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DC dummy load circuit calibration
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VEGETA:
Here is a quick trial in attachment.

I made 5x1R and op-amp per branch. However, there are still issues like inaccuracy for nearly all ranges. Like if I calibrate it to 1A, the 100mA will be bad.

Also, for our LM324 to output 50mV as top max voltage, I guess it is bad since it cannot deal with near rail voltages. If I want 100mA I would output something like 5mV or so! it cannot do that let alone inaccuracy.

Smallest current it could output in that simulation is about 40mA when I choose 0.001V. So it is terrible. This is where the benefit of 1R shunt comes in, where you can output 50mV to get 50mA and won't be affected too much by op-amp offset voltage.

So what to do now? should we put the divider somewhere else?

EDIT:

I have attached another diy dummy load, look at the mosfets and how they are balanced.
VEGETA:
I have tested putting the 1R back and this time put series resistors for each mosfet branch, it worked nice (nevermind the calibration) but I don't know if this will actually make a difference compared to without series resistors.

I often see this configuration or something similar but the current feedback is always taken above the final shunt.
VEGETA:
Here is a newer schematic in attachment, I used 2n2222 with 10k resistor to act as an active adjuster to each branch. If more current gets threw one branch then higher drop voltage happens across 0.2R which turns on the gate which in turns pulls the MOSFET's gate to ground to make it shut down for a moment to regulate current.

In the previously mentioned schematic, he used -5v instead of ground but I use ground since there are no negative rail here. What do you think now? should we call it the final circuit?  :-//

I need to finish this in order to make a PCB for it to complete the project and make a video about it  :popcorn:
Ian.M:
Unfortunately the 2N2222 gate pulldown circuit, although it prevents any individual MOSFET drain current exceeding Vbe/0.2A (about 3.2A) will cause its own share of stability problems.  As each MOSFET goes into limiting, the loop gain of the control loop changes, which is likely to result in transient response problems, and the gate pull-down current is drawn from the control circuit, not the load circuit, and passes through the current shunt, so it introduces a small inaccuracy in the measured current.  There is also no provision to trim out the OPAMP offset voltage so accuracy (and linearity) at very low current settings is likely to be poor.

Returning to the 'classic' circuit where each MOSFET is in its own OPAMP's feedback loop, that disappointed you when you couldn't get good accuracy at low currents, the devil is in the details - unless you are using expensive precision FET input low voltage OPAMPs with guaranteed low offset, rail-to-rail outputs, and input common mode range extending down to fractionally below their negative rail, you *will* need to trim each OPAMP + MOSFET  to compensate for the OPAMP offset voltage to get them all to cut off exactly at Vctrl=0V.   However its very easy to over-trim, so the best option is to trim them for matching Id at a very low control voltage.

I've redrawn your four MOSFET schematic to add the necessary refinements using LTspice's multiple component and bussed connections notation - basically any part or thick wire with [1:4] in its name represents four separate instances of that component.   Where they are connected to a thin wire they are all connected in parallel.  When you have run the sim and you click anything with [1:4] in its name you will be asked which instance you wish to probe.   Uncomment the .step param range to see what it does at different full scale currents

Its using the diode biassed negative rail + a pull-down resistor on each OPAMP output to help the jellybean BJT OPAMPs you are using to have a reasonably good chance of staying in control right down to Vctrl=0V.   The 1K trimmers for the offset voltage should go between Vcc and Vee so they have a little negative trim range.   You *may* need to reduce Ro[1:4] to 220K  if there is insufficient trim range.

There is a small interaction between the individual offset trims and the full scale trim - you should re-trim the offsets if you make any large adjustment to the full scale trim.

To use the sense resistors in series with each MOSFET source for current measurement and display, you need to average all the source voltages.  To do that, tap off each one via a 10K resistor, all going to the in+ of an OPAMP buffer.  As the total sense resistance is 1/20 ohm (if considered all in parallel) you'll need to set up the buffer for a gain of x20 to get 1V per A.  If you want to go up to 5A, you'll need to use a lower scaling factor for current measurement e.g. x2 for 0.1V per A.
VEGETA:
Thanks for the info.

The circuit now became complicated... I now need multiple pots for many times of calibration, will be hard for others to do it too. You didn't mention the Rs of each branch, is it 5x1R = 0.2R or what?

Can't we do it easier? like the last one I posted with some modifications to eliminate the need to calibrate 4 branches. Especially that you seem to require shorting Vgs of each other branch to do so... which will be difficult if i got a PCB done for the project, which I will.

You say that I should put 4mV in control voltage then short 3 mosfet gates to ground (or negative rail?), after that adjust it until it has 1mA. Then repeat for other 3... this is after the global trimmer which in turns I don't know how to trim and based on what?


--- Quote ---There is a small interaction between the individual offset trims and the full scale trim - you should re-trim the offsets if you make any large adjustment to the full scale trim.

--- End quote ---

This is even harder now. Why don't we use our previous method of having all op-amps sense one resistor which is 1R power resistor? then do something about the 2n2222 circuit that you seem to dislike. I found it to be working in ltspice so I got confident, especially that Scullcom guy did it and it works perfectly with him despite using only one op-amp for the 4 mosfets.

One other minor problem is using lots of resistor values which I hate, I wanted to just stick with 1k, 10k , and so on. So 220k, 470k are hated. Plus, using 2 caps... couldn't we ditch them?

Also, the global trimmer is tricky since V_ctrl is supposed to come from the 10-turn 10k pot. So they will be the next stage after the 10-turn pot.


--- Quote ---To use the sense resistors in series with each MOSFET source for current measurement and display, you need to average all the source voltages.  To do that, tap off each one via a 10K resistor, all going to the in+ of an OPAMP buffer.  As the total sense resistance is 1/20 ohm (if considered all in parallel) you'll need to set up the buffer for a gain of x20 to get 1V per A.  If you want to go up to 5A, you'll need to use a lower scaling factor for current measurement e.g. x2 for 0.1V per A.

--- End quote ---

The display here is the panel meter which is the 0.005R resistor before all the mosfets. Doing this measurement is a kind of nightmare compared to the straight forward one. Even with averaging there will still be an error. It is good that I am using the panel meter for this project.

_______

Scullcom project uses only one op-amp to drive 4 mosfets, is it better than using 4? Plus, it is AD8630 which is a rail-to-rail input and output so he would not worry about lower voltages like us.

However, how far can we go with LM324/358?
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