AC fan speed control by periodic on/off switching

--- No, you should use a fuse which blows (or a circuit breaker which breaks) if the steady-state current into your load is exceeded for 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:

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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.

-- JF

The result: (direct link to 127kB jpeg picture)

I made it using the double 555 solution on

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.