Controlling 120 vac with 5 vdc relay

Discussion in 'The Projects Forum' started by StudyingEE, Jun 25, 2012.

  1. StudyingEE

    Thread Starter New Member

    Dec 3, 2011
    My project is similar to the ones already started, but my question is in regards to placing the ssr on a through hole board with both 5 vdc and 120 vac. The relays I am using are alz51b09 from Panasonic and I am using an MSP430 to control the switching current to the coil. Normally closed relay that opens upon energizing. These are rated for 16A @250 vac and the coil is rated for 9 vdc but can be operated on 5 vdc minimum which is what I plan to do. I am planning to use either a bjt to control the 5 volts to the coil, or an opto coupler to prevent any over currenting of the circuit. How do I connect the 120 vac to the board and the relay without burning up the board or the relay contacts? Am I in the ballpark or somewhere out in left field on this one?

    My ultimate goal is to be able to control individual 115 vac receptacles based on current draw or time of day using a program. Each receptacle is individuly wired with the hot side being switched. The fail mode allows each receptacle to be used, and energizing the coil on the relay disables the receptacle it is attatched to. I am using current transformers to measure the current of each receptacle. The idea is similar to a ground fault interrupt receptacle, but it can automatically be reset from the microcontroller instead of a mechanical reset.

    Any information/suggestions would be very much appreciated.
    Last edited: Jun 25, 2012
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Nope.. Pickup voltage is 70% of nominal...5VDC will not be enough to energize the relay. Use 9V or buy the proper relay..

    The typical reason you need to use a transistor or similar to power a relay is based on the amount of current a micro pin can provide (many can only provide 20mA which is typically not enough to energize a relay.)
    Here is how its typically done.. (don't forget the diode)
    and one (4 channel) with LED's to indicate the relays are on.

    When working with higher voltages on a circuit board you just need to ensure the spacing between traces,etc... is sufficient for the voltage you are using.. (You can google "creepage and clearance distances" for more information and suggestions on sufficient spacing.)
    Typically you would include screw type terminal blocks or some sort of connector and put the relays close to that using traces wide enough for the current you intend to switch and spaced far enough that there are no shorts that develop.
    (personally I'd shoot for NO less than 1.5mm clearance for 120VAC between traces for a DIY project)
    Last edited: Jun 25, 2012
    StudyingEE likes this.
  3. takao21203

    Distinguished Member

    Apr 28, 2012
    I have a small dc/dc converter PCB on eBay which can actually power 12v relays from 5v.
  4. StudyingEE

    Thread Starter New Member

    Dec 3, 2011
    Thank you for the information. With the parts I currently have, I believe it would be best to use a 9 vdc transformer to provide the power I need for the relays instead of searching for new ones at 5 volts. I can use a voltage regulator to obtain the 3.3 volts for the MSP430. Still researching the clearance and creepage so I can mill a board.