Driving Relays with 3.3V signal from microprocessor

Discussion in 'The Projects Forum' started by donaldm444, Oct 25, 2011.

  1. donaldm444

    Thread Starter New Member

    Jul 16, 2010
    I am looking to drive electromechanical relays (3 V and 100 mA to turn on) with an Atmel microprocessor. The micro output pins give 3.3 V @ 8 mA, so I looking for a suitable driver to use.

    I am considering the ULN2803A. I will be driving 9 relays (not simultaneously) with the micro.

    If anyone has any suggestions I would greatly appreciate it.

  2. Externet

    AAC Fanatic!

    Nov 29, 2005
    The micro output pin can drive a transistor to saturation and energize a relay.
    donaldm444 likes this.
  3. 19900110

    New Member

    May 20, 2009
    If design itself is not that critical, then you can obviously saturate a BJT to get enough current level. However, you need to concern about the beta of the BJT. The current you get from saturation is approximately beta X base current (output of the micro-P).

    If you are really serious about the design, I suggest a compatible driver. I am using a Futaba servo and the suitable driver in listed on the servo datasheet. I believe you can find the same thing in your relay datasheet.
  4. crutschow


    Mar 14, 2008
    You do not use the data sheet beta value for determining the base current required for saturation. That beta is only for small signal AC applications. To make sure a transistor is well-saturated you should use a beta value of no more than 10-15.

    The ULN2803A should work fine for the OPs application.