Why won’t my P-Channel MOSFET work? Arg!

Hi all, hope this is the right place and appropriate for your forum. I’m a hobbyist trying to build a digital controller / power monitor for a green power generator project built from an auto alternator. Hoping to use an arduino microprocessor & some simple transistor circuits to power an alternator’s field coil which at 13v can draw up to 10amps of power. From what I’ve read I think I need high side PWM switching because the alternator shares a common ground with its other electrical connections preventing me from using my usual novice NPN based bag-of-tricks. From my reading I assume some sort of PNP or P-Channel MOSFET is in order (components I sadly have no prior experience with). Alternativly I've read about using fancy high side controller ICs to drive regular NPNs but don't want to go that route.

So far everything seems to work except I clearly can’t figure this P-channel MOSFET out (IRF6540)! No matter what I try I always get a high voltage at the drain pin (full-on condition) regardless of what I do to the gate pin. Attached is an example test circuit that I thought should work from my limited understanding and online tutorials.
- The NPN driver in the drawing (2N3904) by itself works when tested with a voltmeter and toggling the input
- If I replace the p-channel mosfet with a PNP transistor it work as expected. Shouldn’t this mosfet work pretty much the same? My puny TIP127 PNP can't handle the current if I were to hook up the load.
- I’ve tried several IRF6540 components
- I’ve tried connecting the MOSFET’s ground pin via a current limiting resistor directly to ground. Also via the resistor to 13v. No voltage change on the drain pin in either case.

Clearly I’m missing something embarrassing and fundamental. I mentioned I’m a hobbiest, right - my day job is managing global marketing programs for fortune 500s and don't know anyone that knows anything about electronics! After years of electronics & robotics projects I’ve yet to be stumped to the point of needing help til now - amazing how much you can self-teach via google and such. This one has me beat – hope someone can help as I have no idea what to do next! Truely appreciated...

The PFET is not turning on because the transistor isn't turning on. A transistor is a current amplifier. There is no current flowing in the base emitter junction, thus no current is flowing in the Collector Emitter junction.

I have seen your circuit if your ground is negative and 13 volt is positive it will work. At the same time Digital pulse must be Positive going. In that case you must uses N channel MOSFET. if any difficulty please contact.

You have the right idea regarding the use of a P-Channel MOSFET but your 2N3904 is not in the proper arrangement.

Try one of these (in each case the 1Ω resistor under the "Alt_Field" flag represents your Alternator's field coil -- i.e. replace the 1Ω resistor with the field coil):

https://www.circuitlab.com/circuit/bv5q5h/eeweb_pmostoaltfield_555asoutput/
https://www.circuitlab.com/circuit/3fqm2u/eeweb_pmostoaltfield_compsymout01/
https://www.circuitlab.com/circuit/57vscm/eeweb_pmostoaltfield02/
https://www.circuitlab.com/circuit/ej34q7/eeweb_pmostoaltfield01/

Hi

try this put this together quickley this afternoon Data sheets for devices included You will have to parrallel the devices other wise on maximum load ie 10 amps or more They will sparkle errupt lots of smoke and blow up

Schematic is also fused You can use pwm to control vin to alternator coils Ive run simulations on this.

Schematic is clearly labeled for you

This should be a ok The test is to build it and find out but I cant see any reason why you should have problems with this

In the diagram ive used 400khz as pwm frequency this according to various articles should be in the region of 20khz this seems to be the correct frequency for pwm for these type of applications There is a method for calculating what pwm frequency you should use based on L/R of the windings There is also a trade off btw using lower frequencies for pwm

Note im not referring to the duty cycle which controlls level of voltage / current applied to the alternator field windings

Your pwm output if you are going to use one should have F set to around 15KHz to 25KHz This seems to the accepted norm then obviously you vary your duty cycle accordingly i.e 0% to 100%

I see Steves used a 100Hz which may produce undesirable results After spending a number of hours this afternoon researching various parts of pwm Most sites say to use roughly 15 to 25 KHz as you pwm frequency This delivers the most satisfactory results for regulation

That should now finalise what you want to achieve

See this site for further info on pwm control motors etc

http://www.societyofrobots.com/member_tutorials/book/export/html/234