7volt fan speed reducer cable getting quite warm

[Phil2008]

I bought a areocool dead silent 40mm fan (1500rpm USA version) and decided to use the 7volt reducer cable that was included, as the motor of the fan and was quite loud and annoying.. After about 2mins of using the volt reducer cable, the cable was very warm to hold at 1 end.

So I thought I try another fan reducer cable as I have 3 of these "dead silent" fans in my case, with the other 2 running at lower speeds because they are the uk versions.. So I swapped the reducer for another one and the same thing happened again, but the cable may not be quite so warm.

Anyway I have left it connected, its been running for about 3hrs now and no smoke or flames so far.. So guys is it normal for volt reducer cables to get quite warm?

 

[Skynet5]

I would say so. If it is resistor based it has to dissipate the power somehow, usually through heat...

There are other ways but I don't think some are good for the PSU.


Personally I've always gone with PWM and a controller.

When you say hot...can you keep your fingers on it constantly? Or does it get too hot to handle?

 

[Rroff]

They do get warm - you are probably dissipating 1-2 watt as heat across that resistor.

 

[Skynet5]

Have a read of this for some molex magic.

http://www.overclockers.com/forums/showthread.php/372297-Guide-to-the-7v-Fan-Mod

 

[Phil2008]

When you say hot...can you keep your fingers on it constantly? Or does it get too hot to handle?

It doesnt get too hot to handle, but its on the hot side rather then warm, if you get me.

Have a read of this for some molex magic.

http://www.overclockers.com/forums/showthread.php/372297-Guide-to-the-7v-Fan-Mod

That just goes flying over my head... I like keeping things simple, plus I have to get others todo my computer stuff because my hands are not that steady.

 

[Skynet5]

I think it'll be fine.

 

[t31os]

I'd be surprised if you can't just manage the speed via the bios cooling profile, standard 3 pin fans are generally voltage controlled by the motherboard, else they'd be maxed out(12v) all the time. That's where PWM works better, more granular control over the speed instead of simply alternating between 3 different voltages(5/7/12v).

 

[Skynet5]

I'd be surprised if you can't just manage the speed via the bios cooling profile, standard 3 pin fans are generally voltage controlled by the motherboard, else they'd be maxed out(12v) all the time. That's where PWM works better, more granular control over the speed instead of simply alternating between 3 different voltages(5/7/12v).

There's always one person stating the obvious stuff. Lol.

 

[Habakkuk]

When designing PCBs a 10-degree increase for traces is acceptable.

You can go up to a 30-degree increase depending on application.

Cables will get warmer with the current load - in this case, it's the resistor dropping current.

So long as it doesn't get melty, it's not only fine, it's behaving as it's designed to.

 

[BongoHunter]

Voltage reducers like the ones you are using get warm by design - if not comfortable with this see if you have BIOS options for reducing voltage to the connector

 

[Phil2008]

Well guys, my pc has been on 3d printing now for 24hr and I am stll alive with no burning smells. So I would say im ok, wouldnt you?

Yeah the ideal way to sort the prob out would be just to connect them all to the motherboard, but the less cables I can see the better and its taken me and my mum hrs to hide as meny cables as possible, leaving the main tidy ones showing.

These pics were taken before I changed the cpu fan to the "dead silent" fan, so now all my fans are the same. I cant see whats so silent about them though, but they look the part.

 

[PaulCa]

A resistor on it's own does not drop voltage. If you connect 12V to one end of a resistor and measure it at the other you will still have 12V.

However all resistance in a circuit will consume all voltage potential in that circuit. This is Kirchhoffs voltage law.

It is likely the 7V reducer cables, if they are indeed resistors, are making assumptions about the resistance provided by the fan, so the resistor and the fan combine to create a voltage divider. Hopefully 5V drops across the resistor leaving 7V for the fan. This means the fan must have a resistance equivalent to 7/5th of the resistor. A fan that consumes 100mA at 12V has a resistance of 120 Ohms. So the resistor required would need to be 85 Ohms. 5V through 85 Ohms results in 5*5/85 = 294mW of heat. Of course the fan might not have a resistance of exactly 120Ohms when running at 7V!

However, if your fan actually has a resistance of 50 Ohms or a resistance of 300 Ohms then you will not get 7V to it.

~300mW of heat is likely to make the wire quite warm to touch, but probably not dangerously so.

 

[Rroff]

I've not measured what the actual draw is but most PC 12V fans are rated for around 250-300mA at full speed.

EDIT: Most of the maths still comes back to around 280mW of heat got a bit confused with the power rating above.

 

[Habakkuk]

Tried running Octoprint on a raspberry pi?

Much lower power consumption!

my pc has been on 3d printing now for 24hr

 

[EsaT]

That's where PWM works better, more granular control over the speed instead of simply alternating between 3 different voltages(5/7/12v).

So that's the latest lie what PWM marketroids say?
Voltage control doesn't alternate between different rails.
It adjusts voltage linearly!

In fact it's PWM which is more ambiguous if you want simple accurate speed control.
There's no guaranteed fixed relation between PWM duty cycle and at what percentage from full speed fan runs with different makers having differences in it.

 

[EsaT]

but the less cables I can see the better and its taken me and my mum hrs to hide as meny cables as possible, leaving the main tidy ones showing.

These pics were taken before I changed the cpu fan to the "dead silent" fan, so now all my fans are the same. I cant see whats so silent about them though, but they look the part.

With the amount of time wasted for marketing those fans are likely one of the worser for airflow per noise...

If you don't want cables to visible side maybe that case could take something like Aquaero LT behind motherboard?

I've not measured what the actual draw is but most PC 12V fans are rated for around 250-300mA at full speed.

There's lots of variation in current depending on friction of bearing and fan speed.
For example Gentle Typhoons had very low current:
http://www.scythe-eu.com/en/products/fans/gentle-typhoon-120-mm.html

Though some might use also momentary spin up draw while others use that average current once fan is running at steady speed.

 

[PaulCa]

It adjusts voltage linearly!

In fact it's PWM which is more ambiguous if you want simple accurate speed control.
There's no guaranteed fixed relation between PWM duty cycle and at what percentage from full speed fan runs with different makers having differences in it.

The trouble is 12V brushless fans are not meant to be controlled with varying voltage. They are meant to be controlled by altering the timing of the pulses.

Of course I believe most PC brushless fan motors are fixed pulse per RPM based on hall effect sensors.

PWM wins because dropping voltage at 100-300mA of current produces lots of heat as we discussed. Even using a variable voltage regulator rather than a resistor produces significant, although less, heat. PWM simply pulses the voltage on and off (though the PWM signal from the fan controller is only a signal for the fan to switch the actual voltage in and out).

PWM is no marketing hype it is the correct way to control current to a device, especially one designed for a fixed voltage.

 

[LuckyBenski]

The trouble is 12V brushless fans are not meant to be controlled with varying voltage. They are meant to be controlled by altering the timing of the pulses.

Of course I believe most PC brushless fan motors are fixed pulse per RPM based on hall effect sensors.

PWM wins because dropping voltage at 100-300mA of current produces lots of heat as we discussed. Even using a variable voltage regulator rather than a resistor produces significant, although less, heat. PWM simply pulses the voltage on and off (though the PWM signal from the fan controller is only a signal for the fan to switch the actual voltage in and out).

PWM is no marketing hype it is the correct way to control current to a device, especially one designed for a fixed voltage.

You're right about all this factually, but it's worth putting it into context. I don't think anything in a PC uses direct PWM voltage. If you have a regulated voltage output on a fan controller, there will be an RC filter so that the fan sees a DC power source right? So either way, motherboard control is using PWM and only inline resistors are wasting energy/producing heat.

I'd be surprised if you can't just manage the speed via the bios cooling profile, standard 3 pin fans are generally voltage controlled by the motherboard, else they'd be maxed out(12v) all the time. That's where PWM works better, more granular control over the speed instead of simply alternating between 3 different voltages(5/7/12v).

Nothing in a standard PC switches a fan between 5, 7 and 12 volts. DC control supplies a continuously variable DC voltage. However I haven't found "standard 3 pin fans are generally voltage controlled by the motherboard" to be true at all. One motherboard out of 5 I've used in the last couple years has DC 3-pin control.

 

[PaulCa]

If you have a regulated voltage output on a fan controller, there will be an RC filter so that the fan sees a DC power source right?

Not necessarily. Remember the fans are brushless anyway so they are not actually DC they are multi phase pulse AC. The hall effect sensors determine the locate of the motor and pulse current to the correct magnet coil to pull or push the fan one more bit of rotation. (This is why they "cog" at lower RPMs were they simply twitch back and forth instead of spinning).

When you provide the fan with a regulated 12V and a PWM signal, it could simply use a logical AND via a transistor or mosfet to only provide current when both the PWM is high and the particular sensor says a coil should be on. No capacitors needed. In fact a capacitor would cause the brushless mechanism to have a delay softening the rise and fall times on the pulses which might not be preferential.

 

[Rroff]

Nothing in a standard PC switches a fan between 5, 7 and 12 volts. DC control supplies a continuously variable DC voltage. However I haven't found "standard 3 pin fans are generally voltage controlled by the motherboard" to be true at all. One motherboard out of 5 I've used in the last couple years has DC 3-pin control.

I've seen quite a few that "have" voltage control for the 3 pin headers in the BIOS... sadly I've only found about 2 boards in the dozens that have it where it actually seems to work properly if at all and a few boards where it is very glitchy when it does work.

so that the fan sees a DC power source right?

Pretty sure stuff like the BeQuiet Silent Wings PWM fans operate fine from a pulsed input.