Question Do PWM fan hubs exist that will convert 12v to 5v ?

[HairyOnion]

I bought some 5v Noctua NF-A20 PWM fans. I didn't know about motherboard fan headers being 12v so I shouldn't use them. I want to be able to vary the fan speed. I want to be able to connect a PWM fan hub to the 12v PWM headers on the motherboard. So for this to work the fan hub would need to be able to convert the 12v to 5v. Does such a device exist ?

If not , could I use a Buck Converter to convert the 12 v to 5v and use that on the 5v and ground wires from the 12v motherboard fan header, whilst still contecting the sense and PWM wires ?

I have 4 of these NF-A20 fans that I will be using on an external radiator, the main aim is to have a quiet system so I must be able to run the fans at their lowest rpm.

Please help!!

 

[Lutfij]

Welcome to the forums, newcomer!

I want to be able to connect a PWM fan hub to the 12v PWM headers on the motherboard. So for this to work the fan hub would need to be able to convert the 12v to 5v. Does such a device exist ?
https://noctua.at/en/na-fh1

 

[HairyOnion]

Funny thing is I bought that, I should have mentioned. It only lets me run the fans at 800 rpm

 

[HairyOnion]

update:

The fan hub passes 12v to the fans I tested with a multimeter.

The fan hub doesnt pass the pwm signal correctly. This is how I know...

The fans come with these usb adapters so I used one of them to power the hub. I then decided to connect the sense and pwm pins from a 12v motherboard header to the hub whilst its being powered via the usb connector which delivers 5v .

Changing the speed via the software called "fan controller" from 0 to 100% does not increase the fan speed when all the wires are connected to the hub. 0-10% report 311rpm. anything greater than 10% reports 0 rpm although the fans carry on spinning.

If I connect all the wires directly to the fan I have full control and the rpm is reported correctly from 0 to 100%

I will send it back for a replacement. I now cross my fingers and hope that I can power all 4 fans at 100% from the usb connector. Maybe the new fan hub will convert to 5v(I doubt it though)

 

[Zerk2012]

Funny thing is I bought that, I should have mentioned. It only lets me run the fans at 800 rpm

https://noctua.at/en/nf-a20-pwm/specification

From that yes 800RPM.

 

[dwd999]

Can you add the low noise adapters that Noctua provides with the fans to your circuit to get a lower speed?

 

[HairyOnion]

Can you add the low noise adapters that Noctua provides with the fans to your circuit to get a lower speed?

I think so, but i want to set a curve so the rpm increases and descreases , I think the low noise adapters are resistors that reduce the the rpm by a set amount which would probably prevent them hitting full speed if that was needed.

 

[Paperdoc]

That Noctua Hub does NOT convert power down to 5 VDC for your fans. If I understand its description correctly, it is a normal 4-pin PWM fan HUB for use with fans requiring 12 VDC power, so that is what the fans will get. NOT suited to your use.

BUT is also has another feature of its design. IF you do NOT connect it to a SATA power supply from the PSU then it gets only what comes into it from a mobo header via its signal input cable. So IF your mobo header is DESIGNED for 5VDC fans AND PWM speed control, then the unit acts like a SPLITTER (that is, it takes NO power from the PSU and relies on the power from the header). So IF your special mobo header supplies only 5 VDC from Pin #2, plus the PWM signal on Pin #4, this unit CAN be used with your fans. However, IF your mobo had such an odd header, you would not need this unit!

You can NOT power your 5VDC fans from a USB connection. A typical 12 VDC fan consumes up to 0.15A current, or 1.8 W. For a 5VDC fan of similar airflow capacity, 1.8 W would require 0.36 A current PER FAN. A standard USB2 port can supply up to 0.5 A; a USB3 port can supply up to 0.9 A. So youd need to dedicate one USB port to EACH of your four fans. THEN you get fans at full speed but NO speed control.

I do NOT recommend for your situation the Low Noise Adapters supplied by Noctua. They are just series resistors in the power supply line to the fan. Each is designed for a Noctua fan requiring 12 VDC and a max current of about 0.1 A, and with a resistance value just right to reduce the VOLTAGE at the fan from 12 VDC to NOT less than 5 VDC so it will run slow but not stall. Recognize here that the Voltage reduction the adapter achieves depends entirely on the CURRENT flowing through the circuit and that depends on the motor "resistance". The adapter is NOT a Voltage Regulator.

For the following assume you use the adapter with LESS speed reduction, so the Voltage to the fan is maybe 9 VDC. Now the FAN motor very roughly acts like a resistance of 120 Ohms (12 / 0.10). So suppose the adapter for will cut fan voltage to 9 VDC from 12 when in series with the motor. It would need a resistance of about 40 Ohms to do that. NOW assume you use that with your 5 VDC motor that has a different "resistance" of about 5 / 0.24 = 21 Ohms. (That is calculated by assuming the motor WATTS is also 1.2 W like the 12 V motor to deliver similar air flow.) Put that 21 Ohms in series with a 40 Ohm resistor from the 12 VDC line of the fan header, and you find that the MOTOR will receive about 4.1 Volts, and would run but not at top speed. HOWEVER, all that makes a LOT of "ASSUMED" and "GUESS" numbers. We do NOT know what really IS in the adapters and the real motor characteristics. Without that you take a HUGE gamble!

NOTE that doing all that is for having only ONE fan on that adapter. So you would need FOUR such adapters for your four fans. FURTHER, you still have the issue of getting speed control for them via the PWM signal.

Technically, the much better way to do this is to replace those fans with 12 VDC models. If you cannot, there's a lot of work to do to adapt to using them.

 

[HairyOnion]

That Noctua Hub does NOT convert power down to 5 VDC for your fans. If I understand its description correctly, it is a normal 4-pin PWM fan HUB for use with fans requiring 12 VDC power, so that is what the fans will get. NOT suited to your use.

BUT is also has another feature of its design. IF you do NOT connect it to a SATA power supply from the PSU then it gets only what comes into it from a mobo header via its signal input cable. So IF your mobo header is DESIGNED for 5VDC fans AND PWM speed control, then the unit acts like a SPLITTER (that is, it takes NO power from the PSU and relies on the power from the header). So IF your special mobo header supplies only 5 VDC from Pin #2, plus the PWM signal on Pin #4, this unit CAN be used with your fans. However, IF your mobo had such an odd header, you would not need this unit!

You can NOT power your 5VDC fans from a USB connection. A typical 12 VDC fan consumes up to 0.15A current, or 1.8 W. For a 5VDC fan of similar airflow capacity, 1.8 W would require 0.36 A current PER FAN. A standard USB2 port can supply up to 0.5 A; a USB3 port can supply up to 0.9 A. So youd need to dedicate one USB port to EACH of your four fans. THEN you get fans at full speed but NO speed control.

I do NOT recommend for your situation the Low Noise Adapters supplied by Noctua. They are just series resistors in the power supply line to the fan. Each is designed for a Noctua fan requiring 12 VDC and a max current of about 0.1 A, and with a resistance value just right to reduce the VOLTAGE at the fan from 12 VDC to NOT less than 5 VDC so it will run slow but not stall. Recognize here that the Voltage reduction the adapter achieves depends entirely on the CURRENT flowing through the circuit and that depends on the motor "resistance". The adapter is NOT a Voltage Regulator.

For the following assume you use the adapter with LESS speed reduction, so the Voltage to the fan is maybe 9 VDC. Now the FAN motor very roughly acts like a resistance of 120 Ohms (12 / 0.10). So suppose the adapter for will cut fan voltage to 9 VDC from 12 when in series with the motor. It would need a resistance of about 40 Ohms to do that. NOW assume you use that with your 5 VDC motor that has a different "resistance" of about 5 / 0.24 = 21 Ohms. (That is calculated by assuming the motor WATTS is also 1.2 W like the 12 V motor to deliver similar air flow.) Put that 21 Ohms in series with a 40 Ohm resistor from the 12 VDC line of the fan header, and you find that the MOTOR will receive about 4.1 Volts, and would run but not at top speed. HOWEVER, all that makes a LOT of "ASSUMED" and "GUESS" numbers. We do NOT know what really IS in the adapters and the real motor characteristics. Without that you take a HUGE gamble!

NOTE that doing all that is for having only ONE fan on that adapter. So you would need FOUR such adapters for your four fans. FURTHER, you still have the issue of getting speed control for them via the PWM signal.

Technically, the much better way to do this is to replace those fans with 12 VDC models. If you cannot, there's a lot of work to do to adapt to using them.

Thankyou Paperdoc for the detailed explanation, my mother board does have a 5v d.led header that can supply 2 amps so I could mix use the 2 pins from there and 2 pins from one of the 12v pwm headers, but it is stiill a terrible solution. I have decided to send the fans back and buy the 12v AF-A20 PWM fans.

Buying the 5v was a mistake on my part, I didnt at the time understand the implecations of a 5v fan in the set up. I guess for some they are useful but for me 12v pwm is the way to go. Thanks again and hopefully some1 else will stumble on this thread in the future and save themselves the trouble.