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Relays

Discussion in 'Power Electronics' started by Goldhelmeth, Apr 15, 2020.

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  1. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You refer to an h-bridge, do you? Using changeover contacts on a single relay is the safe way to do it, agreed (see image in the article I linked). However, you can use separate relays if you can ensure proper timing (first open the first set of contacts, then close the other set of contacts). This can be done with the bluetooth module by sequentially controlling the contacts and having a proper delay (e.g. > 10 ms, depending on the relay) between opening and closing instructions.
     
    hevans1944 likes this.
  2. Goldhelmeth

    Goldhelmeth

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    Apr 8, 2020
    I beleive that I should add details about the general project.... Also I might change some inputs I have given upper in this thread.... The bluetooth relay will be replaced by 16 channel relay.... This relay will be controlled by a raspberry pi and "regulated by two ULN2803A, the folowing schematic shows what I would like to do. In my skill deprived mind, I had the naive idea that I was going to be able to use 2 channels to control the actuator movement and with a third channel I would be able to control a 12v fan.... But I have only one 12v battery to do all this... I suppose that my idea for the wiring of the relay-actuator is impossible...
     

    Attached Files:

  3. Goldhelmeth

    Goldhelmeth

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    Apr 8, 2020
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    With this board you do not need separate drivers. The relay drivers (ULN2803) are already on the pcb. In additioon the inputs of the board are decoupled from the relay drivers by photocouplers. The inputs are TTL compatible (5 V logic levels) but possibly work with 3.3 V logoc levels, too.
    Control signals are connected to the black 2 × 10 header pins.
    12 V are connected to the blue screw terminal. This 12 V supply is required for the coil side of the relays. Each relay coil dissipates as far as I could find out max. 0.35 W, so the 12 V supply must be able to handle 16 × 0.45 W = 7.2 W equivalent to 0.6 A.
     
  5. Goldhelmeth

    Goldhelmeth

    30
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    Apr 8, 2020
    So if the battery is 7A its fine if I understand. But I am not very familiar with relays yet.. Even if most of the clouds were dissipated. How would I link the actuator to the relay ? I would like to make that when one channel is on it goes forwards and when another is on (and the first one off) the actuator goes back. I could buy an actuator with built in limit switches if it makes things easier.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,883
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    Nov 17, 2011
    This were only possible if the actuator had separate inputs for forward and backward motion. If the direction is controlled by applying the operating voltage in reverse direction, you need to use 4 relays in an H-bridge configuration (see my post #21.
    Limit switches are a good idea in any case. They will not help with the reversal of the direction.

    Unfortunately the ebay offer has no datasheet to find out more detail about the actuator. This is generally a problem with offers from that kind of platform (be it ebay, aliexpress, bangood, ... - you name it). If you are not familiar with a component, you need a datasheet to learn how to use the component. In this case it pays (pun intended) to buy from an authorized seller who offers documentation
     
  7. Goldhelmeth

    Goldhelmeth

    30
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    Apr 8, 2020
    But why 4 ? All of the tutorials use 2 channels to control dc motors (forward and backward motion) including actuators :

    Im sorry to ask so much but I am truly lost could you draw over the following picture the connections because I am confused. Thanks

    The idea to add the ULN2803 comes from a forum on raspberrypi.org because the pins of the raspberry are 5v intollerent and many pies have fried because of relays...
     

    Attached Files:

  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,883
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    Nov 17, 2011
    2 channels can be used with the changeover or spdt relays as shown in the video you linked.
    Previously I assumed you were using spst relays.

    The H-bridge is constructed this way:
    upload_2020-4-18_18-51-58.png
    I can't draw the connections into your diagram as I have no idea what it shows.
    "coil for relay 1" is a placeholder for the control of relay 1 via the input pins on the relay pcb. Same for relay 2.
    This circuit definitely requires limit switches because without power to the relays the actuator will see the full supply voltage.

    Here is an improved version of the circuit:
    upload_2020-4-18_18-54-30.png
    Note that the nc and no contacts of relay 2 are swapped here. The advantage of this circuit is that without power or control signals to the relays both sides of the actuator are at the same supply voltage (positive in this case) and therefore the effective voltage across the actuator is zero. The actuator will not move.
    To move the actuator, you either activate relay 1 or relay 2, never both at the same time.
    Assuming the circuit as shown above, activating relay 1 will apply neg. voltage to the left side of the actuator and pos. voltage to the right side.
    Activating relay 2 instead (and de-activating relay 1) will apply pos. voltage to the left side of the actuator and neg. voltage to the right side.
    Limit switches will prevent the motor within the actuator from overload when the actuator reaches either end of its travel distance.

    Basically correct, but as you can see on the photos of the pcb these drivers are already installed.
     
  9. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    Perhaps this drawing wil help you:

    relays_board.png

    Bertus
     
    hevans1944 likes this.
  10. Goldhelmeth

    Goldhelmeth

    30
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    Apr 8, 2020
    Well I do not know if they are spst or spdt... I linked the relay upper in this thread. But I don't understand what is the point of relays if I have to power the relay board and power the actuator separatly. Why do I have to have an actuator power supply ? Why can't I use the power that comes from the board ?
     
  11. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    The relays are single contact.
    When no power is on the relays, the NC is connected to the COM.
    When there is power on the relays, the NO is connedted to the COM.

    Bertus
     
  12. Goldhelmeth

    Goldhelmeth

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    1
    Apr 8, 2020
    Thanks, but what about my question on power supply ?
     
  13. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    Did you see this picture on the page you posted:

    relays_board_connections.jpg

    On the bottom right there is the power shown for the board, wich powers the relays.

    Bertus
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

    9,883
    2,096
    Nov 17, 2011
    No power comes from the board.
    The board needs to be powerede,
    Relays are switches where the switch position is controlled by another, usually lower voltage. You need to understand how relays work: What is a relay?
     
    bertus likes this.
  15. Goldhelmeth

    Goldhelmeth

    30
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    Apr 8, 2020
    If I understand this correcly, the only difference between 5v relay board and 12v is the power supply of the board ? So can I use a 5v relay board to control a 12v motor ?
     
  16. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    Yes. If the powersupply for the motor is separate from the 5 Volts.
    The 5 Volts will be used to activate the relays.
    The contacts of the relays may carry the 12 Volts, they are isolated from the board.

    Bertus
     
  17. Goldhelmeth

    Goldhelmeth

    30
    1
    Apr 8, 2020
    But then why do 12v boards exist ? After all 5v power supply are cheaper. So for my 12v board I will need two different power supply ? Or can I use the same one ?
     
  18. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    For the 12 Volts board and the motor is also 12 Volts, the same power can be used.
    You can connect the 12 Volts to the contacts of the relays, to control the motor power.

    In post # 28 @Harald Kapp showed how to connect the motor using two relays, for bi-directional control.

    Bertus
     
  19. Goldhelmeth

    Goldhelmeth

    30
    1
    Apr 8, 2020
    I don't understand I thought you told me that the supply that powers the board cannot power the motor through the channel because it is only a switch... I tried to apply his schematic of the motor connected to the relay and I made this ... this thing.... Will it work like that ?
     

    Attached Files:

  20. bertus

    bertus Moderator

    541
    179
    Nov 8, 2019
    Hello,

    Can you get this?

    relays_board_motor.jpg

    Bertus
     
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