch module based on a CH217 is coming to elab "soon".

Made at ELAB by Marek Baczynski. Find him on the ELAB slack if you have any questions