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Carlos Garcia
Carlos Garcia
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Given the following circuit to sense voltage.

I am trying to calculate the value that the highlighted resistor should have (or to figure out if it is needed at all)

enter image description hereschematic

The MCP23018 is an IO port expander via I2C that I will connect to an ESP32. Since the 3.3V comes directly from the power supply, my intuition tells me that I should limit current before the MCP23018. In the datasheet it says that the input will be clamp at 20mA, so I used that to calculate my resistor:

*[Edited]
V = 3.3V - 0.6V (Darlington voltage drop) 
R = V / I
R = 32.3V7V / 20mA = 165Ω
135Ω

enter image description here

Do I need that resistor? If yes, Is my calculation correct?

Links
LDA210 datasheet: link
MC23018 datasheet: link
My full schematic: link

Given the following circuit to sense voltage.

I am trying to calculate the value that the highlighted resistor should have (or to figure out if it is needed at all)

enter image description here

The MCP23018 is an IO port expander via I2C that I will connect to an ESP32. Since the 3.3V comes directly from the power supply, my intuition tells me that I should limit current before the MCP23018. In the datasheet it says that the input will be clamp at 20mA, so I used that to calculate my resistor:

R = V / I
R = 3.3V / 20mA = 165Ω

enter image description here

Do I need that resistor? If yes, Is my calculation correct?

Links
LDA210 datasheet: link
MC23018 datasheet: link
My full schematic: link

Given the following circuit to sense voltage.

I am trying to calculate the value that the highlighted resistor should have (or to figure out if it is needed at all)

schematic

The MCP23018 is an IO port expander via I2C that I will connect to an ESP32. Since the 3.3V comes directly from the power supply, my intuition tells me that I should limit current before the MCP23018. In the datasheet it says that the input will be clamp at 20mA, so I used that to calculate my resistor:

*[Edited]
V = 3.3V - 0.6V (Darlington voltage drop) 
R = V / I
R = 2.7V / 20mA = 135Ω

enter image description here

Do I need that resistor? If yes, Is my calculation correct?

Links
LDA210 datasheet: link
MC23018 datasheet: link
My full schematic: link

Source Link
Carlos Garcia
Carlos Garcia
  • 1.3k
  • 1
  • 13
  • 34

What resistor value should I use to input 3.3V into MCP2301x

Given the following circuit to sense voltage.

I am trying to calculate the value that the highlighted resistor should have (or to figure out if it is needed at all)

enter image description here

The MCP23018 is an IO port expander via I2C that I will connect to an ESP32. Since the 3.3V comes directly from the power supply, my intuition tells me that I should limit current before the MCP23018. In the datasheet it says that the input will be clamp at 20mA, so I used that to calculate my resistor:

R = V / I
R = 3.3V / 20mA = 165Ω

enter image description here

Do I need that resistor? If yes, Is my calculation correct?

Links
LDA210 datasheet: link
MC23018 datasheet: link
My full schematic: link