MOSFET turns on with gate voltage.

Hi all,
i am trying to light up an LED using a NPN MOSFET.
my circuit is something like...
LED(+) is connected to BATTERY 5v(+)
LED(-) is connected to MOSFET source.
MOSFET drain is connected to BATTERY5v (-).
MOSFET gate is kept floating initially, but then too my LED lights up.
i connected my MOSFET GATE to (+) as well (-), but the led does not gets OFF... it stays ON continuously..

gauravkothari23 said:

MOSFET gate is kept floating initially, but then too my LED lights up.

Click to expand...

The led may light if the gate is disconnected from anything. The gate can pick up electrical noise. This includes 50/60 cycle hum which causes the mosfet to turn on and off at mains frequency, so that the led lights halfway bright.

It's a good idea to put a safety resistor inline with the led. Did you make the led flash brightly, too brightly? It may be ruined due to overmuch current.

Hi,

LED(-) is connected to MOSFET source.
MOSFET drain is connected to BATTERY5v (-).

Click to expand...

Interchange source with drain.

SOURCE needs to be connected with battery -.

Klaus

BradtheRad said:

The led may light if the gate is disconnected from anything. The gate can pick up electrical noise. This includes 50/60 cycle hum which causes the mosfet to turn on and off at mains frequency, so that the led lights halfway bright.

It's a good idea to put a safety resistor inline with the led. Did you make the led flash brightly, too brightly? It may be ruined due to overmuch current.

Click to expand...

i have even tried adding a pull down resistor.

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KlausST said:

Hi,



Interchange source with drain.

SOURCE needs to be connected with battery -.

Klaus

Click to expand...

Klausst... its a NPN mosfet and i have connected to negative side of battery.
how will it work if i connect the source to battery and drain to LED(-)

- - - Updated - - -

The LED also lights up full brightly.....

gauravkothari23 said:

Klausst... its a NPN mosfet and i have connected to negative side of battery.
how will it work if i connect the source to battery and drain to LED(-)

Click to expand...

:?:you won't follow advise of expert !
your schematic is wrong

Source to - battery
Drain to led - , led + to R , R to + battery
R between Gate and source so, accross your external commande 0V to Source + cde to Gate

give reference of your mosfet ..

1. The device is called NMOSFET rather than npn MOSFET. There's no p semiconductor material involved in the active transistor design, just a p substrate.

2. By connecting drain to negative battery terminal, the substrate diode which is included with any 3-terminal MOSFET is forward biased, shorting the switch.

You didn't yet show a circuit with interchanged source and drain, as suggested by KlausST.