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20A screw block-terminal

Discussion in 'Datasheets, Manuals or Parts' started by Chris T, Jan 24, 2016.

  1. Chris T

    Chris T New Member

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    Hi,

    I'm currently designing an HVAC PCB board and I'm having a hard time finding the right terminal that works to connect the wires to the PCB. The board is designed to work directly with the HOT line from an HVAC (24VAC) and it can tolerate a current of up to 20A (apparently some machines chew up quite a lot of power). Now I'm looking at choosing the right terminal blocks to hook the wire onto the PCB board, but I can't really seem to find something reasonable.

    I found the following https://www.phoenixcontact.com/online/portal/us?uri=pxc-oc-itemdetail:pid=1730528&library=usen&tab=1 which is rated for 24A, but then the UL, cUL listings put group B @ 20A, but group D at 10A. I'm not sure what's the difference between the 2 groups. That said, CSA approval sets it to 10A. They're all rated for 300V.

    I'm wondering if I'm not sizing it too high. ie: Because the ratings set this to >= 10A/300V and I'm operating on 24V, can I get a lower-rated terminal? ie: Is it more about Wattage than separate voltage & amperage?


    Thanks,
    Chris
     
  2. Les Jones

    Les Jones Well-Known Member

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    Chris,
    The size of the metal parts of the connector are goverened by the current rating. They need to be large enough to take cable rated at 20 amps and the metal parts must also have a large enough cross sectional area to carry the current. The thickness of the insulation is determined by the voltage rating and the required physical strength. I do not think a terminal rated at 24 volts (If manufactured.) will be signifacantly smaller than one rated at 300 volts. I would have thought that the 24 volt circuit is only a control circuit. Not the circuit that powers the machine. More details of what the PCB that you are designing does and what it connects to would be a great help.

    Les.
     
  3. Chris T

    Chris T New Member

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    Thanks for the reply & explanation. That makes sense. I originally thought that it wasn't a control signal but what drove the motor, but upon further research, I figured out that it is indeed a control signal and in my circumstance, it runs at 40VA (ie: 1.666A), so I can get away with a lot less. This may be somewhat of a problem however. My plan was to run the 24vac into a step-down switching regular to generate 5vdc & power a raspberry pi along with a set of triacs & a 16x2 LCD screen. I believe that the RPI alone is rated @ 1A.

     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    A switching power supply that outputs 5V at 1A will only draw 0.040A at 24V dc.
     
  6. Les Jones

    Les Jones Well-Known Member

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    Hi Ron,
    I would like to know where to buy those switching power supplies. I normally assume about 85% efficiency. 520% efficiency is very good.
    Seriously, I calculate the input current at about 0.21 amps at 100% efficiency. (About 0.25 amps at 85% efficiency. - 6 watts.)

    Les.
     
  7. Chris T

    Chris T New Member

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    Yah I totally agree. I'd love to buy some of those as well. My math also lines up with yours Les (5/24)A + inefficiencies.
     

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