Output voltage at Nmos

Hi,

I am using Nmos IRFZ44N and STP36NF06L as a switch, opening and closing a circuit with 5 volts power suply, a 82R resistor and two Red Leds in parallel. But, I can't undestand why when I use it physically, with gate connected to ground (i.e. OV in the gate) the voltage drops are:

-OV in the resistor (Obviously, no current driving)
-10mV at the LED
-3.6 V from drain to source (which is connected to ground)

Where are the other 1.4V???

Any point will be welcome.

Thanks.

 

[Phil Hobbs]

On 11/23/2012 09:28 AM, d12122749@mydit.ie wrote:

Hi,

I am using Nmos IRFZ44N and STP36NF06L as a switch, opening and closing a circuit with 5 volts power suply, a 82R resistor and two Red Leds in parallel. But, I can't undestand why when I use it physically, with gate connected to ground (i.e. OV in the gate) the voltage drops are:

-OV in the resistor (Obviously, no current driving)
-10mV at the LED
-3.6 V from drain to source (which is connected to ground)

Where are the other 1.4V???

Any point will be welcome.

Thanks.

Your voltmeter draws a small amount of current, so in order to measure
the drain voltage, the LED has to be forward biased a little. Try
measuring from the positive supply downward instead.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Tim Wescott]

On Fri, 23 Nov 2012 12:26:59 -0500, Phil Hobbs wrote:

On 11/23/2012 09:28 AM, d12122749@mydit.ie wrote:
Hi,

I am using Nmos IRFZ44N and STP36NF06L as a switch, opening and closing
a circuit with 5 volts power suply, a 82R resistor and two Red Leds in
parallel. But, I can't undestand why when I use it physically, with
gate connected to ground (i.e. OV in the gate) the voltage drops are:

-OV in the resistor (Obviously, no current driving) -10mV at the LED
-3.6 V from drain to source (which is connected to ground)

Where are the other 1.4V???

Any point will be welcome.

Thanks.

Your voltmeter draws a small amount of current, so in order to measure
the drain voltage, the LED has to be forward biased a little. Try
measuring from the positive supply downward instead.

Or to put it another way, the node where the source and LED cathode come

together in floating when the FET is off, and will float wherever the
voltmeter pulls it.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com