Testing a MOSFET.

Development Boards Arduino
1

I’m learning about MOSFETS right now. I have one I am experimenting with:

http://www.jameco.com/webapp/wcs/stores … 669943-1

So I have a breadboard started:

http://i.imgur.com/qUfsT2H.jpg

http://i.imgur.com/XQZiKJG.jpg

http://i.imgur.com/hfJpVU2.jpg

So right now I am running 2V and 0.01 amps through the gate pin as my power supply is telling me. My multi meter probes are on the drain and source and I don’t get any reading yet for volts or amps across the drain and the source.

I’m very new to Mosfets and I am having a bit of trouble understanding what the datasheet is trying to tell me. Am I not giving enough power to the gate? From my reading of MOSFETS I think I understand that I have to power the gate and then a circuit can be created across the drain and the source by using the drain and the source as a positive and negative lead where the MOSFET becomes the power source.

Thanks for any help on this…

2

Can’t read, pictures too big.

3

I adjusted the size of the images. Hopefully things are readable now.

4

A diagram or schematic would be nice. I can see a resistor in series w/the FET but the rest would be guesswork.

A FET is analogous to a BJT (‘typical’ transistor) except that the gate-source voltage controls the current flow (drain-source). The gate impedance is very high. A BJT is controlled by the current flowing base-emitter and so current must flow through the base. Negligible current flows through the gate.

So if you have +2v from gate to source (>Vgs threshold) then the resistance between the drain and source should be small … thus allowing current to flow through the resistor, provided you have applied a voltage across the resistor & FET. IOW the FET is acting as a switch, open or closed via/Vgs.

I don’t know what you mean by “where the MOSFET becomes the power source”.

5

You are misunderstanding what the FET actually does. Though Mac is right of course, you may not understand those terms yet. Please see the bottom of this post.

From what I can see in the first image, you have the big resistor as a load. This should make the resistor get warm. Do you have the supply set to 2.1V? I think I already answered that question, but it seems to that you set the power supply to 2.1V. Common supplies are variable voltage. Voltage does not get pulled. So it’s not pulling 2.1V, you are pushing 2.1V. Current pulls it’s power. So if you have a 5A motor, you can’t push the 5A to the motor. The motor will only use what it needs. So, to determine what voltage the Gate needs, you need to look at the datasheet. If you can post what the model the FET is, we can tell what that voltage is.

The Fet is just a “switch.” The gate turns off or on the Source to Drain. So let’s say that your Gate voltage is supposed to be 5V, then you would need to provide 5V to the Gate to “close” the switch. Think of the Gate as the stem on a toggle switch but instead of using your finger to flip a switch, you are using voltage to switch it.

So, for you to see action when you apply the 5V, you need to have another circuit on the Source and Drain. You can experiment this by using a regular LED and resistor.

As you can see, the resistor labeled “Load” is simulating load. So if you want to turn and off an LED, just place a LED in series between the 12V source and the resistor. Don’t be worried about the 12V source, it could be any voltage (please do not use the house mains).

Note the green “Net” labeled "Arduino GND. This is telling you that the Drain on the FET needs to be connected to the Arduino’s Ground pin.

So once you apply 5V (this could be a OUTPIN on an Arduino and set to HIGH to provide the voltage source), your LED will turn on (and off if you set the pin to LOW). This is how we switch high current/voltage loads that an Arduino can’t handle on it’s own.

The 1K resistor on the Gate pin protects the Arduino from damage.

*This is an important note to remember. If you are building a circuit with an Arduino, Grounds must be connected.

You can replace an Arduino for whatever microcontroller you are using. If you don’t have a controller, you can just apply 5V from a supply.

You can also power everything from one supply source.

So if you have +2v from gate to source (>Vgs threshold)…

Vgs = the voltage to fully turn on the FET. Different FETs have different Gate voltage.

the resistance between the drain and source should be small

He is just telling you that the "switch" is toggled on. That any amount of current can pass through it. *Note that "any" is not a statement to use +20A. The FET can only handle so much current before it starts heating and can burn your fingers. This is where a heatsink comes in.* The datasheet tells you the maximum ratings.
6

The TIP120 is a Darlington BJT, not a MOSFET.

7

Not completely clear, but it sounds like you want your MOSFET to power some kind of load. In this application the MOSFET isn’t really a power source, but acts as a switch between your load and the power source.

The attached schematic shows N-channel and P-channel MOSFETs in typical load switching applications. Usually, an N-channel MOSFET is used as a low-side switch (on the low side of the load), and a P-channel is used as a high-side switch. There are other configurations, but these are the simplest.

https://forum.sparkfun.com/download/fil … &mode=view

From your photo it looks like you only have a power supply connection to the gate of the device and your volt meter attached across the source-drain. This is essentially like putting your voltmeter across a switch and flipping the switch on/off. If there’s no voltage source attached, you’re not going to see anything change.

You need to configure a circuit to look something like the N-Channel low side switch circuit in the attached schematic. DO NOT connct your power supply directly across the source-drain. You need to have some kind of load between the drain and the power supply (e.g. resistor+LED).

The voltages in the attached schematic are just examples. You should be able to safely use 5V for both the supply and gate voltage so you only need a single supply.

8

leon_heller:
The TIP120 is a Darlington BJT, not a MOSFET.

It was an example. I didn’t make that schematic just for this post…

I wonder how many more people will post the same thing Mac and I did.

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codlink:
…I wonder how many more people will post the same thing Mac and I did.

Not trying to step on toes. Thought it may be helpful to hear some of the same info in a different way. Sometimes that’s what it takes for me to ‘get it.’ …but then again, I’m kinda simple. :think: :wink:

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lol, I was just being sarcastic. It’s rare to find this many people on one forum that can help someone. I’m pretty sure that someone else here can tell it in a different way.

Personally, I would prefer to learn by hands on. So, for example, reading Mac’s reply, his explanation really confused the hell out of me and I know how a FET works (no offense Mac).

11

All good posts. Thank you for those replies.