So I have some logic level MOSFET's on order, they should be here middle of the week next week.. but I happened to be near Radio shack today, so I went in and got the only MOSFET they carry.. IRF510. By reading it, I thought (as I was to learn) that VGS of 5v would be plenty. After reading all over the net and even here, I now understand that's minimum threshold, not full on. 5v compatible I wanted, and it isn't. Crap.
So, I have a bunch of pn2222, I just put a 1k from base to arduino, collector to 12v (common ground), and the emitter to the gate of the MOSFET. The MOSFET drain is taking the ground side of an LED array drawing three amps at 12v (current regulated), and the source to ground. I've got a 470k resistor between the gate and ground also to bleed off the charge. When I meter it, the pn2222 puts out a solid ten volts plus from the emitter.
I can't seem to get the MOSFET to turn on fully given this setup, what gives? When I meter it, the pn2222 is feeding the gate over ten volts.. more than enough to turn it on.. but I am only getting dim output. I've even added a resistor to drop the 12v going to the collector, bringing the emitter voltage to around 8v. Still no dice. If I short the source and drain, the array is fully on, but through the mosfet, it's greatly reduced.
Thoughts? Am I missing something? (likely). As it stands, the MOSFET gets VERY hot, and isn't doing half the job a darlington pair did without getting even a little warm... I'm guessing there's some kind of aspect of MOSFET I'm not understanding..
It does not make sense that the gate is getting 10V. Are you sure?
With 5V from the Arduino coming to the PN2222 base, the emitter should be at 4.3V...it won't get up to 10V in theory. That would also explain why the MOSFET is not turning on fully.
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just metered it running, right. Vbe 5v.. man this is going to be a pain, isn't it? I can see why people don't use this and go for a logic-level instead.
What's a good solution to this, or should I just ignore the whole mess and wait for the logic level FET's?
hmm. Well, I did have an optoisolator (NPN) around so I tried that and got somewhat more power.. but that transistor circuit on the bottom looks like it should do the trick.
If I understand it properly, when the base goes high, the current passing through the 4700ohm resistor is instead directed to ground.. yes? Huh.. that even made sense..
