Hi, does this simple circuit look in order for my switch?
Load: 50mA haptic motor
Voltage: 3.3V
Arduino: 3.3V
I found this in the forums here, but added the 10K pull-up on the base. That should keep the PNP transistor off during power up?
Yeah, looks good to me. And maybe, very maybe (since it's such a small motor), add a flywheel diode.
So the pull up resistor may just be a thing needed in MOSFET switches, not in this scenario.
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I will add in a tiny flyback diode
True.. 
- With the BJT and the MOSFET the resistors are added to help in switching.
BJT, the resistor is used to discharge the base emitter capacitance hence speed up switching.
MOSFET, same, and prevents a floating gate which can cause gate open circuit problems. - Best design would be keep the resistor.
- BJT Vce Saturation is high, best to use a logic level MOSFET if you have one.
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Speed would be important for the haptic feedback in my project. I have often used this MOSFET in 5V scenarios, will it work for 3.3v? I looked at the data sheet but am not sure what chart really tells you that.
PNP MOSFET
Here is a design circuit I drew up for it. See any issues with this?
Thanks
There is no chart to tell you that. If the data sheet doesn't specify a maximum Rds(on) at a Vgs of 3.3V or less then you can only guess what it might be.
Why don't you use a different MOSFET or use the transistor.
thanks!
I looked at the datasheet again to find those tables you referenced. ...why are the numbers negative? Would they be positive for an N channel MOSFET? I know to turn on the P channel, I need to set the gate low, so the Vgs at 3V -- would that mean 3 volts lower or negative than the source?
If it's too hard to explain, no worries.
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If we talk about an N channel MOSFET, the current enters the Drain and goes out on the Source; we define MOSFET current as IDS (current drain to source).
In a P channel MOSFET, the current enters the Source and out on the Drain, hence IDS is negative; just think about current as an absolute value and you’ll be okay
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Also, to turn on an N channel MOSFET the Gate to Source voltage needs to be positive. i.e. you connect your black lead (GND) of a voltmeter to the Source and the red lead to the Gate; we then read a positive number on the meter.
When you have a P channel MOSFET, the black lead still connects to the Source and the red lead to the Gate, hence we see a negative number on the meter.
That makes sense. Thanks.
The easiest way to think about it is to consider the reference point. For MOSFETs, the reference point is typically the Source terminal.
To turn on a MOSFET, the Vgs (Voltage between Gate and Source) must exceed the threshold voltage:
- For an N-Channel MOSFET, the gate needs to be a few volts positive relative to the source.
- For a P-Channel MOSFET, the gate needs to be slightly negative relative to the source. In this case, a multimeter would show a negative voltage.
One common challenge is understanding that the reference point for Vgs (Voltage between Gate and Source) is not always ground; it moves with the source. The gate typically has a positive and negative voltage rating; the MOSFET will only turn on when biased with the correct polarity relative to the source.
Hopefully, this explanation helps clarify how to properly bias a MOSFET.
No, it ‘s just a MOSFET I’ve used before in an application that had to be high side switched.
In this instance I’m just driving a haptic feedback motor of low current (less than 100mA)
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