BS1 switching 2N2222 Relay Driver

Hello,

I have a BS1 connected to a 2N2222 relay driver transistor's base through a 390 ohm resistor. The emitter is grounded. Vdd goes to the relay coil and protection diode and their other ends go to the 2N2222 collector. I measured the current at the emitter and got 40ma. after holding the relay closed for about 10 seconds, the 2N2222 is so hot it could give one a blister. Is this normal? Thanks.
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  • 7 Comments sorted by Votes Date Added
  • edited May 2012 Posts: 0Vote Up0Vote Down
    What kind of relay do you have? What's its coil voltage and resistance? Are you sure everything is hooked up as you've described? The transistor shouldn't get that hot.
  • Chris SavageChris Savage Parallax Engineering
    edited May 2012 Posts: 0Vote Up0Vote Down
    If the protection diode is connected wrong it could cause excessive current to be drawn across the transistor. Please refer to the following diagram for the connection. Not sure where this image came from, but it does show the general connections for an NPN transistor driving a relay. The component values can vary.

    diopro.png
    Chris Savage
    Engineering Tech, Parallax Inc.
    (916) 624-8333 x3005
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  • edited May 2012 Posts: 0Vote Up0Vote Down
    Jeepers, Chris is right. Is your protection/flyback/freewheeling/snubber/suppressor/catch diode getting hot too?
    You'll find me in the new Robotics forum.
  • edited May 2012 Posts: 171Vote Up0Vote Down
    Turns out it was my error. On my powered proto board, I had incorrectly applied 15 vdc to the circuit, instead of 5 vdc! One of those nights. With 5 vdc applied, the transistor does not even get warm. Thanks for all of you replies.
  • edited May 2012 Posts: 0Vote Up0Vote Down
    Functional circuit plus 3X design voltage = space heater! :)

    Glad you got her going without releasing the magic smoke (hopefully).
    You'll find me in the new Robotics forum.
  • edited May 2012 Posts: 0Vote Up0Vote Down
    Something to consider,,, the 2n2222 has a worst case hfe rating of 30 ... The hfe is basically the current gain ratio between the B-E junction to the E-C junction.

    If you have a 5V supply off of an I/O going through a 390 Ohm resistor to the base of the transistor, then the B-E current is going to be about 11.3mA

    I = (Vsupply - 0.6V) / {Base resistor} = (5V - 0.6V) / 390 = 11.3

    With an hfe of 30, this means that the transistor can only saturate and deliver 339mA. As long as your load doesn't pull more than 339mA you should be fine, but if your load does pull more than that, the transistor won't be able to saturate, and will operate in it's linear mode giving off heat.

    Note: 30 is a worst case scenario and is load dependent, so it could be much better than 30... Still, always calculate the hfe for the worst case scenario unless you specifically hand pick the hfe on the transistor by physically measuring it before hand.
    "Irony hides in the deception of your current view of the situation" - Me ....
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