Hello,
this episode has been proven extremely useful to me, can't believe i'd already watched it before and forgot about it.
Anyway, sine i need to set it up for far more current than the example i setup to do some simulations as the parts can get quite expensive and wanted to try some variables...
first thing, Dave, where did you pull out that Vgs to Vs graph from?, i've tried to find it in the datasheet and couldn't find anything, the closest is the transfer characteristic(but doesn't has much detail on the interesting part we care about)
http://www.datasheetcatalog.org/datasheets/90/489299_DS.pdfSo, i set up the circuit on multisim, changing the opamp to a rail-to-rail output one(shouldn't affect it much) and went to test it...
And found some oddities, specially, the output to hte gate of the mosfet reads "7v" at full power when power is 5V
, way to go multiFAIL!(unless i'm doing something WRONG)...
other than that, the circuit behaves as expected drawing the same ammount of current as the voltage in the divider.... so far, so good.
So i went about to change components, first a higher power transistor in a TO-247 package(i need to check the PSU parts bin in my office to see if i can scrounge something from there!). i found Vishay IRFP048 device to comply(and far exceed) my power needs
http://www.vishay.com/docs/91198/91198.pdfi reran the simulation and all values remain(gate voltage again returns whatever crazy value it can come up with, 7.95v this time)
Then a r-t-r opamp that can take >12V (since the gate voltage would probably far exceed 5V), i'm having some trouble here as a first model using LM6142 started oscillating wildly -suspect the gain bandwidth is too high-, so i changed to an LMC6462 with 15.5vcc max (i have to see if i can get this component here) but i can't get the simulation to output more than 4.93A at 15Vcc.
Revising the parametrics in natsemi i can't find something with low BW gain, dual/quad package and high VCC with r-t-r output and DIP package(the 4MHz gain ones are soic)..
but oh wait, the LM324D can get to 30V, so if i supply the system with +12V.... and yes.. looks like it could work!
am i missing something, could this work?