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[on:]

I have a quick question about FET's.  I'll be talking about P-channel here.

A short while ago someone told me on a datasheet the Vg when quoting Rds(on) identifies if its a logic level FET.  I recently bought and am using without problems a FQP27P06 (P Channel FET).  In this the Vg is quoted as -10v with an Rds(on) of 0.07ohm.

I thought the Vg quoted at Rds(on) would have to be -5v or is this particular FET ok to use at logic level because of the Vth (-2v to -4v) and the low Rds(on).  Meaning that at logic level the Rds(on) will be higher than that quoted in the datasheet but because its so low to begin with that the FET won't heat up as much as other FET's with say a 0.4 ohm resistance at Vg -10v being used at logic level and is therefore safe to use.

I guess to sum up what I'm asking is.  Can a P channel FET be called logic level and used with Arduino if the Vth is -2v to -4v and the Rds(on) is still low enough at logic level compared to the Rds(on) @ -10v quoted in the datasheet?

[Reply #1 on:]

Errr...maybe.

You do not have a logic-level FET. The Vgs rating at 2V-4V says that if you ONLY need to supply 250uA through the FET (from source to drain) then you won't need any more than -4V on the gate-to-source junction. Beyond that, they don't tell you anything until you have -10V gate-to-source, at which point you could source 13.5A and have an on-resistance of 0.07 ohms.

In between...you just don't know (well, you can predict and model, but there are no guarantees). The reason you are using it "without problems" is likely because you do not source a lot of current through the FET, thus your -5V gate voltage is good enough for the job, but I really doubt the FET's on resistance (if you measure it) will be near 0.07 ohms.

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[Reply #2 on:]

Thanks.  At most I draw 800mA through the FET.

So pretty much do as im told and look for -5v (P) and 5v(N) for Vg when the datasheet is quoting Rds(on).  That way im pretty certain I'll get the given ratings for the FET.  Non logic level FETs could work but you risk not knowing the actual ratings.

The site I got the FET from said arduino compatible which I mistook for logic level and then got confused looking at the datasheet.

[Reply #3 on:]

Right. And datasheets being what they are, you will probably find Vgs rated at -4.5V for logic-level P-channel FET's (instead of exactly -5V).

Whatever site said your FET is "Arduino compatible" was playing fairly fast and loose with the truth IMHO.

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The Basic Motor Driver: simple, inexpensive motor driver for 1 stepper motor or 2 DC motors

[Reply #4 on:]

The datasheet of that mosfet at http://www.sparkfun.com/datasheets/Components/General/FQP27P06.pdf gives typical characteristics in Fig 1. From that figure, we can see that at Vgs=5v (next to bottom curve), the voltage drop (Vds) will be 0.1v at 1A and 0.25v at 2A. Above 2A, Vds rises rapidly with increasing current. So typically, switching up to 2A is OK if you are delivering 5v to the gate. This explains why you are not experiencing problems. However, this is typical performance, not worst-case performance. You might not be so lucky if you buy another batch of those mosfets. Also, if the 5v supply to your Arduino is on the low side, it will be even more marginal as you can see from the bottom curve in that figure.

[Reply #5 on:]

Thanks again Rugged and dc.  I got the FET from proto-pic http://proto-pic.co.uk/p-channel-mosfet-60v-27a/, sneeky people.

The FQP27P06 is a very common MOSFET with very low on-resistance and a control voltage (aka gate voltage) that is compatible with most 5V microcontroller

I only got 3 so I'll just remember not to try and use them for more than 2A.