Getting incorrect voltage measurements overt resistor divider bridge

[Károly]

Dear Rolf,

Thank you for the explanation. This is the thing I was missing, now the problem is much more clear. This is the first time I'm using a DAQ and I did not even think about the input capacitance at all.

[Károly]

Dear Henrik,

Thank you for all the useful information again. I only need to measure average power consumption over longer periods so I am not that concerned about small voltage spikes. The measurements on the 5V and 3,3V rail seem to work fine, it is only the 12V rail that I need to scale down somehow for the DAQ card. Also as I said before, this is part of a dissertation project so at this point I just need a working proof of concept and maybe somebody else who is an electrical engineer can take over and make it more accurate in subsequent years.

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@Henrik_Volkers wrote:

you will need buffers and you can't go with a low side  power measurement

---

By buffers you mean the high side current shunt amplifiers that we already discussed before right?

Here are some pictures as you requested:

First one shows shielded twisted pair cables soldered onto a PCIe riser for the SSD. On the second one you can make out the shunts on the right from the big fan in the computer as well as the shunts on the HDD next to the electrical tape.

With all this in mind here's the schematic of just the 12V rail:

As you can see I need to measure total power, power on an HDD and power on an SSD. 

For the first solution without external amplifiers:

Unfortunately I cannot easily make my shunts bigger so for now I have to try to make these work. So here you are suggesting to just build up four bridges (two above and below the 2,3m resistor and one below the 10m and 20m resistor each) similar to the 10M ones I had before just with 1k resistors this time. These would draw a more significant current of 12V/2k=0,006A but I can correct for that in software. Do you see any problems with that now in the complete circuit or do you expect it to work just like that?

For the second (better) solution with external amplifiers:

Again I cannot make my shunts bigger easily so I have to stick with these.

- The schematic above shows the currents I'm expecting (70A total can be reduced to 49A if necessary). With this, the voltage drops on the shunts could be anywhere between 0V to 0,161V

- The common mode is about 12V

- I am taking continuous samples with a rate of 10 measurements per second per channel. But this could be reduced if it is necessary.

- The amplifier needs to come in a package that can be hand soldered, either by itself on a matrix board or with some dip converter board. If I understand it correctly I would need to connect three of these one to each shunt (or find a three channel one).

Considering these I have stayed up all night researching amplifiers as I am not very knowledgeable at this topic. I looked at the TSC103IPT and to me it seems all right (even has features I don't need like adjustable gain) but maybe a bit too small to hand solder wires to. Two other potential candidates I found are the INA282AIDR from TI and MCP6C04 from Microchip and maybe something from analogue devices maxim amps (if you look at the sidebar they have many options listed) although I'm sure there are even many more options I did not even consider. Do you think any of these could satisfy my demands or can you recommend an alternative one that I did not consider?

Thank you,

Károly

[Jay14159265]

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@Károly wrote:

Dear Henrik,

Thank you for all the useful information again. I only need to measure average power consumption over longer periods so I am not that concerned about small voltage spikes. The measurements on the 5V and 3,3V rail seem to work fine, it is only the 12V rail that I need to scale down somehow for the DAQ card. Also as I said before, this is part of a dissertation project so at this point I just need a working proof of concept and maybe somebody else who is an electrical engineer can take over and make it more accurate in subsequent years.

---

@Henrik_Volkers wrote:

you will need buffers and you can't go with a low side  power measurement

---

By buffers you mean the high side current shunt amplifiers that we already discussed before right?

Here are some pictures as you requested:

First one shows shielded twisted pair cables soldered onto a PCIe riser for the SSD. On the second one you can make out the shunts on the right from the big fan in the computer as well as the shunts on the HDD next to the electrical tape.

With all this in mind here's the schematic of just the 12V rail:

As you can see I need to measure total power, power on an HDD and power on an SSD. 

For the first solution without external amplifiers:

Unfortunately I cannot easily make my shunts bigger so for now I have to try to make these work. So here you are suggesting to just build up four bridges (two above and below the 2,3m resistor and one below the 10m and 20m resistor each) similar to the 10M ones I had before just with 1k resistors this time. These would draw a more significant current of 12V/2k=0,006A but I can correct for that in software. Do you see any problems with that now in the complete circuit or do you expect it to work just like that?

For the second (better) solution with external amplifiers:

Again I cannot make my shunts bigger easily so I have to stick with these.

- The schematic above shows the currents I'm expecting. With this, the voltage drops on the shunts could be anywhere between 0V to 0,161V

- The common mode is about 12V

- I am taking continuous samples with a rate of 10 measurements per second per channel. But this could be reduced if it is necessary.

- The amplifier needs to come in a package that can be hand soldered, either by itself on a matrix board or with some dip converter board. If I understand it correctly I would need to connect three of these one to each shunt (or find a three channel one).

Considering these I have stayed up all night researching amplifiers as I am not very knowledgeable at this topic. I looked at the TSC103IPT and to me it seems all right (even has features I don't need like adjustable gain) but maybe a bit too small to hand solder wires to. Two other potential candidates I found are the INA282AIDR from TI and MCP6C04 from Microchip and maybe something from analogue devices maxim amps (if you look at the sidebar they have many options listed) although I'm sure there are even many more options I did not even consider. Do you think any of these could satisfy my demands or can you recommend an alternative one that I did not consider?

Thank you,

Károly

---

Based on the requirement that: "The amplifier needs to come in a package that can be hand soldered, either by itself on a matrix board or with some dip converter board." and the 70A max rating on the 12V rail. 

It sounds like you prefer a ready made solution that you can plug directly into your DAQ, Why not try one of these: https://cdn.automationdirect.com/static/specs/acuampdct.pdf

[JB]

I would use current transducers from LEM.

[Károly]

Dear JB and Jay,

thank you for the replies and the suggestions. Unfortunately these parts seem very expensive and if I understand it correctly I would need three of these transducers for my application. Why would you prefer this solution over the amplifiers that others have suggested?

[Károly]

Dear crossrulz, Henrik and rolfk,

I have looked even more into the amplifier option that you were suggesting. I have come up with a list of some potential chips that I think could work:
MAX4372 and MAX4173 from analogue devices
INA240 and INA282 from texas instruments
MCP6C02 and MCP6C04 from microchip
TSC103 from ST

Can you help me determine if any of these would really work based on the below requirements from my previous reply? Can you explain to me how you would derive the required specifications (bandwidth, slew rate etc) from the requirements if you would meet this problem in the industry?

The problem in detail again (EDIT: I can use converter boards from surface mount to dip so package size is not an issue anymore😞

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@Károly wrote:

Dear Henrik,

Thank you for all the useful information again. I only need to measure average power consumption over longer periods so I am not that concerned about small voltage spikes. The measurements on the 5V and 3,3V rail seem to work fine, it is only the 12V rail that I need to scale down somehow for the DAQ card. Also as I said before, this is part of a dissertation project so at this point I just need a working proof of concept and maybe somebody else who is an electrical engineer can take over and make it more accurate in subsequent years.

---

@Henrik_Volkers wrote:

you will need buffers and you can't go with a low side  power measurement

---

By buffers you mean the high side current shunt amplifiers that we already discussed before right?

Here are some pictures as you requested:

First one shows shielded twisted pair cables soldered onto a PCIe riser for the SSD. On the second one you can make out the shunts on the right from the big fan in the computer as well as the shunts on the HDD next to the electrical tape.

With all this in mind here's the schematic of just the 12V rail:

As you can see I need to measure total power, power on an HDD and power on an SSD. 

For the first solution without external amplifiers:

Unfortunately I cannot easily make my shunts bigger so for now I have to try to make these work. So here you are suggesting to just build up four bridges (two above and below the 2,3m resistor and one below the 10m and 20m resistor each) similar to the 10M ones I had before just with 1k resistors this time. These would draw a more significant current of 12V/2k=0,006A but I can correct for that in software. Do you see any problems with that now in the complete circuit or do you expect it to work just like that?

For the second (better) solution with external amplifiers:

Again I cannot make my shunts bigger easily so I have to stick with these.

- The schematic above shows the currents I'm expecting (70A total can be reduced to 49A if necessary). With this, the voltage drops on the shunts could be anywhere between 0V to 0,161V

- The common mode is about 12V

- I am taking continuous samples with a rate of 10 measurements per second per channel. But this could be reduced if it is necessary.

- The amplifier needs to come in a package that can be hand soldered, either by itself on a matrix board or with some dip converter board. If I understand it correctly I would need to connect three of these one to each shunt (or find a three channel one).

Considering these I have stayed up all night researching amplifiers as I am not very knowledgeable at this topic. I looked at the TSC103IPT and to me it seems all right (even has features I don't need like adjustable gain) but maybe a bit too small to hand solder wires to. Two other potential candidates I found are the INA282AIDR from TI and MCP6C04 from Microchip and maybe something from analogue devices maxim amps (if you look at the sidebar they have many options listed) although I'm sure there are even many more options I did not even consider. Do you think any of these could satisfy my demands or can you recommend an alternative one that I did not consider?

Thank you,

Károly

---

[Jay14159265]

---

@Károly wrote:

Dear JB and Jay,

...

Why would you prefer this solution over the amplifiers that others have suggested

---

There is a saying, "nothin in life is free" you are either going to pay in cash or pay in your time. If you need to spend a week implementing/debugging a circuit to take a measurement, that is a big time investment. It might be worth your time to just pay for the 'off the shelf' solution and spend that same week of engineering time doing something else.

The unfortunate thing for all engineers is that there is never an unlimited budget or unlimited time for a project. If you want to design a circuit, that is awesome but it will take some time but maybe that is the balance your project needs. 

I saw in the discussion that there are some op-amps that are being considered,  I would draw another schematic (in LT spice) of what you are trying to do and post it here. If you are going to do low side current sensing you can use some of those solutions. If you still want to do high side current sensing, you need to deal with the common mode voltage. There are some IC level solutions out there like this one: 

https://www.ti.com/lit/ds/symlink/tmcs1101-q1.pdf?ts=1686075794885

Others have brought up the issue with the time constant for the switching MUX, the reason this is an issue in the original circuit all goes back to the high impedance (10M resistors and the current limit you are imposing by using them) It will take a long time to charge up capacitance if you only have 900nA of current. If you go to an op-amp buffered measurement you have much more current to charge up capacitance so the time constant for the RC circuit at the input of the DAQ is much faster  ( this is also something you can model in SPICE to see what is going on ) 

[JB]

Hi Károly,

Have you checked the price for the LEM transducers ? They are inexpensive, but require a power supply and the creation of a small interface board.

One point not to be overlooked is the galvanic separation offered by these modules. This will prevent damage to the DAQ board.