Long term effect of supplying less than starting Voltage?

blackworx · Post by **blackworx** » Mon May 12, 2008 3:29 am

I have a controller whose minimum output is 6V (it will not "switch off") and a fan whose starting Voltage is 9V.

My plan is to have the fan in question not running 99% of the time but use it as an extra intake when system temps get higher, e.g. during gaming.

Would it be safe in the long-term to feed 6V usually, knowing that the fan will not start at that Voltage, then start it when needed by having the controller go above 9V? Or should I put a 9V Zener in series?

The fan itself does not buzz when I give it 6V so there is no immediate noise issue.

VanWaGuy · Post by **VanWaGuy** » Mon May 12, 2008 7:27 am

That is an excellent question. I do not know about the motors specifically used in fans. I work on products that use small motors, and I have burned up small motors in just that way, and others are much more robust to that.

I will try and find out more on that if no one else answers this.

Post by **MikeC** » Mon May 12, 2008 8:50 am

Since the current is running through the fan coils, there has to be some degree of heat buildup. Whether this would be any higher than when the fan is spinning is hard to say. Perhaps when the fan is spinning, the air turbulence and flow provides cooling.

You might just check the temperature of the central hub while it's sitting still with 6V running through it.

How? Maybe just a finger touch test done periodically -- every minute or so for as long as you have the patience for... or some kind of data-logging temp monitoring device with a sensor stuck on the hub?

Felger Carbon · Post by **Felger Carbon** » Mon May 12, 2008 9:16 am

No-spin fan current draw is highly variable, depending on the fan. Fans that use the simplest, oldest, cheapest drive ICs have no overcurrent protection, and can burn up because the IC is tiny, the same shape factor as cheapo 3-lead plastic transistors (not power transistors).

In these ICs, the hub will never seem to overheat because if the teeny tiny fan control IC gets hot enough to heat the huge (by comparison) hub, the control IC will long since have melted down.

Newer fan ICs have no-spin detection, shutting down the current. The IC then periodically tries to re-start. All PWM fans work this way, and that feature has worked its way into some simpler fan controller ICs.

If your fan is an old design, you can bet it does not have this new feature. If it is a cheap fan, it will not use this new feature. So, most fans don't have the new feature... unless they are PWM fans.

Remember that when powered up but not spinning, most of the power dissipation is in the control IC and not in the resistance of the fan motor's copper wire. That's because the IC cannot provide unlimited current. If it could provide unlimited current, then the IC would dissipate no power (it would be a perfect switch), and the copper wire would dissipate all the electrical power input.

VanWaGuy · Post by **VanWaGuy** » Mon May 12, 2008 9:21 am

Thanks Felger,

I thought that the cheaper fans were just wired directly to a motor.

blackworx · Post by **blackworx** » Mon May 12, 2008 2:22 pm

Thanks everyone, especially Felger for such a concise answer.

My plan didn't work anyway. It was fine on paper, but the "controller" in question (my IP35-Pro) is too clever by half!

HAHA · Post by **HAHA** » Wed May 14, 2008 5:05 am

I burned a fan recently, presumably because of undervolting below start rpm. It was one of those 80 mm Revoltec led fans that wasn't very quiet. The fan made a high pitched whine when struggling with the too low startup voltage.

All is good because now I just get the nice light without any noise ....