SMD Mosfet to switch +5v

one of those basic questions, google is just confusing me.

i have 2 lcd’s, one is a character type where i have access to both + and - connections for the backlight, so that’s simple, feed + 5 volts from the usb supply, and switch the negative backlight connection through an N channel mosfet like a 2n7002 (which i have some of)
switched with a pin on the arduino.

The other display is a 7 segment LCD, and i only have access to the +5v connection for the backlight, i.e. the other side of the backlight is connected internally to negative / ground,

Am i right in thinking i must use a P channel mosfet to switch the +5v supply to this blacklight?

or can i use an N channel, putting the backlight + connection to the backlight where the gnd would go on the mosfet (Source) and connect drain to +5 volts from the usb,

If i must use a P channel mosfet, … sorry for this… but any recommendations?
BSS84?
i only need to switch about 30mA,

Does the backlight switch on when you connect 5V to thet pin? Then it’s indeed a high side switch.

Do measure (or check with the datasheet) the current you really need. If <20 mA you can use an Arduino pin to do this directly. Same for the backlight of the LCD.

Otherwise a BSS84 will do fine, as will a traditional PNP transistor such as the BC557. The BSS84 comes only in SOT23 package, the BC557 in TO-92 - of course there exist equivalents in SOT-23 package as well if you prefer that. Mind that your backlight will be active low now.

gazz292:
Am i right in thinking i must use a P channel mosfet to switch the +5v supply to this blacklight?

high-side switch is PNP or p-channel MOSFET.
low-side switch is NPN or n-channel MOSFET.

Switching configuration is always common-emitter (BJTs) or common-source (FETs)

Cheers.
ordered a few BSS84’s,

yep, the display in question is a HT1621, it has a solder bridge on the back to switch from backlight being on all the time, i.e. connected to the main +5v in pin, to route it to a seperate pin, which you apply 5 volts to,

Threre’s probably a way to cut a trace and solder on a negative switching arangement, but i’m trying to make something others can copy if they want, so plug and play is best,

Which thinking about that, i’d best design 2 pcb’s, one with smt components on it that i am fine with soldering, and one using through hole, as i just know people will not even consider smt stuff, even tho i find it easier at times.

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