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Old 11-17-2011, 05:06 AM
jobycm jobycm is offline
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Default AC fan pwm control using MOSFET

I am trying to control ac fan using mosfet driven by a pwm signal. I have ended upon a circuit like the one attached with this. Is there any problem with this circuit regarding leakage of photo transistor during off time? Do i have an alternative? Please help.... Thanks.
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Old 11-17-2011, 01:22 PM
elec_mech elec_mech is offline
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Welcome to AAC.

No, unless there's a property of bridge rectifiers I'm unaware of, this will not work. Bridge rectifiers as well as rectifiers in general work in one direction only. They are used to rectify AC to DC. They will not allow you to use DC to control or vary AC. Look here for more information: http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

To change the speed of a single-phase AC motor - I assume that is what you have - you need to change the frequency. In the U.S., a single-phase, 120VAC motor operates off of 60Hz. However, as you decrease the frequency, the motor will consume more current. We attempted to power a 60Hz motor with 50Hz at work and if it wasn't for the thermal fuse, the motor would have burned up.

Before attempting to build a circuit to change the speed of an AC motor, do some research on single-phase AC motor speed control. In the little research I've done and past experience I've had, I don't think you can safely change the speed of a single-phase AC motor by much, if by any at all. Three-phase is a different story.
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Old 11-17-2011, 01:28 PM
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Well, I think this might actually work, because this is not a standard bridge rectifier, it is more of a "shorted" bridge rectifier.

You donīt say anything about the target voltage, but plese DONīT use this with mains electricity. Even though there is the optocoupler to save you from a nasty shock, the transitor will die in a spectacular fashion if you connect it to 120 or 230 Vac.

I would place the zener before the the opamp and mosfet, so that you have a stable 15V rail which you then switch to the mosfet. The zener in your configuration will probably be too slow to keep the mosfet from dying.
Also the optocoupler in your configuration is subjected to 320Vdc when off and will die as well.
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Old 11-17-2011, 03:51 PM
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Quote:
Originally Posted by elec_mech View Post
No, unless there's a property of bridge rectifiers I'm unaware of, this will not work. Bridge rectifiers as well as rectifiers in general work in one direction only. They are used to rectify AC to DC. They will not allow you to use DC to control or vary AC.
..................
As kubeck noted the circuit shown could work. The bridge output is DC but the input is AC and the input of the bridge is in series with the load. Thus the MOSFET on the bridge output will rather act as a variable resistor in series with the motor. It's similar to varying the AC voltage to the motor, which is not the best method of controlling fan motor speed but is often used.

It's similar to used a Triac type lamp dimmer to control the speed. A Triac would be more efficient than the MOSFET but often causes noticeable motor noise due to the high frequency harmonics generated by the Triac switching.

One thing to note is the the MOSFET could be dissipating a fair amount of power, depending upon the amount of speed reduction so it needs a good heat sink.
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Old 11-17-2011, 04:46 PM
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I think the circuit is meant for PWM, otherwse you wouldnīt need an opto, nor call the control input PWM

Running the mosfet in linear region is a horrible idea.
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Old 11-17-2011, 05:30 PM
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Originally Posted by kubeek View Post
I think the circuit is meant for PWM, otherwse you wouldnīt need an opto, nor call the control input PWM

Running the mosfet in linear region is a horrible idea.
Whether you control the circuit linearly or with PWM you would still want isolation between the control signal and the circuit.

But I certainly agree that PWM control is much better than linear. I forgot about the thread's title.
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Old 11-18-2011, 03:53 AM
jobycm jobycm is offline
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tanks for the welcome elec_mech and i tank all otrs 4 ur suggestions.
yes i did beliv tis s horible one. tats y i postd it.. :P
m usin 230VAC. triac based regultor seems to hav a noisy control. i am on a smal reserch in tis. nw am on a pulse transformer to triggr a irfzp460. i ll need all ur suggstions on the noise effects on tis kinda transformers. wot if i go for a isolated mosfet driver..??
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Old 11-18-2011, 12:45 PM
elec_mech elec_mech is offline
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Thank Kubeek and Crutschow, I still have much to learn.

Quote:
As kubeck noted the circuit shown could work. The bridge output is DC but the input is AC and the input of the bridge is in series with the load. Thus the MOSFET on the bridge output will rather act as a variable resistor in series with the motor. It's similar to varying the AC voltage to the motor, which is not the best method of controlling fan motor speed but is often used.
I'm still lost on this. So the bridge rectifier, as it is shown, is in series with the load. It is somehow acting as a switch to complete the AC circuit going to the load when the MOSFET effectively shorts the DC output? So, if you connect an AC load in series with a bridge rectifier and short the DC output, the bridge will "safely" act as a closed switch and complete the circuit to power the load? Is this an acceptable or common practice or one of those it could work but there are better ways?

For my own curiousity, what type of circuit or control would you recommend to control the speed of a single-phase AC motor? For a DC motor, speed is controlled by voltage, roughly speaking. For an AC motor, speed is controlled by frequency. At least, this has been true in my limitied experience.
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Old 11-18-2011, 01:23 PM
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Quote:
Originally Posted by elec_mech View Post
I'm still lost on this. So the bridge rectifier, as it is shown, is in series with the load. It is somehow acting as a switch to complete the AC circuit going to the load when the MOSFET effectively shorts the DC output? So, if you connect an AC load in series with a bridge rectifier and short the DC output, the bridge will "safely" act as a closed switch and complete the circuit to power the load? Is this an acceptable or common practice or one of those it could work but there are better ways?
Imagine the transistor is closed and see where the current goes for both polarities of the AC.

Quote:
For my own curiousity, what type of circuit or control would you recommend to control the speed of a single-phase AC motor? For a DC motor, speed is controlled by voltage, roughly speaking. For an AC motor, speed is controlled by frequency. At least, this has been true in my limitied experience.
I donīt have much experience with AC motors, but I am sure that changing the frequency is not the only way to control them. I used a typical triac dimmer with a power drill and it worked just fine.
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Old 11-18-2011, 02:06 PM
elec_mech elec_mech is offline
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Quote:
Imagine the transistor is closed and see where the current goes for both polarities of the AC.
I'll have to study this - thank you.

Quote:
I donīt have much experience with AC motors, but I am sure that changing the frequency is not the only way to control them. I used a typical triac dimmer with a power drill and it worked just fine.
Doh, I have seen this before. They make cheap speed controls for wood routers which are variacs and thus change the speed by changing the voltage. That's probably how they do it with corded drills that have a speed adjustment knob on the trigger. Good point, thank you again.
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