Electronics > Beginners
MOSFET HEXFET Help please
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andybarrett1:
Hi All .... Looking for a little Help

I am using a similar  circuit as below to switch a High Power Led from a PIC Processor using a IRL520 HEXFET

All is working fine. However I would like the LED  to be switched from the positive side so effectively one side of the LED is 0v and not 12v as in the picture. if it was a BJT it would be PNP not NPN am looking for?

Is there a P-Channel of the IRL520 I have missed..... Do I need to do something else maybe?

Thanks for reading

Andy

RES:
IRF9520
Psi:
Yes, you need a P-Channel fet.

Not all N-Channel fets have a matching P-Channel version.

You will need to look at the specs you need. max voltage/current gate threshold etc. and select a P-Channel fet which meets your requirements.

Be aware that the resistor will then go to VCC instead of GND and your logic will be inverted.
Ian.M:
... and for supply voltages greater than the MCU's Vcc, you'll also need a level shifter.  e.g. with a 12V supply, you'll need to take the gate between +12V for Off and at least 2x the Vgs threshold voltage negative of that for On.  As the supply voltage is significantly less than the max Vgs rating, you can pull it all the way down to 0V, so a simple discrete small signal N-MOSFET will do for the level shifter if you aren't PWMing the load at a high frequency.   It adds another stage of inversion so you are back to logic high for On.  N.B. The load turn-off time is then determined by how fast the gate pullup resistor can discharge the gate - you have to tradeoff fast turnoff v wasted power when on.

With higher supply voltages, pulling the gate of a high side MOSFET all the way down to 0V could exceed its max Vgs rating causing gate oxide breakdown so your level shifter would need to be considerably more complex to limit or clamp the Vgs applied to the MOSFET.
Peabody:
My understanding is that basic stamps are 5V.  So it seems to me that the circuit is already the way it needs to be for controlling a 12V source with a 5V processor I/O pin.  If you invert everything, wouldn't you have the P-channel gate pullup resistor connected to 12V?  That's not going to be good for the I/O pin.  Could someone post a schematic of how this would look using a P-channel?
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