Triacs (I would have used OR Gates)

Discussion in 'The Projects Forum' started by dhalse001, Mar 21, 2014.

  1. dhalse001

    Thread Starter New Member

    Mar 16, 2014
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    Hi all,
    Apologies in advance. I’m a noob and have searched but only read some of the stuff on here.
    I am going to control a small fan, which will run on AC mains power.
    I have four live feeds coming in (all AC, just mains power again), that will be live at different times (different controllers will, depending on temperature, humidity etc, turn these wires live individually), and at any time that one of these wires is live, I want the fan to be live (turned on). I did some electronics at university (only one year), and all DC at 5V, and here we would have used an IC with four OR gates. But it's AC 230V (UK power).
    I can’t just connect all the four positive (potentially live) feeds to the live of the fan, because one live feed will not only make the fan live, but also all of the other remaining 3 inputs live too. I can’t use diodes, because we’re talking AC power.
    I thought about using an SSR for each of the four live feeds, which would keep their state independent of the states of the other wires regardless of which of the other four are live, but then that seemed excessive and expensive.
    From what I can tell, the answer is to use four Triacs. Without knowing much about them, I guess for each of the four (potentially live) wires, I should feed that to the input and also the ‘gate’ of one of the four triacs, and the output of the triac should feed to the live wire of the fan.
    Does that make any sense? Is this the best idea? :confused:
    Many thanks for your helps
    Dave.
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    With four 230 VAC sources that are not necessarily at the same line or neutral potentials, and a 230 VAC fan, you are correct, you need a 4-in OR gate. Actually, you need a dual 4-in OR gate with full isolation - four relays.

    AC relay, 230 VAC coil, DPST. Each coil goes to one AC source, and for each relay the two common contacts go to each end of the coil. Then the four sets of NO contacts connect to the fan in parallel.

    ak
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Do all the four feeds originate from a common 230vac source or fuse/breaker or share the same neutral.
    If so you could use four steering diodes to energize a DC relay.
    Max.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    If your using the same Neutral , use four diodes and a 120V dc relay, the relay will activate when any live is powered.


    see diagram
     
    Last edited: Mar 21, 2014
  5. dhalse001

    Thread Starter New Member

    Mar 16, 2014
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    Thanks guys so far.

    Sorry - but the fan is 230V AC as well! So a DC solution won't work.

    It's actually a fridge that I'm converting to a meat 'curing chamber'. In it will be a heater, a humidifier, and the DHC100+ controllers on top and STC1000 controller on top will control the fridge.

    So one AC standard plug from the wall, powering all of this.

    When the fridge runs, or the humidifier, or the heater, or the dehumidifier (which is effectively running the fridge again), then I need the fan to run.

    Thanks again.
    Dave.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Clearly your not thinking use the relay to power the fan with its own contacts!!!
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Get 220VAC coil relays for each . Like one for heater one for humidifier etc.

    Connect each relay coil to each unit AC voltage terminal respectively.
    Parallel all the NO contacts accordingly and connect the AC and fan through the contacts which are all common to all relays.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Post's 3 & 4. Use one DC relay to switch the fan!
    Max.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Rereading this thread, it sounds like you have only one AC source that goes to four different control devices that switch the source to four different loads. In this case you probably still can use a common neutral and diodes as in DD's schematic. The diodes pick off the switched AC after the control devices. A DC relay coil might chatter, especially at the oh-so-British 50 Hz. A small capacitor rated for 400 V and connected directly across the coil should quiet it down.

    If the control devices had double pole contacts that break the neutral as well as the line, then you'll have to get your neutral connection where AC enters the system before the controllers.

    ak
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Why do you need diodes?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The diodes also know in this application are steering diodes, it is a common trick to isolate different AC sources, in this case it would be to detect any live conductor but keep them from feeding back to each other.
    A cap should not be needed if a reverse diode is used.
    Max.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    A single diode for isolation will generate a half-wave signal at the relay coil which, I would think, would cause the relay to chatter, even with a reverse diode. Or is that not true?
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    See excellent response #9.

    The only reason I put in the words "excellent response" is because the message is too short without it.

    ak
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It has worked for me in the past, I don't remember if I ever tried on 50Hz.
    Max.
     
  15. dhalse001

    Thread Starter New Member

    Mar 16, 2014
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    Of course. You're right! Silly me!

    Thanks.
    Dave.
     
  16. dhalse001

    Thread Starter New Member

    Mar 16, 2014
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    Nice one thanks - good point about the capacitor.

    The controllers output is a single wire in each case, which is either live or not live. So there is no problem with the neutral (common), it is connected to everything with no break anywhere.

    Thanks,
    Dave.
     
  17. dhalse001

    Thread Starter New Member

    Mar 16, 2014
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    If I joined all four 'feeds' that are potentially live to the single input into the fan, then when one of the four feeds becomes live, it would make all of the other feeds live.

    So for example, if the heater went live, not only would the fan begin to run, but also the fridge, humidifier, etc.

    Thanks,
    Dave.
     
  18. dhalse001

    Thread Starter New Member

    Mar 16, 2014
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    Ok, I don't know what a reverse diode is... I've tried to do a search on it. The best I could come up with is this:

    "Reverse Recovery Time is the time it takes a diode to reduce the reverse current to zero when going from forward bias to reverse bias. Contrary to popular opinion and idealized models diodes do conduct in the reverse direction, but only for a short while."

    Does anyone have a good link or better explanation?

    Thanks,
    Dave.
     
  19. dhalse001

    Thread Starter New Member

    Mar 16, 2014
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    Ok, so overall, it seems like there is a general consensus to go down the track of diodes and a single relay as per dodgydaves diagram in an earlier post.

    Everyone seems to be talking about mechanical relays and not an SSR (is that because the SSR switching time is too fast, or are they just using the terms interchangeably)?

    My original idea of just using 4 Triacs (in place of the 4 diodes, and no relay would be required), doesn't seem to have gained any traction.

    Any views appreciated.

    Cheers,
    Dave.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are many examples out on the web, Google flyback diode, reverse emf diode, snubber diode.
    The collapsing field from the relay during the zero voltage transitions is recycled through the diode, this causes current through the coil after the voltage is removed or transitioned to zero, this has the effect of retaining it long enough to carry it through until the next positive pulse,
    Max.
     
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