From 5fc89ef84591c6c237596e4a1839fd3ab7bd19c7 Mon Sep 17 00:00:00 2001 From: youen Date: Sat, 22 Feb 2025 23:17:11 +0100 Subject: [PATCH] Minor README.md improvements --- README.md | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/README.md b/README.md index 8a79082..8fa8060 100644 --- a/README.md +++ b/README.md @@ -12,13 +12,13 @@ Files on this repository are meant to be used with [KiCad](https://www.kicad.org The voltage divider made by `R1` and `R2` act as a 3V pull-up for the PWM signal. This is because PWM output is open-collector. -When the PWM signal is high (open), capacitor C1 is charged up to 3V through diode `D1`. When the PWM signal is low (connected to ground), the capacitor `C1` remains charged because `D1` blocks discharge. It will discharge itself over time, but slowly enough that for a 25kHz PWM signal it will remain permanently at almost 3V. However, if the PWM signal remains constantly low (when the motherboard wants to shut the fan down), C1 will eventually discharge. +When the PWM signal is high (open), capacitor C1 is charged through diode `D1` (so a little less than 3V due to `D1` voltage drop). When the PWM signal is low (connected to ground), the capacitor `C1` remains charged because `D1` blocks discharge. It will discharge itself over time, but slowly enough that for a 25kHz PWM signal it will remain permanently at almost fully charged. However, if the PWM signal remains constantly low (when the motherboard wants to shut the fan down), C1 will eventually discharge close to 0V. The 555 timer `U1` is configured as a Schmitt trigger (voltage hysteresis). The resistor `R3` connected on CV configures the internal voltage thresholds of U1 to be 0.8V and 1.6V. When the voltage of `C1` goes above 1.6V, the output Q of `U1` goes low. When `C1` goes below 0.8V, Q goes high. When it's in between, Q keeps the same state. The output Q of `U1` is connected to the P-channel MOSFET `Q1`, so that when Q is low, the MOSFET is conductive, and when Q is high the MOSFET is blocked. -The MOSFET controls the fan through resistor `R4` which limits the fan speed. `R4` could be shorted for max speed, or replaced by another value if you need another fan speed. +When conductive, `Q1` powers the fan through resistor `R4` which limits the fan speed. `R4` could be shorted for max speed, or replaced by another value if you need another fan speed. The diode `D2` protects the MOSFET when the fan stops (since it's an inductive load and I don't know if it already has a free-wheel diode).