Hi
I'm setting up a minecraft server PC. It's an AMD FX 8 core 4GHz CPU but the motherboard is somewhat basic, I only have 2 fan headers, one for the CPU and one for the system fan. I bought a push/pull type cooler from eBay but the problem is that the 2 fans on the cooler are wired together to a single 3 pin plug and as a result they always just run at top speed as there's no PWM. I want to get 2 PWM fans to replace the ones the cooler came with but I'm worried about the current draw from the single header (although I am reasonably sure it is OK up to 4 fans) and if the PWM signal will work if it's shared between two fans. My mental concept is as follows:
Fan1-12V -> Fan2-12V -> PSU Molex-12V
Fan1-Gnd -> Fan2-Gnd -> PSU Molex-Gnd
Fan1-Spd -> Motherboard Header-Spd
Fan2-Spd -> No Connection
Fan1-PWM -> Fan2-PWM -> Motherboard Header-PWM
Connecting the fans to the PSU should eliminate any worry about current draw from the header. I was concerned that wiring the speed together for both fans would confuse the motherboard so just wiring one should be okay, they are the same type of fan so their speed should be similar for a given duty cycle anyway right? Of course I won't know if the undetected fan fails, but I can live with that. My main worry is if the PWM will be okay for both fans, my intuition is that it will be, I'm assuming the fans use a pulled down p-type/pnp setup for this and so should be a high resistance on the base/gate of the driver? But I'd just like to confirm all this with folk more in the know about this type of thing. I appreciate any help you can give.
Cheers
Marc
EDIT: Done a bit more research and I think I can make do with the existing fans. Looks like I was wrong about the p-type setup, they seem to use an n-type. That to me seems a bad idea, switching the gnd instead of switching the +ve is sketchy. I could, i expect use a mosfet and a resistor to turn both fans into PWM capable ones. Although I'd probably want to invert the motherboard signal with a BJT and use a p-channel fet so the fan is constantly grounded. Although that would be dicey at 25kHz, I'd need a pretty low value rail pull resistor on the inverter tranny. Hmmmm.
EDIT2: Wait, from the diagram I'm looking at, the n-fet is pulled up, so the fan is on full if the PWM is not connected I assume. So the motherboard signal would already be inverted right? But that is at 3.3V which is not enough to turn off a p-fet so I'd need a driver as well. Then I'm back to the low value resistor because of the 25kHz. I'm actually curious at to what n-fet these fans use that can be turned on at 3.3V? Even signal level fets are dicey at that voltage.
I'm setting up a minecraft server PC. It's an AMD FX 8 core 4GHz CPU but the motherboard is somewhat basic, I only have 2 fan headers, one for the CPU and one for the system fan. I bought a push/pull type cooler from eBay but the problem is that the 2 fans on the cooler are wired together to a single 3 pin plug and as a result they always just run at top speed as there's no PWM. I want to get 2 PWM fans to replace the ones the cooler came with but I'm worried about the current draw from the single header (although I am reasonably sure it is OK up to 4 fans) and if the PWM signal will work if it's shared between two fans. My mental concept is as follows:
Fan1-12V -> Fan2-12V -> PSU Molex-12V
Fan1-Gnd -> Fan2-Gnd -> PSU Molex-Gnd
Fan1-Spd -> Motherboard Header-Spd
Fan2-Spd -> No Connection
Fan1-PWM -> Fan2-PWM -> Motherboard Header-PWM
Connecting the fans to the PSU should eliminate any worry about current draw from the header. I was concerned that wiring the speed together for both fans would confuse the motherboard so just wiring one should be okay, they are the same type of fan so their speed should be similar for a given duty cycle anyway right? Of course I won't know if the undetected fan fails, but I can live with that. My main worry is if the PWM will be okay for both fans, my intuition is that it will be, I'm assuming the fans use a pulled down p-type/pnp setup for this and so should be a high resistance on the base/gate of the driver? But I'd just like to confirm all this with folk more in the know about this type of thing. I appreciate any help you can give.
Cheers
Marc
EDIT: Done a bit more research and I think I can make do with the existing fans. Looks like I was wrong about the p-type setup, they seem to use an n-type. That to me seems a bad idea, switching the gnd instead of switching the +ve is sketchy. I could, i expect use a mosfet and a resistor to turn both fans into PWM capable ones. Although I'd probably want to invert the motherboard signal with a BJT and use a p-channel fet so the fan is constantly grounded. Although that would be dicey at 25kHz, I'd need a pretty low value rail pull resistor on the inverter tranny. Hmmmm.
EDIT2: Wait, from the diagram I'm looking at, the n-fet is pulled up, so the fan is on full if the PWM is not connected I assume. So the motherboard signal would already be inverted right? But that is at 3.3V which is not enough to turn off a p-fet so I'd need a driver as well. Then I'm back to the low value resistor because of the 25kHz. I'm actually curious at to what n-fet these fans use that can be turned on at 3.3V? Even signal level fets are dicey at that voltage.
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