logic level mosfet switching

Thread Starter

Amit Ika

Joined Jul 4, 2016
10
Hey guys,

I'm somewhat new to circuits and im confused right now. I have a logic level mosfet. Here's the data sheet:

https://www.fairchildsemi.com/datasheets/IR/IRL640A.pdf

Anyways, I am trying to switch 12 volts with this. I am giving the gate a 5 volt signal via an Arduino (and this is for sure. I've measured it many times), yet I am only able to get a source-drain voltage of about 4 volts. What am I doing wrong? How can I increase it to 12 volts without increasing the gate voltage? I am open to using another transistor but I've used a few others and its the same outcome. Does anybody know what the problem could be? Thanks.
 

crutschow

Joined Mar 14, 2008
24,730
An N-MOSFET can only switch between the load and ground.
If you want to switch a grounded load then you need a P-MOSFET and a transistor driver.
 

Thread Starter

Amit Ika

Joined Jul 4, 2016
10
I know this picture is kind of ridiculous but this is essentially my set up. The voltmeter is reading a max of 4 volts when the gate is at 5 volts and the voltage source is 12v.
 

Thread Starter

Amit Ika

Joined Jul 4, 2016
10
Thanks for all the replies. Am I correct in saying that the transistor is in the wrong place? This should work If I switch between ground and load rather than between the voltage source in the load? Thanks.
 

jpanhalt

Joined Jan 18, 2008
9,111
In the design you post, it will be hard to make the mosfet source current. The reason is simple. When the mosfet conducts, the voltage drop across it is almost nil. That makes the voltage across the load equal the supply voltage, which is what you usually want, but that makes the source pin of the mosfet equal to the supply voltage too and means that the gate has to be even higher. There are ways to do that (e.g., charge pumps), and you can make a charge pump by toggling a pin on your MCU, but MCU's are often not tolerant to voltages that high (typically supply +5V or so).

So, I recommend switching to a P-channel mosfet or connecting your N-channel Source to ground, Drain to the load (meter), and the other load(meter) terminal to the positive supply.

I believe that is what you describe in your Post #7, which is correct.

John
 
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