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DC dummy load circuit calibration
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VEGETA:
After seeing that I obviously won't go with this circuit as it is... but it is good that we came this far.

I put the current sink as you mentioned and yes it does ruin the supply for IDK reason.

In a panic, this is what I came with as a solution:

Suggestion 1: Using 7660.

I found it on JLCPCB supplier (https://lcsc.com/product-detail/PMIC-AC-DC-Converters_HTC_TJ7660D_TJ7660D_C126092.html) so they can deliver it with the PCB itself.

We could hook the negative of 12v supply to ground which in turns will eliminate all these peaks of noise as I tested. However, we use a simple resistor divider to get approx. 5v or so then feed it to 7660 to get -5v which becomes our negative rail.

I think this is very simple if it does not introduce anything else. I knew about this all along but I didn't want to use 7660 since it is not so available to others (even for me now xD).

We can simulate the -5v supply easily by making a 5v supply with its positive to ground and its negative is -5v. As for the LED I removed it and installed the 100 Ohm back... Now if i want it I can connect it anywhere else like on the main 12v rail with proper resistor dividers and so on.

If we want an extra cost we would use 5v linear regulator 7805 to give 5v and power the LED while at the same time give 5v to 7660.

I know 7660 charge pump has switching but I don't think it will affect us here, or will it?

How about this?

Suggestion 2: use another LM317

or better yet 7805 hooked to 12v and the negative of 5v regulator is hooked to ground not to negative rail. Here we can avoid the negative rail all together.


Suggestion 3: op-amp rail splitter

this is not even a solution to this design but rather a completely new way to avoid the dodgy negative rail of ours, perhaps it is the cause of all this. I didn't build this yet.


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I saw from other designs that they use suggestion 1 since it gives them 12v full voltage while having also -5. Getting that extra IC maybe worth it if the design is working and all these problems are solved. Scullcom design uses suggestion 1.


--- Quote ---If you go with this design without resolving the issue with the Vbe multiplier and the meter supply current, I'm washing my hands of this whole project and ignoring this topic.
--- End quote ---

Don't give up on meeeee! I am trying also xD. Originally I wanted this to finish in a week or so but looks like it took a lot more. I saw Dave's project so simple, also the one from mjlorton so I thought I could do it similarly.

I am good at electronics but not too good, that is why you see me asking a lot in details. I work as an instrumentation planning engineer right now so it is a different field.

Looking forward to read you opinion about the suggestions above. I hope we can finish it soon.


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EDIT:

Suggestion 4: put the panel meter across 12v.

Meaning from positive (+9v) and negative rail (-3). This way it won't interfere in our dodgy negative rail and ground config.

However, I remembered that it must share a common ground with the measuring signal so this suggestion is bad... forget it.


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EDIT 2:

I tried different resistors and caps with 2n2222 circuit and yet it refuses to work! looks like suggestion 1 is the way to go. getting 7660 is doable and if we are to do so then we would get tl431 to replace lm317 if we want.

I need to read your opinion about suggestion 1, if it works or not.

I really want to finish the project xD. Don't you?
VEGETA:
I have tested the opamp splitter and it did not work! same problem as before.

I used 30k with 10k to get 9v and -3v but the same with other values.

Here, there are no transistors at all so what is the cause of this error?!
Ian.M:
If you use a 7660 charge pump for the negative rail, it needs a good stiff power supply to work well, so don't even think about a resistor divider to power it.  You could go back to using a USB charger for 5V supply, but that limits the maximum positive swing of the OPAMP driving the gate and will limit the max current you can get with a decent heatsink.   Using a regulator is a far better choice, but I recommend a 9V one as 7660 chips are rated for operation at up to 10V, and 3V headroom is adequate for all common regulators.   Then you can use the regulator output for the OPAMP's Vcc supply which will keep all the noise from the LED meter's changing current draw out of the sensitive part of the circuit.    That also means you don't need the TL431.  You only need a really common 7809 9V regulator - preferably the low current 78L09 variant - or a LM317 + two resistors to set it to 9V.

Run the power LED with a series resistor from the regulator output - 1K would be good which will give about 7mA through the LED - plenty bright enough. 

For 2A max load, you need 0.11V at the top end of the pot, lets say 0.12V to have a little margin to be certain you can reach 2A.   Therefore it needs 12uA flowing through the pot.  This is a very low current, so the PCB must be cloean and if you mount the pot off-board, please use screened cable to hook it up with the screen connected to ground and the pot body if it is metal, but *NOT* to either end of the track or the wiper.     If you feed the pot via a dropper resistor from the +9V regulator output, you'll need 740K in series.   680K and a 100K preset will give you enough adjustment range. 

For the zero adjust at the bottom end of the pot,  as the 7660 output isn't very well regulated, I'd suggest using a 10K resistor and a diode to get a reasonably stable 0.6V-0.7V negative voltage, then using a 10K preset feeding a 10R resistor to divide that by up to 1000:1 to provide the zero adjust.

An OPAMP + BJT based rail splitter would be another possible option - if its designed to sink enough current it could cope with the varying supply current of the LED meter.  If you want to go down that road, I have some thoughts on the rail splitter design I need to develop in LTspice.

What OPAMP splitter are you experimenting with?  Schematic please with component values, measured voltages and exact description of the fault.
VEGETA:
opamp splitter version below, it does not work.

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How about sticking to LM317 with a fixed resistor + 10k pot for adjustment -> put it to 9v. Since 9v regulators are not as available..? Then feed that 9v to 7660 followed by 100uF caps for filtering... and also powering the meter from 9v lm317 connected to circuit ground as well as the power LED (1k series resistor as you suggested). This eliminates all problems on the expense of getting lm317 IC + 7660 IC + one extra 10k pot! 

^
until we could get a better solution if opamp splitter worked.


__

I will mount the 10-turn pot on the front panel then connect it by wires to the PCB, thus getting a screened cable is an extra annoying step. Is this due to EMI? You remind me of our instrumentation wiring that requires this type of cable but here I want it to be very simple. If this is gonna introduce some error then it is easily offset by the zeroing pot.

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getting the 2A will change as you explained since now it is 9v not 1.25 volts... So 740k is a must now and getting 100k pot is also an extra pain but we have to tolerate it.

____


--- Quote ---For the zero adjust at the bottom end of the pot,  as the 7660 output isn't very well regulated, I'd suggest using a 10K resistor and a diode to get a reasonably stable 0.6V-0.7V negative voltage, then using a 10K preset feeding a 10R resistor to divide that by up to 1000:1 to provide the zero adjust.
--- End quote ---

I didn't understand. Do you mean ditch the 7660 completely and rely on diodes? like putting 2 in series to get -1.4? but here it will connect to 12v negative which in turns is ground so it won't work unless we put a divider like before which got us in this mess in the first place.
VEGETA:
I've been trying these ideas and I came up with this in attachments.

I hope this is the final or near final version, since I don't see any of the previous problems so far.

I've been able to get 1mA out of it which is fantastic, but you gotta know that current meter resolution is 10mA so we have more accuracy than our meter. It is completely safe!


Looking for your response.
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