# AC fan speed control by periodic on/off switching

Discussion in 'Electronic Design' started by ectoplasm, May 3, 2007.

1. ### John FieldsGuest

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No, you should use a fuse which blows (or a circuit breaker which
some amount of time.

ISTR that your fan dissipates 60 watts, so the current it needs from
the mains will be:

P 60W
I = --- = ------ = 0.5 ampere
E 120V

However, the inrush current could be up to 10 times that high,
gradually decreasing to the running current as the motor comes up to
speed. Which could be a problem if you don't let the motor run/rest
long enough per ON-OFF cycle to get rid of the extra heat generated
during startup.

In any case, if that turns out not to be a problem, I suggest you
use a slow-blow fuse:

http://www.littelfuse.com/data/en/Data_Sheets/313_315.pdf

Take a look at the chart on the bottom right of the page and you can
see that a 2 amp fuse will carry 5 amps for 7 seconds before it
blows, but then it also has to cool down, so you may need an
electronic solution.

Before you get into that though, I suggest that you run the thing
with various duty cycles for long enough to determine whether the
motor will overheat.

2. ### ectoplasmGuest

The result: (direct link to 127kB jpeg picture)
<http://img512.imageshack.us/img512/5526/img1236spw3.jpg>

I made it using the double 555 solution on
<http://casemods.pointofnoreturn.org/pwm/circuit2.html>

And the MOC3041 triac driver to switch on zero crossings, controlling
a BT138X triac (12A, so it could be used for a number of purposes).
The red LED is on when the triac is on. Both the period and pulse
width can be independently controlled by the potentiometers from about
0.25 to 4 seconds (pulse width should not exceed period of course).
For any purpose, there's a switch that allows switching to a larger
time base of about 3 to 100 seconds (pulse width up to 45 seconds).
The red switch is to bypass the thing so the appliance is turned on as
normal.

Anyway, it does exactly what I needed it to do, and it might also be
useful in other applications.