Hi,

I found a post on this forum (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CEkQFjAA&url=http%3A%2F%2Fforum.allaboutcircuits.com%2Fshowthread.php%3Ft%3D71159&ei=h7SYU7C4KMePyATL0oCIBg&usg=AFQjCNH25cG0gKZzCizE1zAINozXLXRBzA&sig2=RP9IwU6xpn7iNhCGiz0W1g) that I've been following to make a relay circuit using an MSP430G2553 MCU and a 12VDC relay for 120V circuits. My problem is the transistor appears to be on, even when the input voltage from the MCU is zero. There is always about .7V on the base of the BJT and about 8-9V on the collector (NPN). I'm not sure why. Here is my schematic:

 

Look at using a FET, e.g.2n7000.
Max.

 

But why isn't the OP's circuit working?

I don't see the problem.

 

Disconnect the 2k res and see what happens?
Max.

 

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There is always about .7V on the base of the BJT and about 8-9V on the collector (NPN). I'm not sure why.

Two possibilities: Either the voltage at the left of the 2k resistor (RELAY EN)is not zero or R8 doesn't go to ground.

 

Are the grounds connected to the MCU ground?

 

Is the 10K really needed to keep it low? That is the only difference I see from how I normally drive relays with an NPN. I often use 2n2222's with a 1K between the base and the output of whatever I am using to turn it on.

 

Maybe your transistor connections are incorrect, especially if you are using 2N2222. There are two different lead identifications for 2N2222.

 

There is a possibility that the transistor is shorted ( my be the fly wheel diode is badly connected). 0.7 volts and a 2 K resistor to the base will never trigger the the transitor.

Check with attention all your connection, and replace the transistor with a new one. Make sure that the diode is connect directly accross the coil terminals of the relay!

Cheers

Alberto

 

So the output on RELAY_EN is 3.3V so I have a voltage divider to get the required current on the base of the BJT to put it into saturation. I was leaning towards something is shorting the BJT to force it into saturation, but I checked the diode and it has 12V across it. The ground is the same ground as the MCU. I will try removing the 10k resistor like a couple are suggesting. Thanks for your help!

 

Leave it in place and disconnect the 2k to see if it turns off completely.
Max.