can we use 3.3V Vgs to turn on/off 2n7000 mosfet. I am using mosfet as a switch.

1. datasheet shows that vgs(th) of 2n7000 is 2.1V min.

2. I had given 3.3V from MCU to gate & source is grounded but mosfet is not turning on.

3. Circuit is to drive a relay as we normally use npn transistor . But I had replaced with 2n7000.
No success till now

 

Hello,

In the datasheet is given that the Vgs should be at least 4.5 Volts for switching the fet on:

Bertus

 

hi aamirali,
As you say the d/s indicates a threshold of approx 2Vgs, which will give a Drain/Source current of approx 100mA at 3.3Vgs.
What is the relay coil specification.? and do you have other 2N7000 on the bench that you could select from,?
E

 

If the Vcc for relay is 5V then you still got the chance to drive the mosfet using 3.3V.
The idea as below, you can try it :
3.3V → (-)Diode(+)(-)Diode(+) → Vgs (2N7000)
Vgs adding a 2.2K ~4.7K to +5V.
The diodes are using 2 1N4148 in series.
If it doesn't work then please describe the status.

 

If the Vcc for relay is 5V then you still got the chance to drive the mosfet using 3.3V.
The idea as below, you can try it :
3.3V → (-)Diode(+)(-)Diode(+) → Vgs (2N7000)
Vgs adding a 2.2K ~4.7K to +5V.
The diodes are using 2 1N4148 in series.
If it doesn't work then please describe the status.

Despite the stated gate threshold figure, you probably need about 5V to guarantee the full headline drain current/RDSon.

With a bipolar output, you could also be worried about Vo-low.

A common trick is to tie an external resistor either from Vdd to output or from output to Vss, depending whether the output doesn't go high enough or doesn't go low enough.

 

Despite the stated gate threshold figure, you probably need about 5V to guarantee the full headline drain current/RDSon.

Do you have any reason when the Vds is 5V, the Vgs must need 5V?
The typical Vgs = 4.5V.
When mcu output a 3.3V then Vgs = 3.3V+(0.65V*2) = 4.6V, so 4.6V>4.5V, that will be enough for Vgs.

With a bipolar output, you could also be worried about Vo-low.

The typical Vgs threshold voltage is 2.1V.
A mcu output 3.3V then I will assuming that is CMOS type, the output low level close to 0V, two diodes voltage equal to 0.65V*2 = 1.3V, so 1.3V<2.1V, and the OP mentioned that when the mcu ouptut a 3.3V still can't turn on the Rds, so I think 1.3V as a low level is safe for the Vgs(th).

A common trick is to tie an external resistor either from Vdd to output or from output to Vss, depending whether the output doesn't go high enough or doesn't go low enough.

Have you ever saw any load as relay was used like that way?

 

Do you have any reason when the Vds is 5V, the Vgs must need 5V?
The typical Vgs = 4.5V.
When mcu output a 3.3V then Vgs = 3.3V+(0.65V*2) = 4.6V, so 4.6V>4.5V, that will be enough for Vgs.


The typical Vgs threshold voltage is 2.1V.
A mcu output 3.3V then I will assuming that is CMOS type, the output low level close to 0V, two diodes voltage equal to 0.65V*2 = 1.3V, so 1.3V<2.1V, and the OP mentioned that when the mcu ouptut a 3.3V still can't turn on the Rds, so I think 1.3V as a low level is safe for the Vgs(th).


Have you ever saw any load as relay was used like that way?

You have to look carefully at the datasheet to see exactly what RDSon and Id you get at just the VGSthr and no more.

If the CMOS Vo-low goes down to 0V, you should have no trouble turning the MOSFET off - I wouldn't rule out maybe needing to assist V0-high with an external pull up resistor.

I never mentioned the relay at all - only the interface between the chip and the MOSFET.

 

You have to look carefully at the datasheet to see exactly what RDSon and Id you get at just the VGSthr and no more.

If the CMOS Vo-low goes down to 0V, you should have no trouble turning the MOSFET off - I wouldn't rule out maybe needing to assist V0-high with an external pull up resistor.

I never mentioned the relay at all - only the interface between the chip and the MOSFET.

I was used 74HC02 to simulate the mcu on breadboard, and to output a 3.3V to drive the Vgs of 2N7000, and it worked fine, the R2 could worked for a big range, I was suggested the R2 use 2.2K ~4.7K on 4#, and it also in the range, you may also to try it, I was used a led and resistor connected to the N.O and C of 5V relay, used led as a indicator when the relay is on then the led will be lighting up.

 

I used 2N7000 to switch directly from 3.3V MCU in several projects, and it works fine.

It won't work if you need higher current.

 

I used 2N7000 to switch directly from 3.3V MCU in several projects, and it works fine.

It won't work if you need higher current.

That's pretty much what I was saying - you don't necessarily get the headline RDSon unless you drive the gate a little bit better than VGSthr.