Sanity check - duct intake controller

Discussion in 'The Projects Forum' started by kooganani, Mar 9, 2010.

  1. kooganani

    Thread Starter New Member

    Mar 8, 2010
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    I'm working on a temperature-controlled cabinet. When it gets too warm in the cabinet, a valve will open and a fan turns on, pulling air in from outside. The thermostat and fan are installed, but cold air also blows in through the vent when the fan is off, so I need to install a valve that opens when the fan is on and closes when it's off.

    I'd appreciate a 2nd opinion on my circuit. I know it's simple, but this is my first circuit diagram:

    [​IMG]


    - I did the DPDT relay as two SPDTs because it made more sense to me on paper.
    - Power supply is a 12V AC adapter.
    - S1: Thermostat
    - M1: Fan
    - M2: Valve motor
    - When the thermostat turns the fan on, M2 moves a plate that hits S2 and turns off M2 when the valve is fully open.
    - When the thermostat turns the fan off, M2 moves the plate in reverse, hitting S3 and turning off M2 when the valve is fully closed.

    Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Nice job for a 1st schematic! :)

    However, your limit switches and the thermostat are all carrying motor loads, which isn't so good. Better if relay contacts do the switching of motor loads.

    Also, if the fan motor turns on when the plate is closed, the pressure on the plate will make it harder to open or close the plate. Better if the fan doesn't start until after the plate is open, and stops when the plate first starts to close.

    You say that your power source is a 12V AC adapter - do you mean that it's simply a transformer that steps down mains power to 12VAC, or is it actually a 12V DC power pack or "wall wart" that runs from mains power?

    If DC, it would be a good idea to use diodes across the relay coils and the motors. This will help to reduce reverse-EMF when the current is shut off, and to reduce arcing on the contacts when they break. That won't really help with AC though.
     
    Last edited: Mar 9, 2010
  3. kooganani

    Thread Starter New Member

    Mar 8, 2010
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    Thanks, SgtWookie! It was actually a post you made on a thread about a model railroad component that made me realize that using a DPDT relay and limit switches was a much simpler approach than my original idea of a timer circuit which would have been a giant pain considering my skill level.

    Understood on using relays with the limit switches. If I understand your point, I should add a relay to the fan circuit, so when the "open" valve limit switch is hit, it turns off the valve motor and turns on the fan motor. Correct?

    It actually a 12V DC power pack or "wall wart" that runs from the 120VAC wall power.

    Good advice about the diodes.

    So... would you recommend using two wall warts -- one for the thermostat, switches, and relays, and other for the motors?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, try this one on for size - see if you can figure out how it works. ;)

    [​IMG]

    Hints:
    My S3 is your S1.
    My S1 performs a similar function as your S2, but also controls the ground path of the fan control relay's coil.
    My S2 performs a similar function as your S3.
     
    Last edited: Mar 9, 2010
  5. SgtWookie

    Expert

    Jul 17, 2007
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    I left the diodes off the motors - it would've made it a bit confusing when you're first trying to understand it.
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    You could also just use gravity-closed vent covers, such as typically used on outside walls with small extractor ducts.
     
  7. kooganani

    Thread Starter New Member

    Mar 8, 2010
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    SgtWookie - That looks great, it may take me some time and a few "state" diagrams to get a full understanding. What's the best place to put the anti-emf diodes?

    rjenkins - I appreciate the suggestion, and I wish it were that easy. Unfortunately it's an intake vent rather than an exhaust vent, so it's the state of the fan that matters rather than the direction of air flow.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    For the motor EMF diodes, use diodes rated for the motor current. 1N400n (1A), 1N540n(3A), where n=2 to 7, are possible candidates.

    M1 (door motor):
    Connect two diodes' cathodes to +12v. Connect their anodes to both the + and - motor terminals.

    Connect two more diodes' cathodes to the motor + (red) and - (black) terminals.
    Connect the anodes of these latter two diodes to ground.

    M2 (fan motor):
    Connect the cathode of a diode to the motor + terminal (red).
    Connect the anode to ground.

    Operation:
    When below the temperature of the thermostatic switch S3, the relays will be in their NC positions as shown. S2 is the N.C. door closed limit switch; if the door is still open, RLY2's coil will be energized, supplying 12v to the motor's - terminal while the motor's + terminal is connected to ground via RLY1. The motor will run in reverse until S2 opens, breaking the ground for RLY2's coil.

    When S3 closes, RLY3's coil energizes, removing +12v from RLY2's coil and supplying 12v to the high side of both RLY1 and RLY4's coils. RLY1's contacts toggle states; the + motor terminal is supplied with 12v, and the - motor terminal is grounded via RLY2's NC contact. Until the door is fully open, the coil of RLY1 is supplied through the N.C. contacts of S1; the coil of RLY4 does not have a ground path until S1, the door open limit switch, is depressed.

    When S1 is depressed, the ground path for the coil of RLY1 is broken. The contacts of RLY1 change states, grounding the + motor terminal, acting as a brake for a quick motor stop.

    At the same time, ground is applied to the low side of the coil of RLY4, causing it's N.O. contact to close, supplying 12v to the + terminal of the fan motor, causing the fan to operate.

    When the temperature decreases to the point of S3 opening, the coil of RLY3 loses power, causing the contacts to again toggle states. Power is removed from the high sides of the coils of RLY1 and RLY4. This causes the contacts of RLY4 to change states, grounding the + motor terminal, acting as a brake.

    At the same time, power is applied to the high side of the coil of RLY2, causing the N.O. contact to make. This supplies 12v to the - terminal of the door motor. Since RLY1's NC contact is supplying a ground to the + terminal of the motor, the door moves towards the closed position until S2 is depressed, breaking the ground path for the coil of RLY2. M1 is then braked to a stop, ready for the next heat/cool cycle.

    The relays can be standard automotive-type 12v SPDT relays. Some of these have diodes built-in; if they do, a polarity will be indicated for the coil connection. If they do not, you should add diodes.

    Automotive Relay connections:
    85 - coil
    86 - coil
    30 - Common contact
    87 - N.O. contact
    87a- N.C. contact
     
    Last edited: Mar 9, 2010
  9. kooganani

    Thread Starter New Member

    Mar 8, 2010
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    I just did "live wire" diagrams of the various states, and the operational stages make sense to me now. This is awesome.

    I see what you mean now about not running motor current through the relays. You keep the motor terminals on -/- when they're off, and when they turn on the positive terminal is connected directly to the main + voltage with no relays in between.

    Thanks for the info on the motor diodes.

    I'm playing around with some schematic software. I'll add the motor diodes and post a final version once I've figured it out. Thanks much for your help!

    I'll let you know how the sauerkraut turns out in a few weeks. :)
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, only the coil current flows through switches; no motor current.

    All motor current passes through at least one relay contact.

    This enables you to use miniature switches and low-switch-current thermal sensors. Passing motor current through a thermal sensors' contacts would tend to increase it's temperature.

    No relay coil is energized unless the motor(s) it is controlling is in operation. No more than two relay coils are drawing current at any given time. Only one motor runs at a time. After a door open/fan/door close cycle is complete, no standby power is consumed.

    Relays are relatively easy to replace, if you use relay sockets or a relay harness/pigtail.

    I'll take a Ruben melt on rye, please. :)
     
    Last edited: Mar 9, 2010
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