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MOSFET turning completely on

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Los Frijoles

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3v gate- source voltage mosfet

I seem to be having a problem turning on a mosfet completely. I am using a 2N7000 n-channel mosfet hooked up to a logic gate which is controlling the gate of the mosfet. However, the logic seems to be unable to turn on the mosfet all the way. Until the gate voltage gets above the drain voltage, only about half of the drain voltage makes it through and makes it through unreliably (the LED attached to the mosfet flickers in time to the other LEDs on the board until the voltage gets high enough to turn the mosfet completely on). Is this normal? Is there any way to circumvent this?
 

logic level fet 3v

Los Frijoles,
From your description it appears that you are trying to use the FET as a series switch with an LED as a load. An NMOS enhancement mode FET starts to turn on when the gate-source voltage exceeds the drain voltage by an amount equal to Vgsth, the Gate-source threshold voltage. When you apply a voltage to the gate in this configuration, the source "follows" the gate voltage at a value of approximately Vgsth - Vgth.
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It is much easier to turn on a an LED if you use the FET as a "low side" switch, with the source grounded, and the LED connected from the drain to the supply through the current limiting resistor. In this configuration, you only have to supply approximately Vgth to the gate to turn the FET completely on.
Regards,
Kral
 

dot matrix mosfet

use series resistor too to the gate...because even a 2n7000 will have some gate capacitance (?)
 

mosfet turn-on

That seems counter intuitive...what does putting a series resistor to the gate do?

I can't connect the LED to the high side because the mosfet is functioning as a row driver for an LED dot matrix display and the rows are the anodes with the columns as cathodes.

I ended up doing a ghetto fix by simply running the logic behind the gates about .3V higher than everything else. I don't like this arrangement because now the column sink chip behaves strangely and the circuit draws over 200mA with no LEDs on, so there has to be another way.
 

2n7000 3v

You are driving the FET from a logic device and it didn't occur to you that maybe a logic level fet could have something to offer in this application??

It also depends (obviously) on the logic used and therefore the logic levels, but have a look at the data on the drive device that you have selected and think about what the ON region characteristics mean for your application.
 

n-channel mosfet source-follower

Los Frijoles said:
I seem to be having a problem turning on a mosfet completely. I am using a 2N7000 n-channel mosfet hooked up to a logic gate which is controlling the gate of the mosfet. However, the logic seems to be unable to turn on the mosfet all the way. Until the gate voltage gets above the drain voltage, only about half of the drain voltage makes it through and makes it through unreliably (the LED attached to the mosfet flickers in time to the other LEDs on the board until the voltage gets high enough to turn the mosfet completely on). Is this normal? Is there any way to circumvent this?
Click to expand...

Gate-Source thresold voltage for this N-channel D-MOS transitor is 2.5-3V.
So, TTL/CMOS (5V) can drive this MOS only with source grounding.
If you want to use this MOS as Source-Follower you have to get TTL open-collector output with a pull-up resistor to another 8V supply.
NPN transistor emmiter-follower with 74HC' driver are a good replacement!
 

mosfet turn-on voltage

The max threshold voltage for a 2N7000 Mosfet is 3V and its drain current is only 1mA. The threshold voltage shows when it is almost turned off, not when it is turned on.

It is spec'd with a max on-resistance of 5.3 ohms when its gate-source voltage is 4.5v and its max on-resistance is 5 ohms when its gate-source voltage is 10V.

If it is a source-follower driving a grounded LED then it will barely turn on with only 5V on its gate.
 

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