Today I built the circuit and tested it - one quarter of it. I wanted to test out the MOSFET with the feedback servo and see if it would regulate the load from about 0 to 1.25 Amps. All appeared to work OK, so I will now expand it to all four MOSFETs and that should get me up to 5 amps.
Here is a picture of the test. On the bottom the current meter shows 1A. It was actually 1.25 amp, and I have to zero the meter and tweak the shunt resistance to get that to read correctly. On top is the DMM set to show degrees F. The thermocouple is pressing against the heatsink tab. Here with 1.25 A flowing through it, it stayed below 90 F, I pushed it up to 2A, and it was just a bit over 100 - and that was without any fan blowing on it. With the AC meter I didn't read any AC on the MOSFET's source or gate. I'll scope it when I get the other three wired up.
As far as current control, the pot seemed to give me a good and linear range from 0 to 1.25A, but it wasn't through the full range of control. Upon closer inspection I found my "oops". The 0.25 ohm current sense resistor was mistakenly 0.1 ohms, so by not sensing the full actual current on the MOSFET, the op amp would "go to the rail" trying to turn on the MOSFET more, but it wasn't to be. That would happen at about 1/2 travel of the pot, and anything higher would not get me any more load current.
The 2nd picture shows the circuit built but not wired-up for testing, the 15V supply is on the lower edge of the board.
The 3rd picture shows the actual current the load circuit was handling. I trust the power supply reading much more than the new $5 surplus "ArcherKit" meter I got for the project.
As for the discussion about MOSFETs, this particular MOSFET was primarily designed for switching although I don't know if it is vertical or horizontal or what...the spec sheet doesn't say.
At least it falls into the recommendation towards the end of the article that it should be higher voltage but not high current. Since the IRF840 is only rated for 4A (at 25*C), and at 100*C only rated for 2.5 Amps, I am running these a bit close to the DC current edge, but I hope to keep them running much cooler than 100*C. Of course, we will see...