Friday, November 2, 2012

Controlling an AC load with a MOSFET

In most cases where you want to control an AC load, a triac or SCRs will be used. However, it is not easy to drive a triac or SCR. The drive requirement for the triac or SCR makes it sometimes difficult to control it as we want. One thing is that we can not turn the triac or SCR on or off as we desire, because once we turn it on, it latches and stays on until the next zero crossing or until current stops flowing through it. Also, driving a triac with reference to MT1 (or A1) is not as straightforward as we would want.

However, a MOSFET can be controlled as we want. Set the gate high (with a sufficient voltage) and current can flow from drain to source. Set the gate low and current can no longer flow. Convenient!

However, a MOSFET can only be used to control DC loads since it is a unidirectional switch - current flow can be controlled when it is flowing from drain to source, but can not be controlled from source to drain. So, certainly it can not be used to control AC loads. Right?

Well, directly, it can not be used to control AC loads the same way you'd control DC loads. But, with some clever circuitry, it can be used to control AC loads. And here's how:



 Perhaps you don't see how it works now. But consider the two diagrams below, which show the flow of current during the two AC half cycles. I'm sure you'll get it better then.
 


As you can see, due to the bridge rectifier, the MOSFET always "sees" a DC voltage as the drain is always positive with respect to the source. Thus, with this combination of the bridge rectifier and MOSFET, by controlling a DC switch - the MOSFET, you can control the AC load.

The MOSFET must be turned on fully by driving it high by at least 8V above source level - 8V with respect to source.

So, you can turn the MOSFET on and off at any time and accordingly turn the load on and off as required. This makes driving the AC load so easy!

27 comments:

  1. using this ckt...i can finsh my project...so., im not clear vt components values regarding to this CKT.

    so please give the values of this CKT as early as possible......

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  2. For R1, you can use a 1k resistor. For R2, you can use 10R. A look at your driver circuit would help in suggesting appropriate values, since I did not put in values for R1 and R2 since they depend on the drive circuit. You may try with 10R and 1k for R1 and R2.

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  3. This comment has been removed by the author.

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  4. Great idea. Maybe it should be noted, not for inductive loads.

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  5. hi tahmid may i ask ... if my signal from R2 there about the gate signal ....what is the waveform look like DC PWM duty cycle ??? or using firing angle pulse concept ???? i wait u reply .... because i only know how to do in DC pwm......the firing angle code i not quite familiar to write .... and may i ask this R2 signal able to use Microcontroller to apply the signal?

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  6. hi sir,
    is ti applicable to inductive loads tanks.

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  7. Hi congrats for your articles which have helped me to repair a lot of smps and inverters but for this article,triacs are the best,with triacs you deal directly with ac the costly rectifier is spared especially with three phase dimmers;security i.e. RCD/ELCB protect the triac directly from the source.MOSFET with high current rating are rare on the market and are at high cost including the high rating rectifier which has to be fixed with chassis for cooling.

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    Replies
    1. Hi,

      This has been shown for simplicity of firing. For example, using this, you can implement phase angle control by first firing the device on and keeping it on for a while and then turning it off. This you can't do with a triac.

      That being said, it's up to you to choose. This has just been shown as an idea. But that doesn't mean you have to always use it. Yes, in most cases triac will be better. Nonetheless, control of AC using a MOSFET is an interesting topic.

      Regards,
      Tahmid.

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  8. Hi Tahmid,
    I deal with repairing of all types of dimmers mono phase,three phase etc., with RGB or RGBW LED floodlight MOSFET are efficient in controllling the output,they can be used on smd circuits but for high loads triac are still better.You are wrong as for the firing,using DMX controllers,all triacs can be fired at any time when driven with optotriacs.I can send you details if you want.
    Regards

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    Replies
    1. That's interesting. I'll look them up. It will be helpful if you provide some links.

      Thanks!

      Delete
  9. Hi Tahmid,

    This Article is very useful for me, anyway, with this circuit, can we control the
    AC Voltage by giving PWM signal to the MOSFET's Gate ? therefore we can set the voltage as
    we want

    Regards,
    Suprapto

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  10. For this to work the ground for your low voltage control circuit would have to float at the MOSFET's source voltage (which is AC 240V). How exactly are you driving the MOSFET?

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    Replies
    1. http://sandysplash.blogspot.in/

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  11. To drive the mosfet from a pic , do i have to connect the ground of the pic to the source of the Mosfet ?
    My pic run at 3 or 5 volts, but you said i need at lease 8 volts source to drain.
    How i can then drive the mosfet from a pic ?

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    Replies
    1. Look at this device - PVI5050NS/PVI5080NS. I'm thinking of doing something similar with a PIC. This circuit as anonymous above points out needs a floating input which is what this device appears to give.

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  12. Hi, Tahmid

    excellent work you have done.
    I am trying to make automatic voltage regulator.
    Can you please give me the proteus design of your ckt.
    So than I can simulate it.
    As i want to add some other functionalities as battery backup(some what like inverter) etc.

    It will be very helpful for me.

    Thanks

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  13. Hi Tahmid,
    if we have a load that consume 30mA, what will be the requirements on Vds ?
    thanks,

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  14. When you want to simulate, the simulation mode is using AC analysis or transient mode?

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  15. Could you please post a full schematic diagram for the whole complete circuit? Need it urgently. thank you.

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  16. good idea. Great help.
    I have a doubt. If the load is not connected, the bridge diode and mosfet will get burned, right???

    How will you protect the circuit when the load is not connected?

    Thanks in advance.

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    Replies
    1. when the load is not connected it is basically open circuited and hence no current flow unless you specifically short circuit the load

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  17. i will try this circuit for inductive load but the mosfet are heated and damage this type of fault how to protech mosfet

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  18. Hi,
    Could i use this circuit for control Heater supply??I have developed PWM with 10KHz Frequency. Why Ri1 is used ? Is that any problem without R1? Because my PWM ground level is differ from Ac supply.

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  19. Hi..
    I'm using a PIC24FJ16GA002. I want to generate an interrupt to a constant high signal at every defined interval.Can u please help me out with the code for it.
    Thanks in advance.

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  20. Hi. Thanks for your article. Please I want to ask, where is R2 connected to? And can I use this circuit in combination with a triac for some inductive lads or like air conditioners?
    Thanks.

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  21. Sir, can we use IRF730 in this circuit, if yes then what is the required values of resistor(including watts). Please reply ASAP, its urgently required!!!

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  22. Somebody else too used such kind of approach.
    http://www.dmitry.gr/index.php?r=05.Projects&proj=06.%20RC%20dimmer

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