Author Topic: Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?  (Read 13815 times)

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Offline GrahamTopic starter

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Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?
« on: September 04, 2009, 06:09:11 PM »
Hi there,

Im new to MOSFETs but thought I understood them until building my own (very simple) test circuit.

I have connected a 9V battery positive terminal to the drain of a STP36NF06 N-channel power MOSFET. The source has been connected to an LED, then to a 100ohm resistor, then to the battery return. The gate is supplied with 5V with a voltage divider using the same battery (and resistors in the range of 100ohms). When the gate is supplied with 5V, I would expect the diode to light with current supplied from the drain-source path, but I have to supply the gate with the full 9V for this to happen. Then when I remove the gate voltage, the diode stays lit. If I then supply the gate with 5V (or anything less than about 9), the diode turns off!

Please can someone help!

Also, the voltage across the Drain and Source pins is 7.1V when the diode is off and 3.8V when the diode is on!! I thought this hsould drop to close to 0 when the diode is on as the MOSFET has been turned 'on'.

Again, any help would be most appreciated.

Graham

Offline billhowl

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Re: Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?
« Reply #1 on: September 04, 2009, 07:05:31 PM »
Mosfets have 3 regions of operation that we will concern ourselves:

The first is cut-off mode, when the mosfet does not conduct.

The second is the saturation mode, when the mosfet is switched on and is conducting with a very low drain to source resistance.

The third region is in between cut-off and saturation, and is known as the triode region. This is the region where the mosfet begins to conduct, but has a high channel resistance.

There are 3 key voltages that will determine what mode the mosfet is operating in. These are:

VGS, the voltage from the gate to the source, taken with the gate as positive and source as negative

VDS, the voltage from the drain to the source, taken with the drain as positive and the source as negative

Vt, the mosfet’s threshold voltage. This value is a device parameter, and is roughly given in the device’s data sheets. (Base on the Datasheet of the STP36NF06 MOSFET VGS is 10V to get saturation mode VDS is 25V and Vt is 2V)

For an N-Mos device:

Cutoff when VGS < Vt
Triode when VGS > Vt, and VDS < (VGS – Vt)
Saturation when VGS > Vt, and VDS > (VGS – Vt)

For you case here, you are using 9V supply so the MOSFET is in the triode region therefore have high channel resistance which the voltage drop in the Drain and Source is high.

Read this article for more detailed info on MOSFET
http://roko.ca/robotics/h-bridge-fundamentals/4
Datasheet of the STP36NF06
http://www.st.com/stonline/products/literature/ds/9619.pdf






Offline Soeren

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Re: Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?
« Reply #2 on: September 04, 2009, 09:06:02 PM »
Hi,

Remember a resistor from gate to ground. MOSFETs gates are high impedance, so won't turn off if left floating.
Use a Logic Level MOSFET when you work with such low voltage. Regular MOSFETs need 7V..10V gate voltage to saturate.


billhowl <- something tells me you don't really understand what's going on with MOSFET?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline billhowl

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Re: Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?
« Reply #3 on: September 05, 2009, 12:17:43 AM »
billhowl <- something tells me you don't really understand what's going on with MOSFET?

You get it right, I have not work with MOSFET. My order is on the way, will try it once I get it.
« Last Edit: September 05, 2009, 04:56:55 AM by billhowl »

Offline Joker94

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Re: Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?
« Reply #4 on: September 05, 2009, 05:25:32 AM »
For a h-bridge do you just use N channel mosfets as that is all i can find in the logic level range.

Offline airman00

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Re: Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?
« Reply #5 on: September 05, 2009, 02:39:18 PM »
For a h-bridge do you just use N channel mosfets as that is all i can find in the logic level range.
Usually you boost up the voltage to a higher one and then control the MOSFET. We've done some designs at BugLabs(the place I worked this summer) where we used a MOSFET driver that boosted our 5V signal to a 12V signal in order to switch MOSFETs. MOSFET drivers exist that do this.

Also, the higher the voltage , the less current the MOSFET takes and the cooler ( less hot) it runs.
Check out the Roboduino, Arduino-compatible board!


Link: http://curiousinventor.com/kits/roboduino

www.Narobo.com

Offline Admin

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Re: Beginner's MOSFET: Why is my Drain-Source volt drop 3.8V?
« Reply #6 on: September 06, 2009, 08:45:57 AM »
Look in the MOSFET datasheet, towards the bottom, and you'll see a gait voltage vs source-drain resistance chart. Or something to that effect . . .

 


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