A Transistor Load

RE: A Transistor Load

jc2048 — Sat, 17 Nov 2018 00:03:26 GMT

I've investigated a bit further and found that if I take the compensation capacitor to the junction of the three resistors (ie before the gate resistor rather than after it) it is much happier.

This is how it behaves at 10kHz with that change (with a 22pF capacitor for the compensation)

[View:/resized-image/__size/480x234/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-ad0b6d9b-6288-411c-b08a-f40c7be669ec/contentimage_5F00_189201.png:480:234]

The yellow trace (under the blue trace much of the time) is the input voltage from the function generator. The blue trace is the voltage across the current sense resistor, so shows us fairly accurately the current in the output circuit. The two waveforms should be identical, except that the current can't go negative so, when the input voltage gets down below 0V, the output current sits at 0A. When the input rises above 0V again, the current then takes a few moments (a few microseconds) before it gets to the point where it can follow the input again [the reason for that delay is that the control loop has been trying to force the output current negative by lowering the MOSFET gate voltage and so the gate voltage is some distance away from where it needs to be to pick up again smoothly and there's a limit to how quickly the gate driver can then slew it back to the correct voltage].

RE: A Transistor Load

michaelkellett — Wed, 14 Nov 2018 15:36:44 GMT

You might find it easier to stabilize the operation at low currents and voltages if you used a negative supply for the small signal stages - it looks to me as if you will have some severe changes in device operating conditions as the output voltage at the MOSFET get gets low - possibly even a resistor to ground at that point might help.

This is just a 'gut feel' kind of suggestion - not based on simulation or sums !

RE: A Transistor Load

jw0752 — Tue, 13 Nov 2018 19:20:40 GMT

Hi Jon,

As usual your explorations are fascinating to follow. I believe that one can't truly understand electronics without a good feel for the fundamental components. Thanks to your interest and willingness to share we are all benefiting with a better understanding of transistors and other circuit basics.

John

RE: A Transistor Load

three-phase — Tue, 13 Nov 2018 17:40:14 GMT

Excellent well detailed project

Kind regards

RE: A Transistor Load

genebren — Tue, 13 Nov 2018 16:31:24 GMT

Jon,

Very nice design! I have never ventured down the path of building an op-amp from scratch. I always thought that would make a great school project (i.e. learning exercise), but I mostly, just bought the op-amp and built around it. If you are interesting in converting this a surface mount, and need some parts, I can hook you up with some smt (SOT23) 2N3904/2N3906 parts.

Gene

RE: A Transistor Load

shabaz — Tue, 13 Nov 2018 13:28:39 GMT

Hi Jon,

It was really nice to follow your process as you made changes, and seeing it all documented.

Also you may have established a record on lowest-cost for a DC load.. since each part (apart from the final MOSFET) is about $0.01, you have a sub-$2 project I think.. : )

Also, great idea with the shield for future automation! : )