Hi all

Yet another question about logic-level MOSFET circuits. As of now, my circuit is switching 40-ish V very fine,
but I was wondering if there is any reason why the same circuit shouldn't be able to switch a lower voltage,
like 5V or 12V...or maybe even 3V. Does it seem odd to use 5V to switch 5V? Well, I thought of doing it since
my USB controller has the usual 500mA USB limit.

So, using a logic level MOSFET to switch 5V? Go/no-go?

Thanks,

Nicholas

 

Logic-level MOSFET refers to the gate voltage required. Or to be more precise: the threshold voltage. These MOSFETS can be driven from common logic level signals 3.3V to 5V.

No problem having the gate voltage higher than the drain circuit voltage.

The only thing you need to consider is the ON resistance and the current you are switching. This will determine the drain - source voltage drop when switched on. Should be fraction of a volt.

 

So it will probably be fine? I am going to light a LED or two with one MOSFET. I know a IRL540
is overkill for this, but then I can use the same circuit for different voltages, 5V or 40V without
needing to change anything. Difference is, the 40V application will draw 10 amps or more, the
5V will draw 0.1A or something like that

 

Logic-level MOSFET refers to the gate voltage required. Or to be more precise: the threshold voltage. These MOSFETS can be driven from common logic level signals 3.3V to 5V.
..................

Logic-level refers to the gate-source voltage required to fully turn it on as shown in the data sheet where the Rds(on) value is specified.
It may be related to the threshold voltage but that voltage doesn't determine whether it's a logic-level type MOSFET (as many seem to assume).
The threshold voltage is where the MOSFET barely starts to conduct current.

 

Fdv301 or fdv303, bss138, 2n2007

Easy to work with

 

So it will probably be fine? I am going to light a LED or two with one MOSFET. I know a IRL540
is overkill for this, but then I can use the same circuit for different voltages, 5V or 40V without
needing to change anything. Difference is, the 40V application will draw 10 amps or more, the
5V will draw 0.1A or something like that

You'll be fine.
When the MOSFET is fully on, it acts like a low value resistor [as shown for the Rds(on) value in the spec sheet].
This ON resistance value varies little from zero current up to the MOSFET's maximum current rating.