HELP! backfeed or something!

[yoobi1]

Hi Folks,

I'm a total newbie with electronics, but have been make doing so far until now.

I have two IRF740 MOSFET's controlling a string of LEDS (depicted with single led) one set is blue the other is red.

I dont understand why but there is some backfeed in the circuit where both strings are always on regardless to the the MCU pins being high or low, this even occurs when the circuit (as per attachment) is totally disconnected from the micro.

I'm running out hair to pull out, please help!

I know it's something really basic but i don't see it!!!

thanks in advance.

 

[Gary B]

The MOSFET is drawn as an enhancement type. Try a pull-up resister to make sure the MCU pins aren’t just going from active to open and back. If you have the drawing or specifications and can verify that the pins are not open collector type, then that is your problem.

 

[MikeMl]

According to this plot from the data sheet, you will have trouble turning them on. At 5v Vgs, they are just barely beginning to turn on. These are not "logic-level" FETS!

**broken link removed**

What is the Vdd for your MCU?

Try putting a 10K resistor from gate to source with the MCU disconnected. If they are still on, they are blown, or hooked up wrong.