Using a 2N7000 as a switch

[Andres Rodriguez]

I am trying to use a 2N7000 FET as a switch and have my doubts about the gate saturation voltage. The datasheet states that the max gate saturation voltage is 3V (min is 0.8V). With an output pin high connected the the Gate I am trying to use a voltage divider. I also want to turn on an LED by incuding it in the divider. Will this work? (See attached schematic)

 

[Peter M]

The max Vgs voltage is +/- 20v continuos according to the data sheet. The MAX gate threshold voltage is the maximum voltage you should need to switch it on to conduct 1mA (Id)(test condition).
http://www.fairchildsemi.com/ds/2N/2N7000.pdf
2.1volts is what is typically needed to switch it on at this condition.
These specs are measured over a sample batch and are the typical, min , and max figures reported from that. Not all 2N7000's are created equal, like us where all the same, but different.

Basically your circuit should work, but you haven't actually created a voltage divider with the two 330 ohm resistors as the input to the FET is high impedance, this will limit the current if the gate punch's through though.
I hope you have included a pull up or pull down resistor on j1 for the input to pin2 (leg 5) so that its not floating
Good luck

Sounds like even a few variations on data sheets from later posts so added the one I used

 

[premelec]

I'd add that a good look at the 2N7000 data sheet could help - the Vgs is the threshold voltage and this is different than 'saturation voltage' which applies to bipolar transistors.

Vgs varies with current thru the FET etc... you might just want to take the empirical approach seeing what works and simply protecting the 2N7000 from too much current or voltage spikes... they are cheap if you miscalculate...

 

[Dippy]

I think you (Marmitus) should maybe have a look at webpages concerned with MOSFET/FET spec and use. Loads of people misunderstand it so you're not alone

I assume you think Vgs = Voltage Gate Saturation?
No.
There is no saturation like BJTs (bipolars) just as said by Premelec.

The 'threshold' voltage is an agreed spec which means the Gate Voltage (wrt Source) required to permit a current of 250uA to flow. You'll see this on all MOSFET transistor Data Sheets. It does NOT mean that is the maximum permitted voltage at the Gate. And it does NOT mean the threshold at which the MOSFET acts like a 'perfect' switch.

In fact, it is clearly shown in the Data Sheet that a Vgs of 4.5V results in an Rds(on) of 2 to 5.3 ohms (in the ST Data Sheet).

The absolute MAximum Gate-Source voltage is +/- 18V as shown in the Data Sheet.
So, you don't need a pot/div. And as you haven't got one in your circuit that's fine.

To determine the Gate voltage required you should look at the graphs. Example "Output Characteristics" in the Data Sheet.

They say Gate-Source Voltage (Vgs) as you should have the MOSFET source connected to Ground for a simple switch like you want. Therefore everything is referenced to Ground (0V).

I would assume you will be switching tiny currents so your circuit will be fine, though I don't know what happens after your connector.... so it may not.... remember your circuit will act as a low-sided switch (switch to ground or switch-to-lower-voltage-line).

For greater consistency generally you can get MOSFETs which are better for switching at 5V - Logic Level.

A MOSFET is not a general purpose relay substitute. When used properly it can replace a relay but you can't connect power any-old-way-round.

 

[gengis]

As already pointed out, the current through the mosfet determines what voltage you need to bias it on.

Try it and see. I tried to use a mosfet for my camera shutter and it wouldn't work. Tried several and couldn't get even the LL mosfets to work.

Something hinky inside that camera . . . it absolutely wants to go to ground at less than point six volts, and has a very high current - suggesting a capacitor inside the camera that has to be discharged rapidly - probably their idea of switch denounce for the manual shutter button.

A small signal NPN direct coupled to the shutter button with a 10K pull-up worked. I'm using a Saitek PocketCamX. Also used a Saitek MegaCam 1.3 megapixal and it was able to work with a mosfet and capacitive coupling.