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Understanding this sensing approach

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Red_Micro:
In the circuit attached, what could be the intent of the designer who made this? As far as I understand, if Vcc = 35V, when MOSFET is ON, actuator_sense pin goes to ~GND?. When the MOSFET is OFF, the resistance L1 and R25 are in parallel. So by resistor divider formula, we get a voltage across R27, or actuator_sense pin. I think the designer only wants to know if L1 is present or not? Am I right?

floobydust:
The circuit is for fault detection, it can detect an open L1 or mosfet on or mosfet off, by looking at three permutations for voltage to the A/D.

Red_Micro:

--- Quote from: floobydust on April 26, 2020, 04:30:43 am ---The circuit is for fault detection, it can detect an open L1 or mosfet on or mosfet off, by looking at three permutations for voltage to the A/D.

--- End quote ---

OK. I want to understand the behavior.

When MOSFET is OFF, and L1 is ok, the voltage at the sense pin is determined by L1//R25 + R26 + R27.
When MOSFET is OFF, and L1 is open, sense pin = R25 + R26 + R27.
When MOSFET is ON, sense pin = ~0V.

Is this analysis OK?

Rerouter:
L1 Good, Q1 Off = VCC * R27 / (R26+R27) (L1 is assumed to be a very low resistance)
L1 Good, Q1 On = 0V
L1 Bad, Q1 Off = VCC * R27 / (R25+R26+R27)

so for a 35V VCC

Case 1 = 1.95V
Case 2 = 0V
Case 3 = 1V

You can technically also see if Q1 has failed open or short,
The capacitor C16 would make it difficult, but technically you can also see if D3 has failed, as on switching off the transistor the voltage seen by R26 would exceed 35.7V, meaning you would measure more than 1.95V for some time

Edit: I should note, the D3 would be failing open, if it fails short, well your hopefully going to blow a fuse,

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