P-Channel MOSFET as a high side switch

Other Hardware General Electronics
1

Hello everyone,

I’m working on a high-side switch circuit to control a red LED using a P-channel MOSFET (IRF4905). My goal is to turn the LED on and off with a switch in a high-side configuration, but I’ve run into a problem, the MOSFET never seems to turn off.

From my understanding, a P-channel MOSFET turns off when its gate voltage is equal to the source voltage. In my setup, I’ve confirmed that the gate and source are at the same voltage, but the LED still stays lit. I’ve also tried swapping the IRF4905 with several others from the same batch to rule out a defective part—no change.

I’ve attached my schematic and a photo of the circuit under power. If anyone has suggestions on why the MOSFET stays on, or if there’s something I might be missing with the wiring, I’d really appreciate your advice.

Thank you in advance!

Powered CCT
Powered CCT1920×2560 546 KB

Schematic
Schematic1920×2560 312 KB

2
  • This MOSFET is not meant for 5V operation.
    -10Vgs should turn the MOSFET fully ON, 0V Vgs should turn it OFF
3

Hi Larry,

I switched out the LED's current limiting resistor with a 675ohm one so that 12V could be applied (see photo). Here Vgs is again equal to 0 and the MOSFET is on so it seems the problem persists

Schematic 2
Schematic 21920×2560 501 KB

. Also, if you could explain how you know the IRF4905 is not meant for 5V I would appreciate it since I did not know that and looking at the datasheet can't seem to figure out why.

Thanks!

4

Yes, it should be off with gate and source both at 5V. I believe the pinout, front view, left to right, is GDS. If that's right, you have it wired correctly. So I think that means the mosfet is really something else, or the batch you got is defective. What markings are on the mosfet?

5

The markings are "IRF4905" (at the top), then "Name x327x" (second line), then "B9P0"(last line). Where Name is shown in the attached photo.
image

6

You got it backwards, a P-chan is controlled by negative Vgs.

This particular FET
VGS = -10V

Read the datasheet

1 Like
7
  • I Bought 20 MOSFETs form China a few months back, none of them worked. :grimacing:

2025-03-06_15-22-27
2025-03-06_15-22-27916×309 145 KB

  • 0V on the gate (with respect to GND) turns on a P MOSFET, i.e. Vgs = -12V in you case.
8

...but they were cheap, so it was ok, yes? :slight_smile:

1 Like
9
  • ~ $2.00
    :scream:
10

2 bucks is 2 bucks :unamused_face:

Mouser 20 pcs

Merchandise: 24,20 €

Subtotal: 44,20 €

I guess that you don't always get blanks so it pays off to buy from Chinese shops.

11

To my understanding the MOSFET is indeed controlled by a negative voltage when I want to turn it on. To do this I would have the gate tied to ground (Vg = 0V) and the source tied to power (Vs = 12V). Thus Vgs = -12V and the MOSFET is conducting. However, the issue I am having is that when the gate and source are both 12V (as shown in reply to Larry), Vgs = 0V and the MOSFET should be off since Vgs = 0V is not yet at the threshold voltage of -4V from the data sheet.

12
  • Try connecting the Gate directly to +12V.
13
  • I can still use these faulty MOSFETs when teaching troubleshooting to students.
    :shushing_face:
1 Like
14

Smart idea but still doesn't work

Cheap parts may be to blame :frowning:

15

And the FET is still on? Check your wiring again, that Source and +12 V really is connected. If not the body diode will conduct.

16

Well I just did some advertisement for Mouser then...

17
  • If in your circuit, the MOSFET doesn’t turn OFF when the gate is connected to +12V, give them to the wife for jewelry.
2 Likes
18

Yes unfortunately it is still on... I have checked my wiring many times and tested each part of the circuit with my multimeter. I guess Mouser it is haha

19

FYI

MOSFET Examples
MOSFET Examples2128×1600 433 KB

3 Likes
20

Earrings, or as piercings..

2 Likes