Transistor or Relay?

jcb344

Hi, I recently built an led lighting system in my room. The led's them self are all individually controlled and connected with a common cathode. Because of their power demands I cannot drive them off of the stamps current and voltage, thus I need to use a transistor but not a BJT. I tried using a mosfet transistor to switch the lights on and off but I didn't realize that mosfets can only switch to ground which does not help me control lights with a common cathode. Is there a way to get around this problem?Possibly a diferent type of transistor, or am I going to have to use an expensive solid state relay for my application?

-Jacob

Mike Green

Try using a PNP transistor. These work opposite to an NPN transistor. The emitter is connected to the positive supply, the collector is connected to your load (the LED) and the base is connected through a resistor to ground to turn it on. There are P-channel MOSFETs which are like N-channel MOSFETs except that the source is connected to the positive supply, the drain is connected to your load, and the gate is grounded to turn it on. You would need a resistor (like 100K) between the gate and the source (positive supply) to hold the MOSFET in an off state.

Here's a P-channel switching MOSFET from Jameco: www.jameco.com/Jameco/Products/ProdDS/256057PHILIPS.pdf

Post Edited (Mike Green) : 4/7/2008 5:56:09 PM GMT