Relay Circuit

Discussion in 'General Electronics Chat' started by amatour, Jul 8, 2005.

  1. amatour

    Thread Starter New Member

    Jul 8, 2005
    I have been playing around with this relay circuit. And have no problems when I use a 3 amp relay. But when I use a 12amp relay. I receive errors and have communication problems. I'm thinking that the resistors aren't right or the transistor is miss sized. Any ideas or suggestion or comments will be greatly appreciated.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    I took a look at the datasheet for the 2n4403 and it appears that the device should be able to drive the 12A relay coil. The drive current for the 70 ohm relay coil is about 71 milliamps. The base current established by the 1K resistor is about 4.2 milliamps. That means that the transistor should saturate as long as its beta is 16 or greater. The transistor should be capable of a beta of 30 or better.

    My guess is that the drive current avaiable from your digital control line is not high enough to supply the current needed to the base of the transistor.

    A quick check can rule this explanation out. You should be able to temporarily disconnect the 1K resistor from the digital IO control and connect the 1K to ground. If the relay pulls in then the digital IO will need to be buffered or you will need to look for a transistor with a higher beta than the 2n4403. If the relay does not pull in then you need to double check your wiring or the relay coil resistance to make sure it is 70 ohms or greater.

    If you have a second 2n4403 you may want to try it in the circuit in case the one you have is not meeting the spec.

    Another thought: You need to make sure that the +5V power source you are using to power the relay is not being overloaded and dropping out of regulation when you activate the relay.

  3. n9xv

    Senior Member

    Jan 18, 2005
    Eleminate the 10-K resistor and try a 1.8-K resistor inplace of the 1-K resistor.