[novice] MOSFET not turning on

[Mark Herhold]

Hi everyone,

I am having an issue with a simple circuit using an n-chanel mosfet I purchased. Datasheet: STD12NF06L

I have two voltage sources, one 3.3v from my micro, and one 5v (for testing - it will be 10v later), connected via ground.
I can get my LED to turn on slightly if I touch the mosfet gate pin to the 5v line, so my fear is that my VGS voltage is too low (3.3v) for this mosfet, but need confirmation that this is indeed the issue.

I picked this mosfet because I thought that a VGS of 3.3v would suffice since on the Figure 5. Transfer characteristics chart, it is about in the middle of the curve and would cary FAR more amps than I would need (I will only need 1A max). I am not sure where I went wrong.

 

[Minder]

Why not select a Logic level Mosfet?
M.

 

[Mark Herhold]

I honestly thought this was one. I thought the voltage threshold of 2+ volts as on the transfer characteristics chart would suffice with a 3.3 v source.

Can you recommend one or explain what spec in the datasheet tells me that this could not have worked?

 

[Patrick123]

The Vgs threshold (i.e, the number that you're quoting), is the voltage required between the gate and the source, in order to achieve the listed threshold current of 250uA. So that's essentially the voltage where the transistor starts to turn on. However this is just the start of the turn on. These devices aren't just on/off, they can be any resistance between fully on and fully off. At this min voltage, it's barely (i.e. 2V/250uA = 8k on resistance!). You have to give it more voltage than the threshold to get it fully on.

Another point, is this is the voltage between the gate and source, not the voltage between the gate and ground. In your circuit, the source isn't tied to ground, but instead goes through a resistor and an LED. The LED and resistor are going to drop some voltage and this takes away from the voltage between the gain and source. So if you drive it with 3.3V and the led drops 1V, you've only got 2.3V of Vgs left. As soon as some current starts to flow the resistor drops even more voltage and there's barley anything left to turn on the mosfet.

What you really should do, is put the led and resistor on the drain side of the mosfet and ground the source. That way the full 3.3V will appear across the Vgs junction. Also you don't need a 10k resistor in series with the gate, 100-ohm would be fine

 

[cjdelphi]

An NPN would struggle in this circuit after the voltage drop

Yup switch the side of the LED, but in this instance an NPN is better than a mosfet

 

[dorke]

The 12NF06L is a 60V/12A beast, why would you use it to turn on a tiny LED?
What kind of LED are you using(color and mA rating)?

Do what @Patrick123 suggests :
Move D1 and R2 to the Drain and connect the source directly to GND.

BTW,
R2 seems a bit to high.

 

[Mark Herhold]

Thanks all. Putting the load on the drain side fixed my issue. It makes sense hind-sight... Cheers!