I want to drive the IRF9530 P-Channel FET with 5V PIC microcontroller. My load is 3.2A. schemtic shows one row in my LED matrix. Scanning @ 1/16th rate, so peak current per LED will be 64mA and the average will be 4mA. When one or two LEDs light up in a row, they are bright. However when more LEDs light up inthe same row, they are dimmer.

  • Can the FET can supply require current with the given gate voltage?
  • Is there any simple modification to be done within the given hardware, to supply the required current?

I cannot use another driving transistor to drive the FET (because I have a PCB). Also I cannot find logic level MOSFETs in my area.

Minor changes will be preferred.


  • \$\begingroup\$ Perhaps you should control the ULN2803's instead. Or if this is a global enable even try a relay. It sounds like a one-off, so doing a little 3d sculpture on the existing board in the vicinity of the FET footprint would not be outside of reason. \$\endgroup\$ Mar 5, 2019 at 16:29
  • \$\begingroup\$ Hi I cannot understand what tou mean...!!! The schemtic shows one row in my LED matrix.Scanning @ 1/16th rate.So peak current per LED will be 64mA and the average will be 4mA. \$\endgroup\$
    – Joseph143
    Mar 5, 2019 at 16:45
  • \$\begingroup\$ Seems like there's a lot you left out of your question - that comment should be an edit. But it also seems like you have boxed yourself into a corner and may simply have to suspend the project for whatever amount of time it will take suitable parts to reach your location. \$\endgroup\$ Mar 5, 2019 at 16:46
  • \$\begingroup\$ ”Also I cannot find logic level MOSFETs in my area.” Mouser and Digi-Key ships all over the world, so that’s not an issue. Pick a transistor suited for your application. \$\endgroup\$
    – winny
    Mar 5, 2019 at 17:55

1 Answer 1


Can the FET can supply require current with the given gate voltage?

Not really, you do not have a lot of voltage margin left.

From the left plot I can see that at \$I_D\$ = 3 A and \$V_{gs}\$ = -5 V the FET's \$V_{DS}\$ will be about -1 V.

That leaves 5 V - 1 V = 4 V for the LEDs + series resistor. If the LEDs actually have a 3 V forward voltage then this might just work but often LEDs do have a higher forward voltage especially White LEDs.

You could try to decrease the 27 ohms resistors such that you do get the required current. That will still not eliminate the 1 V drop across the MOSFET though.

This MOSFET is really better suited to a much higher \$V_{GS}\$ like 10 V as then the drop would be about 0.5 V, that's still a lot!

Also note that if you manage to get the 3 A and the MOSFET drops 1 V it will dissipate 3 W so it will get hot! It will need a heatsink!

You really need to consider using a "better" MOSFET with a much lower \$R_{DSon}\$, for example the AO3401. Since the 3.2 A is on the edge for this device I'd use 2 of them and let each power half of the LEDs.

  • 1
    \$\begingroup\$ More importantly, get a "better" MOSFET with a lower Vgs(th), commonly known as a logic-level MOSFET. For that matter, the AO341 is not a good choice, since it has a max rating of 4 amps at Vgs of 10 volts. \$\endgroup\$ Mar 5, 2019 at 15:35
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    \$\begingroup\$ @WhatRoughBeast You're a bit quick with your conclusions, I suggested to use two of those AO3401 each at half of that 3.4 A. At Vgs = -4.5 V Rdson is 60 mohm or less which is good enough for 3.4 /2 = 1.7 A. I never suggested to use the AO3401 at 4 A. Of course there are MOSFETs which need an even lower Vgs for even more current. The suggested AO3401 is very common and cheap and that's why I suggested it. If you have a better suggestion then feel free to add a comment below. \$\endgroup\$ Mar 5, 2019 at 15:44
  • \$\begingroup\$ Ok thanks.I plan to use Two of AO3401 SMD Parts in parallel.I hope I dont need to change the resistors.The One in the schematic 10K pullup & the 100R gate resistor will share both of the AO3401. \$\endgroup\$
    – Joseph143
    Mar 5, 2019 at 23:49

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