How to Undervolt a Fan Inside a Small Device?

[kony]

Placing a cap in parallel with the resistor seems to cure startup
problems.

"Cure"?
To what extent?
I mean, does it actually allow a lower working RPM then or
just offset the overly high resistance?

This is an interesting idea, do you have an example of what
gain there will be for any particular fan?

 

[CBFalconer]

"Cure"?
To what extent?
I mean, does it actually allow a lower working RPM then or
just offset the overly high resistance?

This is an interesting idea, do you have an example of what
gain there will be for any particular fan?

This is probably just plain dangerous to the rest of your system.
It would depend on "giving a kick" at turn-on, provided that the
power supply had a rapid rise time. If the fan ever gets stalled
in normal operation the kick is not there, and the thing just sits
and encourages the rest of the system to cook.

 

[kony]

This is probably just plain dangerous to the rest of your system.
It would depend on "giving a kick" at turn-on, provided that the
power supply had a rapid rise time. If the fan ever gets stalled
in normal operation the kick is not there, and the thing just sits
and encourages the rest of the system to cook.

What I'm suspecting is that if a cap were in parallel, a
bypass for the power to the fan, it would just make matters
worse because whatever the turn-on delay was already, adding
more capacitance will only lengthen that delay.

Ric wrote about it being parallel with the resistor though,
so i presume actually paralleled to it, bypassing it on the
same power lead. While that can be a crude but effective
filter for some noise reduction, I'd not heard of anyone
employing anything like this for fans.

Sometimes it's not even, merely a matter of getting the fan
to spin-up though, with some fans there will be pulsation
below a certain RPM, actually making the fan louder than if
it were running slightly faster-enough to eliminate this
pulsation. Offhand I'd speculate that it pulsating occurs
due to stronger (permanent) magnetic field in some fans than
others. IE- It seems more common in >= 120 x 32mm, large
thick fans.

 

[CBFalconer]

What I'm suspecting is that if a cap were in parallel, a
bypass for the power to the fan, it would just make matters
worse because whatever the turn-on delay was already, adding
more capacitance will only lengthen that delay.

No suspicion needed. It forms a simple high pass filter. Very
straight forward. Once the cap is charged it has no further
effect. The charging current, as the power comes on, supplies the
initial start-up boost.

 

[ric]

"Cure"?
To what extent?
I mean, does it actually allow a lower working RPM then or
just offset the overly high resistance?

For fans that will spin at, for example, 5 volts but have startup
problems, the cap acts as a short at startup, allowing the full
12v to reach the fan, then charging to the resistor's dropping voltage.
I normally use a 10uF, 15v electrolytic. [Get the polarity right!]

This is an interesting idea, do you have an example of what
gain there will be for any particular fan?

Not off hand. No gain in operational function. But it sure cures the
"I have to help my fan start" blues.