Hi Chico,



The idea here is to use a MOSFET as a switch. With Ugs=0 the MOSFET is turned-off, will say the source drain path is high ohmic and looks like an opened switch. To close this "switch" would mean, that the source drain path becomes very low ohmic. The datasheet contains information, what gate source voltage (Ugs) is needed to "close" this switch. In the case of IRLZ24N a Ugs of 4V is enough to fully turn-on the MOSFET. Then, the impedance of drain source path is 0,105Ohm. IF you think of your laser module with a current consumption of 300mA, then only 0.105 Ohm x 300mA = 0.032mV would drop across the MOSFET "switch" and 2.97V, which is nearly 3V, would drop across the laser module. So, this MOSFET would be ideal to switch on your laser module.

Unfortunately, most MOSFETs are not specified at 4V or 5V gate source voltage, but at 10V! Then, it cannot be said, if the 5V of your logic supply voltage will be enough to fully turn-on the MOSFET. It could be, that the MOSFET isn't fully turned-on at Ugs = 5V, but shows an impedance of several Ohms. Then, the 300mA current consumption of your laser module would cause a voltage drop of several 100mV, if not higher, and couldn't be turned-on, eventually.

That's the reason, why I suggested you a "logic level" MOSFET, which is a MOSFET, that is reliably switching already at Ugs = 5V. The "L" in IRLZ24N stands for "Logic Level MOSFET".

Now, there are two ways to proceed:

1. You buy a suited logic level MOSFET (preferred!), similar to the IRLZ24N, to guarantee, that the MOSFET can be fully turned-on, even at Ugs=5V, or

2. You take a standard MOSFET (not at all preferred!), and hope that it can be turned-on enough for your 300mA load at Ugs=5V, allthough the datasheet says, that it will need a much higher gate source voltage. Take care, this can be very risky, when driving a laser module!!! First the laser is off, and suddenly, after a temperature rise of electronics, or else, the laser goes on, which nobody expects...


Try the following:
Connect a 4k7 pull-up directly from output of micro to +5V, as you plan already.
Connect from this output a current limiting resistor of about 2k2 to the gate of MOSFET. And use a logic level MOSFET, that can be reliably turned-on at Ugs=5V!!

Kai Klaas