Hey Mark / pqass --
After posting the video, I went back and spent sometime going through everything and finally - a wiring screwup!! But I learned a lot in the process (more on that later). I had the inverting input STILL tied to the RSense resistor!!
So, as per that video, all those issues I was showing you at 3VDC. Now, totally gone!!
However, I am now seeing some oscillations at 5Vdc (with the 1Ohm resistor) and testing at 1A through the load I(d). That said, this is still the minimum requirement for V(ds), so I tried 10V.
Nope, things look nice and clean now! See attached image.
Other things wrong in the video(1) I had a Fluke showing V(g), yes it was the "Gate voltage", but I also have that on the 'scope (brown colour). I've replaced that scope to monitor V(gs).
(2) Second photo shows the fixed circuit.
However...Remember Peter Oaks' video from your earlier comment (
https://www.eevblog.com/forum/projects/constant-current-dummy-load-ran-a-gutter/msg3181972/#msg3181972) - He talks about using a signal generator and he managed to get a DC-square wave from his "AWG". He mentioned running "500ms pulse, 100Hz, with 2A shift in voltage"?
So it sounds like he's doing something similar to what one would call a "step-response", where we simulate an Input load turning on. Any tips on how to get this going and get _that_ square wave on to the my 'scope?
I'll check my PSU manual on getting its duty-cycle feature working.
In watching your video, it made me wonder how you have the scope probes connected. Especially, the yellow trace and ground.
Do you mean, you have one probe on the 'drain' and another on the 'source'?
(1) Yellow trace is simply the LOAD input voltage, so it is connected to the PSU supply at +/-.
(2) No... I cannot connect the 'scope to Drain/Source as that would cause a short due to mains-earth termination in the Scope probe. I need a differential-probe for that, so thankfully I was cautious and used the Fluke 289 (backlit display) to monitor V(ds).
(3) Green trace: Rshunt.
(4) Brown trace: (Orange really). MOSFET gate voltage, taken right after the 100 Ohm resistor from the Opamp output.
Try shortening the wires (to the power source, to any meters).
I just tested this on my DIY CC load and found that if I had my bench meter (with long leads) measuring DC current in the load-source loop (12V battery as source), that I had terrible oscillations of 4V@54kHz (scope between +load-in and GND) when drawing 2A. But then I replaced the meter with a short 30cm wire (passing through a DC clamp ammeter), the oscillations went away. If I elongated the pair of wires to the battery source by 60cm, the oscillation came back to 2V@54kHz when drawing 2A.
This was the first time I've looked at it with my scope; was relying only on [lying, averaging] meters to inform me when it was built. I only have a 100ohm gate resistor and not the low-pass filter in the feedback loop. I'll be sure to add it later and retest. At worst, maybe this will be another condition of use like any other specification; ie. that shall use only short fat leads.
Also, I think the usual bench setup (with breadboards and all sorts of meters with long leads) lends itself to generating phantom problems (oscillations and ringing). Maybe this is an opportunity to do it the Jim Williams way; air-wire prototyping, using ground planes, short fat wires with higher current lines, [kelvin] sense wires, twisted pairs, etc. Damn parasites!
That makes sense, earlier today before I fixed my wiring blunter, I was seeing oscillation around 62 - 90kHz, possibly in the 2V region.
Cheers for the tip on air-wiring, mmm that sounds interesting!
I should be up in about 8 hours time and will share further info on the oscillations I'm seeing. I'll also use my free time to sort out the CPU cooler heatsink hack.