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Questions about circuitry for making a cabinet fan

Discussion in 'General Electronics Discussion' started by Coolcise, Feb 17, 2016.

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  1. Coolcise

    Coolcise

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    Feb 17, 2016
    I am currently working on a project where I need to mount a computer fan into a cabinet door, but I have a couple questions about the circuitry involved.

    The main idea is to have the fan get power from a wall outlet (using an appropriate adapter), and it will turn on when a computer is turned on. The signal will be delivered by using a USB cable connected to a USB port in the computer.

    I have a couple cabinets that need the Single USB Build (below), and one that has two computers in it that will need a multi-USB build (also below).

    My first question is: Could the single build fry the computer? My assumption is that it should work fine, though input in highly appreciated!

    The second question is: Will the way I'm handling the two USBs work? Using the diodes seemed like a good plan in the beginning, but seeing the actual circuit drawn out made me less sure. Would it be better/safer to just use two relays (one for each USB)?

    CabinetFanCircuitryBuilds.png
     
  2. Minder

    Minder

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    Apr 24, 2015
    Why do you need the outlet fed fan source?
    Personally I would have picked up the relay direct from the P.S 12v connector or the fan direct from it.
    What is the object? Seems a very convoluted way of doing things.
    M.
     
    Martaine2005 likes this.
  3. Coolcise

    Coolcise

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    Feb 17, 2016
    I am trying to get the fan to start when I turn on the computer(s) and not run otherwise. This is supposed to be mounted in a box (thus the convoluted solution with the wall outlet going to the relay and THEN to the adapter). In addition, the relays my boss provided me has a load range of 24-380VAC. It's also supposed to be modular, so that it can just be connected the wall, the computer and the fan.
     
  4. Gryd3

    Gryd3

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    Jun 25, 2014
    First Q:
    It could if you don't put any protection in place. A relay coil is not the cleanest thing to turn on and off. Using a reverse biased diode directly across the + and - terminals on the relay will prevent the relay coil from feeding back into the USB port when it turns off. Ideally, the relay will draw less than 100mA, but USB ports should be able to put out up to 500mA without any additional components.

    Second Q:
    I don't know your electrical situation, but the current design ties the ground of two computers together. A building wiring fault could result in small or large amounts of current flowing between the computers using the negative USB terminal (which is often tied to ground)
    Using two relays will completely isolate the two machines and would be the safer way to go.


    Sometimes it's preferred, the only other solution is modifying a PC to get it's 12V source to the outside of the case... and depending on how this is done, the computer is tethered to the case.
    I like the solution actually... it can be used on any computer system simply by plugging in a USB port.
     
  5. Gryd3

    Gryd3

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    Jun 25, 2014
    Although... I would probably put the relay 'after' the 12V adaptor... the adaptor will still draw tiny amounts of current, but switching 12V will be easier than switching 120V. It will also be much safer and won't require as much isolation... The 5V USB and the 120V side of the relay would be pretty close together, and accidental short and the whole computer could go :p
     
  6. Coolcise

    Coolcise

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    Feb 17, 2016
    It is a solid state relay, not a coil one. I just used a random circuit drawing tool i found online to make it easier to visualize.
    Does that make a difference?

    This is what I was afraid of. Thank you.

    Would this still work with a load range of 24-380VAC though?

    Another question: do you know, in theory, how much voltage I could send through a 3.5mm mono jack connection? I've been googling it, but all the threads only speak of using it for audio (which I guess makes sense). It would be cool to use it to connect the fan to the box to the adapter.

    EDIT: merged two posts
     
    Last edited: Feb 17, 2016
  7. Gryd3

    Gryd3

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    Jun 25, 2014
    It does. Solid State shouldn't have that same back EMF as the conventional relays. Check the specs on the current draw though.

    It very well could. The thing you need to watch for is the current draw and voltage requirement on the input of the relay, and the voltage/current capability of the output. If you have a relay capable of switching 380VAC then you got nothing to worry about.*

    That's a tricky question... mainly because voltage limitations are imposed by insulation.
    Insulation can be any non-conductive material including air... Say there is a 0.25mm gap between the tip and sleeve on the mono jack (That little plastic spacer):
    Air has a dielectric strength of 3MV/m, or 3KV/mm... so in theory in absolutely ideal conditions, you could in theory carry <750V before a spark would jump across the little plastic ring through the air...
    A larger space means larger voltage, and less than ideal environment conditions will bring that voltage down considerably... ei, moisture in the air, dust, debris, etc.
    The next limiting factor is current. The 'size' of the conductor here will determine current capability. Larger wire = larger current.
     
  8. Minder

    Minder

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    Apr 24, 2015
    I see galvanic isolation using a relay or SSR? where is the common ground point?
    I have used a 3.5mm jack to provide power to a small servo system with no problems.
    Also have you considered whether the the fan in this application requires to provide positive pressure to the enclosure?
    M.
     
  9. Coolcise

    Coolcise

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    Feb 17, 2016
    As for the ground, I actually hadn't thought about that, but I think using the wall outlet ground should be sufficient.
    I have indeed considered the second fan, and I don't think it should be needed. I will however make sure I'm right with a single cabinet before applying it to all.

    Thank you both for all the feedback! I believe I have what I need.
     
  10. Gryd3

    Gryd3

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    Jun 25, 2014
    The common ground was on design option #2 when the op had originally designed a circuit for two computers at the same time. The ground from two use ports were tied common, and the positives ran through a Diode.

    There should not need to be a common ground point between the relay input side and relay output side
     
  11. AnalogKid

    AnalogKid

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    Jun 10, 2015
    AC and DC solid state relays are very different internally, so it does make a difference. I think the most simple solution is to use a small 110 Vac fan rather than 12 Vdc, and switch it directly with your donated SSR's. With two relays you have complete galvanic isolation between the two USB port grounds. Your SSR's probably have input current limiting built-in, so each control circuit is two wires directly to a USB port +5V and GND.

    UL has very clear requirements for using air as an insulator. Clearance distance is the air gap between two conductors. Creepage distance is the distance across a surface, such as the distance between two pc board traces or the distance between the input and output pins of an opto coupler. No matter what size or package or power handling capability, AC SSR's have UL minimum creepage between the control and output pins.

    ak
     
    Last edited: Feb 18, 2016
    Gryd3 likes this.
  12. davenn

    davenn Moderator

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    Sep 5, 2009
    I really have to agree with Minder .... Seriously !! ... you really are making life difficult for yourself with all the plugpack, USB port and relay use
    get your 12V from within the PC .... it's going to be active from the moment the PC power switch is turned on

    all that other circuitry is a pointless complication, not to mention the waste of money


    Dave
     
    Martaine2005 likes this.
  13. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Is it a requirement that all connections the PCs to determine their on/off status be through external connectors, or is connecting to an internal, unused disk drive power connector an option?

    Also, what are the manufacturer and part number of the SSR's you have?

    ak
     
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