Optocouplers and Active Low vs Active High

Discussion in 'Embedded Systems and Microcontrollers' started by klangst, Aug 12, 2014.

  1. klangst

    Thread Starter Member

    Jul 5, 2013
    I am working on a project to actuate some moderate amperage relays (30VDC 10A). I can select relays that have either 5V or 12V coils. I am going to be using a PIC24 which has output of 3.3V (on most pins).

    My question is should I include a optocoupler to seperate the MCU output voltage from the voltage to control the relay coil?

    Also, I have been looking at some setups which would actuate the relay coil based on a Active Low input. Is there an advantage to using Active Low versus Active High?
  2. MaxHeadRoom


    Jul 18, 2013
    You could probably use 2n7000 to operate the relays, don't forget the BEMF diode across the coil.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    For any given relay series a device with the higher voltage will require less current then a lower voltage device. It's a power thing, so I'd go with the 12V units for the lower current.

    Another trick is to look at the minimum pull in voltage for the series: oft you can go up one voltage (9V instead of 5V or 15V instead of 12V) and still guarantee proper operation, but use a bit less current. Quite a bit less when using 9V over 5V.

    No real reason for it if you use catch diodes (essential anyway), proper supply bypassing and good layout where the relay power and the CPU power only join at a single point.
    Either way is fine. What is important is to select a device that keeps the relay off when there is no input.

    Why? When a PIC first turns on the I/O pins are all set to inputs, so they act as open circuits to anything outside expecting to be driven. Now for relays the time between power on and outputs active is typically very small compared to the time to change the contacts, but you don't want a big slug of current as they all try to turn n at the same time.