No, it takes them a while to get up to ignition temperature. They smoke quite a lot before that actually fail. Also, they tend to either run near full speed or drop to a very slow speed. They just won't go a about 1/3rd normal speed.
Successfully, yes.
For a fan that isn't really the case. The loading rises smoothly with increasing RPM. At any give voltage it will settle to a speed.
I wonder about putting a capacitor in series. This can be a dangerous idea, however. You don't want to make a tuned circuit that somehow rings up to a large voltage.
There is another problem with the light dimmer idea. They are not designed to work with inductive loads. When the current in the motor stops, the mains voltage won't be at the zero crossing. There will be a large dv/dt right after the current stops that would likely retrigger the triac.
I think it is well within the scope of what a small microcontroller could do. For extra credit, lets see if we can figure out how to get a measure of the speed of the motor.
We have a complex waveform on the motor. If we measure the voltage and current, we should be able to figure out an impedance curve for the motor. The impedance will be very high near the synchonous speed.
Actually, the schematic I found using the double 555
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only sets the absolute duty cycle. I.e. if you change the frequency, the duty cycle in seconds would stay the same, and the % duty cycle is changed. You'd need to adjust it again.
So mr. Fields... if your solution can handle this better, if you could post the schematic anyway? Thanks!
Lets say that you have a fan running with a couple of 555's in series so that the output into the TRIAC driver looks like this:
___________________ __ ___| |___________________|
||| | ||
where each positive pulse is 300 60Hz cycles wide and the period of the waveform is 600 60Hz cycles.
Now lets say that the fan is running too fast and that you want to slow it down. What you would do would be to either decrease the width of the positive portion of the waveform (decreasing the number of cycles into the fan) without disturbing the width of the low portion:
___________ __ ___| |___________________|
||| | ||
or increase the width of the low portion of the waveform while leaving the high portion alone:
___________ __ ___| |___________________|
||| | ||
Either way would cause the fan to stay on for less time than it was on, which is what you'd want to cause happen if the fan was running too fast.
--- Mine handles it in exactly the same way, but the dual 555 way is much less complex to implement so, IMO, that's the better way. In any case, I'll post the schematic in a day or so. :-)
Actually, the schematic I found using the double 555 only sets the absolute duty cycle. I.e. if you change the frequency, the duty cycle in seconds would stay the same, and the % duty cycle is changed. You'd need to adjust it again.
--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
On Sat, 12 May 2007 03:37:55 -0500, John Fields wrote:
--- Aaarrghhhh!!!
I must be getting senile.
You can do it with a single 555 astable: . .VCC>--+---------------------------------+ . | | . [Ra] +---------+ | . | 7 |_ | 8 | . +----------------O|D Vcc|-----+ . | 6 | _| 4 | . +----[Rb]----+----|TH R|O----+ . | | 2 |__ | 3 | . +--[DIODE>]--+---O|TR OUT|-----|--->OUT . | | GND | | . [C] +----+----+ [0.1µF] . | | 1 | .GND>---------------+---------+----------+ .
RaC determines how long the output will remain high and RbC determines how long it will be low.
You could also use a pot (about 1 megohm) to make adjustment easy, and then C would be about 10µF to give you a period of about 10 seconds. The 1k is in there so you don't toast the chip if you crank the pot all the way up to Vcc.
Thanks for the schematic. I plan to test it on a test board; I think it's useful. As for your explanation in your last post where you said that frequency doesn't need to be altered: you are right, as a speed regulation the pulsewidth method suffices. Though I would like to be in control of frequency too (I like to think of the fan spinning up & slowing down as a sound likened to sea waves, when it has a long period of, say >=3D 8 seconds). For the method you aim at, a period of 1 second or less would work, no? (C=3D1=B5F) It would then probably go unnoticable that it's switching on/off to attain low speeds.
--
I don\'t know anything about your fan, so I can\'t say...
BTW, there are a couple of errors in the schematic which didn\'t get
caught and fixed:
1. The 40103 should be a 74HC40103 and,
2. R1 needs to be 4700 ohms.
Thanks for the great help, John! The Fairchild datasheet really helped. And also thanks to ehsjr.
And now that we're talking about power, I think of one thing: do I need to add a fuse in series with the BT137 triac? Since the triac can handle up to 8A and probably has no protection for over-current, I'd have to use an 8A fuse, right?
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