relay question

Discussion in 'The Projects Forum' started by Macabra, Oct 22, 2008.

  1. Macabra

    Thread Starter Active Member

    May 31, 2008

    I got this 5V SPDT relay driven by a 2N2222 NPN transistor. The relay should switch between 3V and 0V depending upon the state of the output from a microcontroller whose output when high is 3V.

    The relay's characteristics are the following:
    nominal current: 89.3mA
    pickup voltage: 3.4VDc
    coil resistance:56 ohms

    since I'm using 3V my nominal current should then be (3V/56 ohm) = 53.6mA

    and the transistor has a current gain (hfe) of 200 so therefore (53.6mA/200)= .268mA for base current. So, just to make sure it saturates it is doubled to .536mA. Finally my resistor then (the one connected between the base and the output pin of uC) should then be (3/.536mA) = 5600 ohm resistor.

    I've physically tried this out and it does not work when at three volts :confused:. Is there something wrong with my math or the manner in which I'm doing this? The picture attached is exactly how I have it where Vin is the 3V and so is 3V from VCC.

    I appreciate the help :)
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    Your pickup voltage is 3.4V - anything less and the relay won't actuate.

    Your supply is only 3V

    You need more oomph. You need more current through the relay coil.