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DC Controlled AC Load

Discussion in 'General Electronics Discussion' started by fatman57, Sep 22, 2013.

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  1. fatman57

    fatman57

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    May 27, 2013
    Hi all,

    I am looking to power an AC device but control it using a DC transistor. I thought I could achieve this by using a rectifier bridge to turn the negative side (after the load) into DC and then control connection to the ground using a Transistor.

    Please see image below to better illustrate what I mean:

    [​IMG]

    Would this work as intended?

    Dan
     
  2. duke37

    duke37

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    Jan 9, 2011
    Looks like a good posibility. The components will need to be sized to suit.
     
  3. fatman57

    fatman57

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    May 27, 2013
    Thanks Duke - by that I presume you mean the Volts/Amps ratings?

    Is this commonly done?
     
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    Not sure if Duke read your description fully

    firstly, there is NO Positive or Negative "sides in AC
    the voltage is cycling at 50/60 Hz
    so you cannot put something in the negative side of the load

    secondly, putting a bridge rectif. into the circuit will break the circuit and there will be no path for the AC voltage

    this is what you said you want to do
    It wouldnt make any difference if it was in the phase or neutral side

    [​IMG]

    Unless I have missed something, I cant see it working

    Dave
     

    Attached Files:

  5. fatman57

    fatman57

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    May 27, 2013
    just thinking... :)

    Is there a way to control an AC circuit with a DC transistor?
     
    Last edited: Sep 22, 2013
  6. duke37

    duke37

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    Jan 9, 2011
    I would think that the diagram as shown by Dave would work, It is used to measure AC current with a DC meter and some TVs used this circuit with two of the diodes replaced with thyristors to control the voltage but here he load was after the rectifier.

    The output can not be connected to a 'common ground'. The output can swing to full mains potential

    Edit
    But the diagram in #5 will NOT work.
     
    Last edited: Sep 22, 2013
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    Duke

    its Monday morning ... brain is still asleep ;) explain to me ....
    how would it work ?
    there's no AC Current path between phase and neutral via the bridge ?

    Dave
     
  8. fatman57

    fatman57

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    May 27, 2013
    Ok, would this work on any one of the wires (P or N) - all I want to do is control if the circuit is on or not.

    To do this I would turn both transistors on or off at the same time. Diodes are there to protect the transistors.

    [​IMG]

    Any better?

    Would the use of a TRIAC be easier/cheaper? I haven't worked with these yet and was told to stick with DC transistors for now...I am also planning to use an Arduino as my controller...

    Also, the image found in #4 - on the DC side, is the black box to be considered negative?
     
    Last edited: Sep 22, 2013
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Why not save time and tell us what you're actually trying to do.

    e.g. "I want the speed of my electric drill to vary with the volume of the output of my battery powered transistor radio."
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Duke is right, it's valid to connect the AC inputs of a bridge rectifier in series with the load so you can switch the load with a DC device such as an SCR. Dave's design in post #4 will work, with the capacitor replaced by the DC switching device. This is a standard technique.

    But both sides of the switching device are at "half mains" potential - neither live nor neutral, so the signal that drives it needs to be isolated. This can be achieved using an SCR driven by an optocoupler (these are available with zero-crossing detection) or by making the whole drive circuit half-live, in which case you can use a transistor or preferably a MOSFET instead of the SCR.

    For the latter option, you can use a capacitor-fed power supply powered from the mains before the load, to provide power to the half-live isolated drive circuitry.

    As Steve says, though, it would be very helpful to know what type of load you are controlling, in what way you want to control it, and where the control signal comes from.
     
  11. fatman57

    fatman57

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    May 27, 2013
    Thanks for the replies...

    I want to turn a CLF light bulb on or off - not fast switching, simply on for a while and then off for a while. The source of power is mains AC and the signal will come from an Arduino.

    Thanks KrisBlueNZ! Wiki says that SCR will only allow current to move in one direction - surely this is not what I need...?

    Would the diagram in #8 work or is this not recommended?
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes, and SCR would be alright on the DC side of the bridge rectifier. The bridge rectifier converts the alternating direction of the AC current into a single direction.

    The diagram in post #8 is not complete enough for me to say whether it will work or not. You would need some way to generate bias for the transistors, and the way they are connected, those two bias sources would need to be isolated from each other. So it's unnecessarily complicated. It's not the way I would go about it.

    Your best bet is probably a solid state relay.

    What is your mains voltage?

    (There's a place for your location in your profile and it can be helpful to us if you fill it out. It also tells us which supplier(s) you will want to buy from.)

    The IXYS CPC1945Y is a solid state relay rated for 400V and 1A continuous, 10A surge, available from Digikey for USD 3.22: http://www.digikey.com/product-detail/en/CPC1945Y/CLA163-ND/565909

    It's controlled by a 5 mA current and can be driven from the Arduino just like a normal LED. In fact it IS an LED internally.

    I'm not totally sure that it's suitable for switching a CCFL lamp, but I think it is. It has a leakage current of less than 1 mA and works with any power factor greater than 0.25. I think these specifications are compatible with a CCFL. If ANYONE ELSE on the forum can confirm or refute this, please do.
     
  13. fatman57

    fatman57

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    May 27, 2013
    Thanks so much! I didn't even know they existed!

    Mains voltage is 230v. I'm in the UK. Found this on RS Online:

    http://uk.rs-online.com/web/p/solid-state-relays/6675225/

    It says the Max control Voltage is 1.5V - isn't that too low? I will have to check, I think the Arduino pins put out 5V, wouldn't a resistor solve this anyway?

    I'll have a look at the profile info.

    Could I use a TRIAC for this application?
     
    Last edited: Sep 23, 2013
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I explicitly recommend you follow Kris' advice and use a solid state relay. Otherwise the arduino will be connected to mains voltage with potentially life-threatening consequences.
     
  15. fatman57

    fatman57

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    May 27, 2013
    Thanks Harald!

    I saw a video last night on TRIACS, looks similar to transistors to work with, but I am a safety first person! I would put as low a value fuse on my mains plug as possible in any scenario. And I am an amateur of course!
     
  16. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A fuse will protect you from overcurrent. If the fuse blows, it's too late for your life.
    You will need at least a Ground Fault Circuit Interrupter to protect you from even weak mains currents (which will be way too low to trigger the fuse).

    Anyway, isolation ist THE way to save you and others (including the arduino) from harm.
     
  17. fatman57

    fatman57

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    May 27, 2013
    I'm sure every home has one of these, no? Still didn't stop me getting electrocuted as a kid! Yes, I did put my finger in a socket once...didn't do it again (as far as I can remember, might explain this twitch I have had for ages...)

    Safety warnings are always welcome! Part of the reason why I post before trying anything :)

    With a TRIAC though, if I use a DC transistor to control the gate connection to the negative, and the Arduino controls the transistor, isn't that isolation enough?
     
    Last edited: Sep 23, 2013
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    no you didnt get electrocuted

    if you did you wouldnt be typing today ;)

    definition: electrocution is death resulting from electric shock

    cheers
    Dave
     
  19. fatman57

    fatman57

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    May 27, 2013
    Is there a technical term for a brief encounter with electricity that locks up your muscles but you manage to escape somehow (by chance or aid from an external source) to live another day?
     
  20. duke37

    duke37

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    Jan 9, 2011
    The term isolation is used to describe the case where there is no connection between two parts of the system.
    A transformer can have isolation if the input and output windings are not connected together.
    A relay can have isolation as the coil pulls in the contacts with an insulating part.
    Triacs, resistors or transistors have connections through them. They do NOT have isolation.
    A solid state relay uses a light source (led) to activate the output. It does have isolation.

    New electrical installations in the UK have earth leakage trips but there are houses without them. You should never rely on safety devices to save your life. Would you be happy to crash your car because you are wearing a seat belt?
     
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