randim Posted January 19, 2005 Report Share Posted January 19, 2005 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 Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted January 19, 2005 Report Share Posted January 19, 2005 Tr1 can be:BC337BC338BC547BC549BC550BF483BF485BF4872N2222A2N3904Any low-power NPN transistor will work.D1 is any low-power diode rated to more than 30mA Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 20, 2005 Report Share Posted January 20, 2005 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. Quote Link to comment Share on other sites More sharing options...
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