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DC dummy load circuit calibration

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VEGETA:
Here is the first schematic done in KiCAD.

Anything we should change?

I remember you spoke about the position of the panel meter current shunt, that we should move it from where it is now. Kindly specify why? I noticed some difference in current between it and the 1R resistor in LTSPICE (in the 1 ma to 10s of uA) so I thought of re-asking you about it.

Perhaps putting it under 1R is better?

Ian.M:
Are you still using this panel meter?

If so, that schematic will *NOT* work - the meter current shunt *MUST* be on the low side so you need to move it to in series with the Gnd pin of the D.U.T. connector J4.   Be very careful of the polarity of its black and green wires - the green CS+ wire needs to go to circuit Gnd and the thick black CS-wire needs to go to J4 D.U.T. Gnd.   Its thick red wire goes to the J4 D.U.T. V+ pin.

Also if you want the meter to be accurate, as the thin and thick black wires are internally connected, to avoid it displaying its own current consumption,  you'll need a separate floating supply for it.    I would suggest a 5V to 5V 1W isolated DC-DC converter running from your main 5V rail, with its output connected to the panel meter's thin red and black wires *ONLY*.  If you don't mind it showing its own current consumption, connect circuit +5V to its thin red wire and leave its thin black wire disconnected.

VEGETA:
Yes, I am using that meter.

Why it must be on the low side? there is no indication that the current shunt negative terminal is shared with the negative of its power supply... unless I miss something. It says "series connect on the power supply cathode" so is it why it must be on negative side of the power supply?

The green wire is actually blue, so it is the I+ and the black is I-. If I put the meter shunt resistance on the negative side under J4 in LTspice it will have a negative current reading... so is this why I+ must be connected to ground and I- to J4 negative? so it could be the returning path of the current from our circuit (1R power resistor) -> to ground -> to J4 negative (which is the supply itself).??

The panel meter will draw approx. 15mA but it won't pass through its shunt resistance, so I don't know why you assumed it would affect our reading.

Even if it does, we can still calibrate it by its own pot... I guess this will work to zero it out right? I don't like using such dc-dc converters since they are not so common unlike all other project parts.

Ian.M:
Yes you are missing something:

--- Quote from: banggood ---Onboard Wire Instructions:

Thin Red Wire (VCC): positive pole of power supply input (3.5-30V)
(Note: if the measuring signal is less than 30V and the power is adequate, can be the power supply of module)
Thin Black Wire (GND): negative pole of power supply input (3.5-30V, common ground with measuring signal)
Thick Red Wire (VIN): positive pole of measuring signal input (0-100V)
Thick Green Wire (I+): positive pole of current input (series connect on the power supply cathode)
Thick Black Wire (I-): negative pole of current input (series connect on the power supply cathode)

--- End quote ---
Also see http://files.banggood.com/2018/04/Direct-power-supply.jpg and note the thin black wire is not used in that configuration.

In this context, chinglish "series connect on the power supply cathode"  translates to "connect in series with the negative terminal of the power supply".

'calibrating' out its own current consumption with the display's on-board zero preset is likely to be unsatisfactory as it will vary significantly with the number of segments lit.   At best you will be able to get it zeroed at a particular voltage then as the current increases from zero it will be grossly inaccurate at small currents as the number of lit segments changes.

If you were using an AC output walllwart, you could power the meter with a quasi-floating supply using a capacitively coupled bridge rectifier.   You could also use a pair of opposite half-wave rectifiers to get + and - supply rails for the OPAMP.

If you dont have an AC output wallwart, take any old-skool heavy (i.e with a real line frequency transformer in it) unregulated DC one with a nominal output voltage between 9V and 12V, that has a case you can open, and remove its bridge rectifier circuit and any reservoir capacitor, reconnecting ite output wires to the secondary.   If it doesn't have a bridge rectifier, stop and ask with photos.  If it has any electronics on the primary side of the transformer it isn't suitable for conversion.

VEGETA:
Ok, how about this?

Of course we can use the current adjust pot on the meter to adjust for the 5mOhm resistance of the meter.

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