How do I start a fan?

[Prune]

I want to run the fan at low voltage to minimize noise, but while the low voltage is enough to run the fan, it can't start it. What's the simplest circuit I can use to do that?

[Evan Shultz]

you might be able to bootstrap the fan to the power supply. that would let the cap supply a larger voltage until it discharges

[acenovelty]

A typical computer box fan runs at 12V dc and most will be in the under .25A range. That would make it possible to run it off a wall wart power supply. If the fan is too noisy at 12V, use a 5 or 6V supply. It will run slower but more quiet. In a computer box with noisy fans at 12V, I rewire the leads from the power supply. From the computer power supply solder the red wire to black on the fan, and the yellow from the power supply to the red on the fan. You now have 7V give or take a skooch.

Prosit

[Stocker]

The bootstrap from the power supply business gives me an idea.

You probably (hopefully) don't need the fan for the first few seconds after power up, yes?

How about this? :
*Power up.
*A starting capacitor is charged from the power supply, perhaps regulated to an appropriately lower voltage.
*When the voltage across the capacitor reaches the level required to kick start the fan, a comparator setup triggers a relay to cut power to the first power supply that charged the capacitor.
*Then a relay powers up the regular operating voltage for the fan.

If you got a break-before-make relay that had NO/NC contacts with the same lead inbetween them (not unusual) you could use it to select which power source went to the fan circuit. Maybe a dual-circuit relay would be easier!

If you can find a relay with a coil that can handle the voltage and NOT actuate with too low a voltage: Use one contact for the higher starting voltage and hook the coil to the fan/capacitor setup. When the capacitor is charged, the coil is charged as well. Then the contacts are switched. Be sure that when this happens, the capacitor discharges to kick the fan started. The fan now has the appropriate "running" voltage on it, and the setup is set to not work again until the next power-up sequence (latched).

Is that clear?

Will that work? Is it possible?

Brainstorm.

[Prune]

Well I was hoping I wouldn't need a relay. Otherwise I can just have a series resistor to a parallel capacitor, which charges the capacitor, but then the relay connects the fan, and the voltage will drop due to the resistor. But then I need a delayed driver for the relay.

[dangus]

If you had 12V available, isolate your normal fan supply from the fan with a diode (1N4005)(bar end towards the fan), then use an electrolytic (few 100 to few 1000 uF depending on what works) between the fan and the 12V supply, + end towards +12V. Capacitor will charge through the fan, giving the fan a kickstart. Once the cap is charged the fan will (hopefully) run off the lower voltage supply coming through the rectifier diode. It would be a good idea to have another diode across the fan (bar end towards fan positive) so that the electrolytic doesn't drive the fan negative when powering down.

(I need a virtual napkin to draw a sketch on)

[Prune]

dangus, what a great idea! I only had a single supply (about 8 V under load) and I used a resistor instead of a diode to drop more voltage. Connecting both the resistor and capacitor to the supply did the trick. Now the 12 V fan starts fine and runs quietly on as little as 3 V.

Now to figure out a way to raise RPM if this starts overheating. Too bad I can't find high resistance NTC thermistors locally, as the 500 Ohms I have is too low to put at the gate of a transistor driving the fan.

[acenovelty]

Fan controller:

http://sunbeamtech.com/

go to controllers/smart fan controller

To buy go to: http://store.pctoys.com/840556051541.html
Sale price: $11.95
Model: 840556051541 is blue led

Model: 840556051558 is red led with
# This latest 'high power' design is capable of controlling 3 to 6 fans.
# Power consumption of 2.0 Amps. (24 watts total fan power)
# The fan levels will adjust automatically to the system needs.
# Several fail safe features.
# 5(five) super bright LED
# Automatic warning alarm(buzzer and LED flash) on high temperature.
# Lighted enclosure in a clear case.

on both models

But then you don't get to invent anything.

Prosit

[dangus]

I'm glad the capacitor worked. As for thermistors... doesn't Radio Shack still sell those little blue thermistors? If size and looks aren't important, maybe there's some off-the-shelf thermostat you could use, like for home or car temperature control. On the other hand, there's gotta be a bunch of different fan controllers for sale at one of Vancouver's hole-in-the-wall computer stores.

[cpemma]

Yet another way of kick-starting that can have a fairly long startup without enormous cap size - assumes you have a 12V supply with some gizmo (eg, resistor, regulator, zener diode, etc) to reduce the fan's voltage to the desired running voltage.

By-pass the gizmo with a highish-gain PNP that will take the fan current (higher the gain, smaller the cap, so a power darlington is good), base connected resistor-cap-ground. With empty cap the PNP is hard on, when base current charges the cap up to near 11.4V (less for darlington) the transistor gradually shuts down and the gizmo takes over.




A bleed resistor R2 can be added if the cap isn't leaky enough, but needs a high value so the cap voltage can get high enough to turn the PNP off.

Or you can add a second transistor to drain the cap quicker.