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DC dummy load circuit calibration

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Ian.M:
That will work for the meter, but what's going on with the 2N2222 transistors?   With 10K in series with the emitters, they can only cause at most a 10% reduction in gate voltage which probably isn't going to be enough to stabilise it once the MOSFETs are hot

Also, due to differing OPAMP input offset voltages, it will be out of balance from the moment you switch it on.

I know you are very attached to the idea of using your high power 1R resistor, but its *really* not helping this design!

VEGETA:

--- Quote from: Ian.M on May 19, 2018, 01:12:13 am ---That will work for the meter, but what's going on with the 2N2222 transistors?   With 10K in series with the emitters, they can only cause at most a 10% reduction in gate voltage which probably isn't going to be enough to stabilise it once the MOSFETs are hot

Also, due to differing OPAMP input offset voltages, it will be out of balance from the moment you switch it on.

I know you are very attached to the idea of using your high power 1R resistor, but its *really* not helping this design!

--- End quote ---

I thought the transistors would solve the problem, what other value do we need? I really don't understand why 10K gives 10%... thus won't be able to pick the proper value.

Ok, then give me a simple solution without the need to calibrate the entire 4 branches  :-// :-// then I will gladly ditch the power resistor  :-+

VEGETA:
I reverted back to your version without your eternal enemy (1R power resistor), just without the calibration of each branch to get 1v per 1A since we are using the panel meter without measuring stuff.

How can this version achieve balancing without the 2n2222 circuit?

VEGETA:
Here is the schematic after doing what I said above.

Strange thing is that the least amount of current that we can get is 36mA even if V_set is 0... this is not what should we have assuming we have the negative rail.

Ian.M:
There is no simple zero calibration solution if you need a 1A per V wide range linear control voltage input to set the current.

If you just want to set  the current with a pot, don't care about control linearity at low currents and don't mind say +/-5% uncertainty for its actual full scale range, because it will always be set by reading the current display on the meter,  you can use 4x the simple OPAMP + MOSFET circuit, with multiple paralleled 1/4W resistors for each MOSFET's current sense, your 10 turn 10K pot feeding all the + ins and a suitable resistor between the top end of the pot and a regulated positive supply to set an appropriate maximum current.
 
Unless you use better OPAMPs, or don't care if you cant get the current right down to zero, you'll need a negative rail for the OPAMPs, so if you want to run it all off a 5V USB charger you'll need to use a boost module to get enough voltage for the OPAMPs   Set the boost module for +12V out, connect the USB charger +5V to circuit ground and you'll have -5V and + 7V rails - perfect for the OPAMPs to give up to 5V gate drive + enough negative swing for full MOSFET cutoff, and the meter will be quite happy running from a +7V rail.

The easiest way of trimming to zero current at the bottom of the pot is to offset it a little negative.  See attached schematic.  Depending on your pot's track end resistance you may need to increase R4 e.g to 10R.   For the full scale trim resistor R5, use a fixed resistor + a 10K preset in series.   

CAUTION - The schematic is designed for a full scale load of about 5A, but your heatsinks wont be good for that with a 30V PSU D.U.T.  They would probably be OK if your D.U.T. is under 10V

To avoid difficulty with non-standard models and keep the runtime reasonable, the sim uses an ordinary voltage source to represent one of the DC-DC non-isolated boost converter modules that I remember from your previous topic.  Similarly it uses a voltage source to represent the LM317 feeding the 10 turn pot with a regulated voltage. If you want, you could substitute a Zener circuit or a voltage reference instead of the LM317 - just change R5 to get about the same voltage across the pot.  R2 is *ONLY* to satisfy the LM317 minimum load requirement. Delete it if using a different way of getting a regulated supply for the pot. 

Personally, I wouldn't use a USB charger to power it, unless I mounted it internally with no externally accessible USB socket, to remove the temptation of ever powering it from the same USB supply as another device its connected to.

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