I’m building a simple LED strobe with a number of high power LEDs but the supply is a 12v supply and I expect to draw up to 5A. I was planning to have the 12v supply to the LEDs with an N-Channel Mosfet being controlled from a RPI Pico on the gate pin. To power the Pico, I was going to use a step-down buck converter to give the PICO its 5v supply. So, I’m having trouble finding a Mosfet that can be controlled from the 3.3v of the Pico. They all seem to have gate threshold voltages at around 4v which I understand is going to be an issue since the pins on the Pico provide only 3.3v. I have managed to find some on RS Components that specify both a ‘Maximum Gate Voltage’ and a ‘Minimum Gate Voltage’. Should I be looking for one that would have a Minimum Gate Voltage under 3.3v (eg 2v) and the Maximum would be anything over 3.3v? Do I have this correct?
Hi Philip
Welcome.
That maximum gate voltage would likely be +/- 20V. What you are looking for is “Gate threshold voltage” which is the voltage where the Mosfet is starting to turn on. I stress the word “starting” here as at 5A you really want the Mosfet to be fully on or close to that. This probably means you would have to look up the graphs in the Data sheets. I think Mosfets that switch at about 2V are available but off the top of my head I can’t think of any particular one as I tend to stay away from 3.3V stuff. Not enough fudge factor at that voltage for me I am afraid.
There are “Logic Level” (5V) Mosfets available that some claim to work at 3.3V but I suspect they could be only partially ON. At 5A you really need the device to be FULLY on or risk overheating. When fully ON any decent Mosfet would only get warm at 5A.
It is possible to use an Opto coupler driven at 3.3V to switch a higher voltage to the gate and think there are ready made modules around for this. If no one else comes up with this I will have a look tomorrow when I have more time.
Cheers Bob
Add on. I think Core have a switch using P channel devices that can be switched with 3.3V. It is a high side switch which means you switch the 12V and not the ground side.
Phillip and Robert, Can suggest using a suitable opto coupler between the RP and the MOSFET. The 5V for the gate drive can be be sorted from your 12V LED supply or another voltage. The pi 3.3V would drive the opto diode via a current limiting resistor.
Regards Bryan
I think this is revisiting the problem I had with switching a solenoid on. We went with your suggestion of a seperate MOSFET switch which has worked exceptionally well.
This one: SKU: DFR0457
Hi Gerard
Yes that is one of the ones I was thinking of. Should be OK.
Remember this is a high side switch.
Cheers Bob
Hi Philip.
The one gerard just linked.
DFR0457
would work.
This is a high side switch. That is you switch the 12V instead of switching to ground. Refer to the example circuit in the product description.
Cheers Bob
It works very nicely.
Hi Gerard
Good to hear
Cheers Bob
Wow! Thanks guys. I’ll order one and see how that works.
