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7/12/2025, 11:39:35 AM
>>2930805
>so am I correct in understanding with your design GND is unified across both power sources and since logic is handled with a PFET the reference to GND isn't impeded so TACH and PWM won't float up?
Yes. Simulate in Falstad or LTspice if you're curious.
Solid state relays generally use an optocoupler of some sort for extra isolation. You could use a solid-state relay here as it would simplify things a bit, or you could use an optocoupler, but either way to get those TACH and PWM signals you'll need to connect the GNDs together.
One thing to consider is what will happen if the computer is turned off but the external PSU is turned on, or vice-versa. It's possible that the PWM signal could damage the fan if the fan isn't getting 12V, so it might be wise to add another MOSFET to that wire, picrel. The fan shouldn't get 12V while the PC is off because of the MOSFET I guess.
>so am I correct in understanding with your design GND is unified across both power sources and since logic is handled with a PFET the reference to GND isn't impeded so TACH and PWM won't float up?
Yes. Simulate in Falstad or LTspice if you're curious.
Solid state relays generally use an optocoupler of some sort for extra isolation. You could use a solid-state relay here as it would simplify things a bit, or you could use an optocoupler, but either way to get those TACH and PWM signals you'll need to connect the GNDs together.
One thing to consider is what will happen if the computer is turned off but the external PSU is turned on, or vice-versa. It's possible that the PWM signal could damage the fan if the fan isn't getting 12V, so it might be wise to add another MOSFET to that wire, picrel. The fan shouldn't get 12V while the PC is off because of the MOSFET I guess.
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