Extreme beginner question with MOSFET

Thread Starter

benchaz

Joined Feb 2, 2009
14
Hey guys, I'm pretty new to circuits

I bought a IRLR7821 N-channel MOSFET, but I can't get anything working with it! :(

Here's what i did

i connected an led connected to 0v (ground) in series with a 1k resistor to the drain of the mosfet. the source of the mosfet is connected to +12v, and the gate is directly connected to 0v (ground).
But the led stays on!!!

Help!!! What am i doing wrong??? :confused:
 

jpanhalt

Joined Jan 18, 2008
11,088
First, you have the mosfet connected backwards. The source of an N-channel mosfet goes to ground.

Try this configuration: resistor to V+, other side to LED anode (+ terminal), negative side of LED to mosfet drain, mosfet source to ground.

The mosfet gate to a resistor (100 to 1K). When gate is grounded through the resistor, LED will be off; when gate is connected to 12V+ through the resistor, LED will be on.

John
 

beenthere

Joined Apr 20, 2004
15,819
If you take a look at the data sheet, you will see that hexfets have inherent reverse polarity diode structures. By connecting the source and drain backwards, you forward biased the body diode and got uncontrollable current through the device. Good thing you have the limiting resistor in circuit.
 

Thread Starter

benchaz

Joined Feb 2, 2009
14
wow, thanks!
how about if i want to turn it on. i know you can just connect it with +v, and it works, but is it better to have a limiting resistor there too? what value?
 

beenthere

Joined Apr 20, 2004
15,819
It's more for circuit stability. The resistance in the gate circuit is good anywhere from 100 ohms - 470 ohms. That's just rule of thumb, though.
 

mik3

Joined Feb 4, 2008
4,843
wow, thanks!
how about if i want to turn it on. i know you can just connect it with +v, and it works, but is it better to have a limiting resistor there too? what value?
A resistance on the gate is useful when you switch the MOS on/off at high frequencies and thus the gate capacitance becomes significant and causes some current. However, you can use a resistor when you drive the gate with DC just to eliminate in rush currents through the gate.
Also, connect a 10K resistor between the gate and the source to turn off the MOS (the resistor discharges the internal gate capacitance).
 
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