SparkFun Electronics Forum

The Rds(on) for the Si4403BDY I mentioned earlier is 32 milli-ohms max (and typically closer to 10), so yes, you can get better performance than you saw with the random Radio Shack transistor (which is more than 10 times that Rds, from your results).

The IRF510 has an RDSon of 0.54 ohms; which is fairly bad. A fet I'm using right now - the IRF1310N has a RDSon of 0.036ohms - and you can get even better than that. That would solve your voltage drop issues.

--David Carne

Next time I order parts, I'll get something more appropriate.

I still think it was a good exercise. Now I understand what to look for, and maybe some others have learned something too.

Thanks for posting the results Jon. With the right selection of MOSFET, this can pretty much be scaled to any current you like with only millivolts of loss. As stated before, with low voltages, it is important to pay attention to the threshold voltage of the MOSFETs (ie, look for "logic level" devices).

I need to verify that my design for reverse polarity protection with P MOSFET is correct.

I have space limitation on my pcb board and i want to avoid the use of zener diode and resistor.

As a result i am looking for a very small smd P MOSFET that will be suitable for my application

Vin = 12V (+-0.5V)
Load current max=3A

Rds(on) is not important for me because i will not use battery for Vin.
Important is to have the lowest voltage drop possible because the board control dc motors that are rated for 12vdc

I want to know if the Vishay SQ2319ES will do the job for me.
http://www.vishay.com/docs/65735/sq2319es.pdf

Thank you in advance.

estelios wrote: Rds(on) is not important for me because i will not use battery for Vin.
Important is to have the lowest voltage drop possible because the board control dc motors that are rated for 12vdc

The voltage drop is directly related to Rds(on), as the MOSFET behaves like a low-value resistor.

estelios wrote:I want to know if the Vishay SQ2319ES will do the job for me.
http://www.vishay.com/docs/65735/sq2319es.pdf

If the load current is really 3A, I'd choose something with a higher current rating and lower Rds(on). That device would drop 0.225V and dissipate almost 0.7W at 3A. Unfortunately, I can't see anything better in a SOT-23 (TO-236) package. You could either try an n-channel device with the appropriate circuit or use a p-channel device in a larger package (eg DMP4015SPS-13).