Newb needs help!

[randim]

Hi everyone,

I'm trying to control a relay using a 68hc908 Microcontroller.

My relay draws 0.28A at 12V while my Microcontroller gives only 5V.

I'm thinking of using a seperate power supply for my relay and the 5V of the microcontroller to control the relay.

My question is, which type of transistor can i use as a switch for the relay... Any suggestions of common transistors that could be used??

Do I need Resistors or anything to protect the microcontroller from getting fried from return current... Or can i just use the transistor directly with the power supplies and the relay??

I'm not sure where to start any help would be great!
Thanks

 

[Alun]

Tr1 can be:
BC337
BC338
BC547
BC549
BC550
BF483
BF485
BF487
2N2222A
2N3904
Any low-power NPN transistor will work.

D1 is any low-power diode rated to more than 30mA

 

[audioguru2]

Hi Alun,
Many of those little transistors have a maximum rating of only 100mA.
Many also have a very low current gain of only 40 with a 288mA collector current, so the value of your base resistor is much too high. I don't know if the microcontroller can output 5mA to drive the base of a transistor. Maybe a darlington transistor or Mosfet should be used.