A simple MOSFET switch....[Solved #6]

The circuit I have attached is expected to power the 3V3 regulator as long as the PB is pressed.

The MOSFET is a P-Channel logic type ( RMV48XP ). I assembled the circuit on bread board and had used a LED in place of the 3V3 regulator. But find that the LED is always ON and when the PB is pressed the intensity just increases a bit.

Will the circuit work as expected ? If yes then I can replace the Mosfet or Transistor or both…

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That should work, if it doesn't then you have either miss wired it or the RMV48XP isn't what you think it is. Google knows nothing of a RMV48XP apart from this thread, so is this number correct?

and had used a LED in place of the 3V3 regulator.

I hope that includes a resistor as well as an LED.

I also am surprised. But searching on NXP site gives this which has a A at the end of the model. Looks like both are " same " ?

http://cache.nxp.com/documents/data_sheet/PMV48XPA.pdf?fsrch=1&sr=2&pageNum=1

PMV48XPA is not the same as RMV48XP which is what you said it was in the first post!

Maybe noob and probably unrelated question: why you use the BJT to drive the gate? Why not to use the switch directly?

I suspect this was because it was copied from a circuit that used a lot more than 5V to power the system and he did not know how to simplify the design for the lower voltage.

The diagram is wrong.
Source of a P-channel fet goes to the supply, and drain to the LED/regulator.
Leo..

Well spotted, I missed that. +1

Wawa:
The diagram is wrong.
Source of a P-channel fet goes to the supply, and drain to the LED/regulator.
Leo…

Also the symbol used is that of an N-Channel MOSFET.

P Channel MOSFET Symbol.jpg

Wawa:
The diagram is wrong.
Source of a P-channel fet goes to the supply, and drain to the LED/regulator.
Leo..

Yes. That nailed it ! Thanks.

Its the Copy-Paste devil at work. Originally was trying with a N-Channel MOSFET but that was a low side switch which meant that the 3v3 regulator was on-line with Vcc always draining the battery. So tried with a P-Channel MOSFET as a high side switch. But forgot to change the symbol and also its always a good idea to show the full symbol with the diode inside. If that had been shown the flaw would have been evident.

And some one asked why use the SL100 transistor directly. Yes it can be but the C-E junction voltage drop is a matter considering that the overhead available for the 3V3 regulator is already pretty low with a 4.5V source.

Mogaraghu:
And some one asked why use the SL100 transistor directly. Yes it can be but the C-E junction voltage drop is a matter considering that the overhead available for the 3V3 regulator is already pretty low with a 4.5V source.

Yes the high side mosfet is the way to go there, as you don't have much overhead for the regulator.

However I don't really see that the Sl100 is needed at all. The push button could have been wired to directly pull the mosfet gate to ground (0 volts).