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Sunday 23 March 2014

Room Management System – Relay shield discovery


I just made a discovery about the available relay shields. If you remember my article about adding the relays to the system, where I was looking also into the possibility of using one of the ready made relay shields instead of putting one together yourself? Well, I just discovered, that most of the relay shields on the market are negative switching, meaning the activate in input LOW which is not of great use for this project since we programmed everything on activating while the output is HIGH. We could change the software I guess but that would mean to have a permanent current going through the output pins of the shift register while nothing is switched on. Personally, I opt for having as less current flow as any possible while nothing is used even we are talking only about Milliamp s. Calculating it together, 16 relays and a few indicator LED's, we are talking about roughly about 1,5 to 2 Amps.
To solve this problem, I will keep on looking for a relay shield which is switching on input HIGH. If somebody knows of one with relays rated 240V and 10A, pleas let me know. Meanwhile I added the schematic of a 4-channel relay shield and a image of the PCB of the 8 channel relay shield I am using for my projects.

 


Above an example of a 4 channel relay shield. For 8 channels just keep on adding relays with there respective components (diode, transistor and resistor). The LED's are optional and only indicating which channel is activated or indicating the presence of the 5V power supply.


Above is the PCB layout I am using for my projects. If you like to copy it, please check it careful cause it's a double sided circuit board and be aware of the used relays because pin positions may vary.

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11 comments:

  1. Suryansh Jain27 March 2014 at 20:10

    This comment has been removed by the author.

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  2. Suryansh Jain27 March 2014 at 20:14

    Is it good if the relay is connected to the mains (240V), 24x7? Or it will damage the relay....or may create some hazardous conditions?

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    Replies
    1. Unknown27 March 2014 at 22:50

      Es long as you don't run more current through the relay as it is rated for no problem. If it's a big load on a low rated relay you may consider controlling a contactor fitting your load with the on board relay.

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    2. Unknown27 March 2014 at 22:58

      Just a small addition to the above. Adding a fuse or MCB to your load lower than the weakest link in the circuit. Like if you are using a 10 Amp rated relay, putting a 8 Amp fuse in will keep you safe anyway.

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    3. Unknown29 March 2014 at 08:08

      For high voltages, you might consider restricting opening or closing contacts to the time when zero crossings occur to avoid arcing and contact welding.

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    4. Unknown29 March 2014 at 15:11

      I don't think there is a need for it in that scope if the load is kept well within the relay specifications. For high load switching I always would suggest controlling with the on board relay a suitable contactor. In places where lot's of switch cycles occur in a short period of time it definitely is worth a consideration to minimize possible malfunction and to extend the life time of the used relays and contactors.

      Delete
  3. Unknown31 March 2014 at 23:58

    Its easier way to solve that , use SSR Relay.

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    Replies
    1. Unknown1 April 2014 at 19:07

      Considering the 5 volt switching of the Arduino, it is quite a challenge to find fitting SSR relays except if you are happy with a max. 2A switching load. Well, there are some SSR relays around switching between 3 and 15 volts with a switching load of 5A. Personally, I had problems in the past with relays switching within a range when the switching voltage is close to the lower limit. Depending on what you want to control with the relay. A optoisolated relay shield with 8, 10 A rated relays you can get for around 14 Euro s, just over 10$. Suitable solid state relays between 2 and 5A rated we are talking about 9.00 and 20.00 $ per relay. I have room management units working with just standard 5A rated relays for over 8 years now with out any problems and after this amount of time I am happy replacing 1 complete 8 channel relay shield for the price of 1 solid state relay. Controlling some industrial equipment where a whole production stops or in some essential medical equipment I would also go for SSR relays.

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  4. Unknown25 January 2016 at 10:55

    please apply a legend for your fritzing diagram - i would like to build a board of my own.

    Many Thanks
    Cameron

    ReplyDelete
  5. Unknown25 January 2016 at 11:16

    please apply a legend for your fritzing diagram - i would like to build a board of my own.

    Many Thanks
    Cameron

    ReplyDelete
    Replies
    1. Unknown25 January 2016 at 18:22

      All Diodes 1N414B
      Transistors BC 337
      Resistors R1 - R4 1 K
      Resistors R5 - R9 330 ohms
      Relays JRC - 21F
      LED's are 3.4V types

      Alternative
      Diodes 1N4004
      and Transistors 2N2222
      work as well.

      As relay you can use everything with a 5V coil. Just make sure your switch contacts can handle what you want to switch with it.

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