Re: Fungus Gnats and CO2

Wim Osterholt (
Sun, 5 Mar 1995 00:06:21 +0100 (MET)


> > He used a little 1" computer fan slowed down to a very slow
> > speed with resistors to slowly diffuse the CO2 amongst his plants.
> > It worked amazingly well and was completely safe.
> >
> This bit caught my eye. One word of warning for people who may be
> inspired to do the same. Be careful as to the sort of fan you buy. A
> lot of the modern fans are "brushless" DC motors which do not respond
> to lowering the voltage (this is what the resisitors are doing -
> lowering the voltage to the fan) in any useful manner.

This is not quite true. These resistors aren't meant to reduce the voltage
on the brushless DC motor. They're meant to reduce the _current_ through
the motor. Anyway, the result is that the motor will get less energy and
will turn less powerfull (slower) until the internal and external friction
match the energy level you provide.
Maybe the result isn't that stable as you were expecting on the long run.

> You need to find a fan that is NOT using a brushless DC motor.

Incorrect. (Please also consider the wearing out of the brushes in your toy
fan motor. It's often copper on copper.)

> Also need I say the selection of the resistor values is empirical at best

Indeed, you must do some playing :-)

Regards, Wim.

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