Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: pterrus on February 10, 2012, 12:27:04 PM

Title: Losing voltage across a MOSFET
Post by: pterrus on February 10, 2012, 12:27:04 PM

I put together a small IR board for a line tracker robot, and my idea was to provide an enable for it so that the emitter diodes aren't always drawing current.

I'm using a 2N7000 (http://www.fairchildsemi.com/ds/2N/2N7000.pdf), with the drain connected to 5v, the gate connected to an I/O pin (the enable), and the souce connected to the positive terminal of the IR board.

The problem is that when I pull up the enable, there's still a voltage drop of over 2V from the drain to the source, leaving only 3V or so for my circuit.  The gate votage is at over 5V when this happens, well above what the threshold voltage is supposed to be for the 2N7000.  What am I doing wrong?

Title: Re: Losing voltage across a MOSFET
Post by: newInRobotics on February 10, 2012, 01:17:09 PM

Hi,

this all happens because You use N-ch MOSFET as high side switch, it should be used as low side switch. Connect Positive side of IR circuit to 5V, GND side of IR circuit to Drain of MOSFET and Source of MOSFET to GND. Don't forget to add pull-down resistor between MOSFET's Gate and GND.

Title: Re: Losing voltage across a MOSFET
Post by: pterrus on February 10, 2012, 01:19:57 PM

Ah, I knew there had to be something basic I was doing wrong.  Thanks!