Need to monitor the current on -12V Supply

Thread Starter

lindula

Joined Sep 23, 2016
90
Hello, I need some helping find a device to monitor the current and voltage of a -12V power rail on my custom board. I have been looking at the LTC2946 Datasheet and on page 22 of the datasheet they have the figure 16 what looks as a circuit to monitor a negative voltage supply. However, I am not clear on High-side and Low-side measurement.

I really like the I2C interface on the LTC2946.

Can anyone give me some help please or suggest a device to do the job.

Thank you
 

ScottWang

Joined Aug 23, 2012
7,278
Using a 1000 mOhms shunt resistor to measure the current and using a multimeter voltage range to measure the voltage in parallel with the shunt resistor.
I = V/1000 mOhms

Edit:
1mΩ, not 1000mΩ.
 

Ian0

Joined Aug 7, 2020
5,945
Measuring the current in a negative supply is a function that the world just assumes you don’t want to do.
So, look under the lid of a positive supply monitor and reverse everything, and you get this:
1DFEFA5C-FEB9-4115-889A-F77E396E3657.jpeg
You need a low offset op-amp, with a common-mode input voltage that includes its negative supply, luckily they are quite common.
You can measure the output with an A/D on your micro, remembering that the result reads negative 0xFFFF is no current and 0 is maximum.
R1, R2 and R3 depend on the current you wish to measure.
The schottky diode prevents the output going negative, the MOSFET is any small MOSFET like BS170. The output is at the junction of R3 and the schottky diode.
 

Papabravo

Joined Feb 24, 2006
19,023
Refer back to Figures 5(a) through 5(d) for the definitions of high side and low side and whether a shunt regulator is required.
1659799492975.png
Figure 5(a) Vin is > 100V with respect to GROUND potential. The chip is sensing current at potentials that are above GROUND potential. The chip is deriving power from INTVcc, Vdd, and GND pins which are all above GROUND potential. So high side current sense with high side shunt regulator.

Figure 5(b) 0≤ Vin ≤ 100 V with respect to GROUND potential. The chip is sensing current at potentials that are all at or above GROUND potential. The chip is deriving power from a voltage that is > 100V through Rshunt and the GND pin which is at GROUND potential. High side current sense and low side shunt regulator.

Figure 5(c) Vin > -100 V with respect to GROUNND potential. GND pin is at Vin. The chip is sensing currents at potentials that are below GROUND potential. The chip is deriving power from INTVcc, Vdd and GND pins which are all below GROUND potential. So low side current sense and low side shunt regulator.

Figure 5(d) -100V ≤ Vin ≤ -4V with respect to GROUND potential. GND pin is at Vin. The chip is sensing currents that are below GROUND potential. The chip is deriving power from the monitored supply. So low side current sense and low side regulator. In this particular case there is no external connection between INTVcc and Vdd. That is because of the restriction on Vin.

Given you -12V supply, I think Figure 5(d) is the configuration that you should use.

I hope this helps. Good luck with your project.
 

Thread Starter

lindula

Joined Sep 23, 2016
90
Refer back to Figures 5(a) through 5(d) for the definitions of high side and low side and whether a shunt regulator is required.
View attachment 273276
Figure 5(a) Vin is > 100V with respect to GROUND potential. The chip is sensing current at potentials that are above GROUND potential. The chip is deriving power from INTVcc, Vdd, and GND pins which are all above GROUND potential. So high side current sense with high side shunt regulator.

Figure 5(b) 0≤ Vin ≤ 100 V with respect to GROUND potential. The chip is sensing current at potentials that are all at or above GROUND potential. The chip is deriving power from a voltage that is > 100V through Rshunt and the GND pin which is at GROUND potential. High side current sense and low side shunt regulator.

Figure 5(c) Vin > -100 V with respect to GROUNND potential. GND pin is at Vin. The chip is sensing currents at potentials that are below GROUND potential. The chip is deriving power from INTVcc, Vdd and GND pins which are all below GROUND potential. So low side current sense and low side shunt regulator.

Figure 5(d) -100V ≤ Vin ≤ -4V with respect to GROUND potential. GND pin is at Vin. The chip is sensing currents that are below GROUND potential. The chip is deriving power from the monitored supply. So low side current sense and low side regulator. In this particular case there is no external connection between INTVcc and Vdd. That is because of the restriction on Vin.

Given you -12V supply, I think Figure 5(d) is the configuration that you should use.

I hope this helps. Good luck with your project.
Thank you very much. One more question please, I see in the datasheet they use isolators on the I2C pins, is this really necessary when using power monitors when monitoring positive voltages and negative voltages. On my positive +12V I'm using an INA233 and I don't see a reference in the datasheet to isolate the I2C pins from the MCU.

Is this a good idea to use isolation?

Thank you
 

Papabravo

Joined Feb 24, 2006
19,023
Thank you very much. One more question please, I see in the datasheet they use isolators on the I2C pins, is this really necessary when using power monitors when monitoring positive voltages and negative voltages. On my positive +12V I'm using an INA233 and I don't see a reference in the datasheet to isolate the I2C pins from the MCU.

Is this a good idea to use isolation?

Thank you
If you had only the negative supply and a processor operating from that negative supply then you could do it without the isolation. I'm guessing that you processor is powered from the positive supply so you need the isolation or you have to use some kind of level shifting so you can talk to the device. Isolation is one way to do this but it is not the only way.

Is it a good idea? It depends on the circumstances. If everything is contained on a single board or boards in close proximity then it may not be necessary. It won't hurt in the absence of a better solution. One thing you might want to consider is that the CTR (Current Transfer Ratio) will degrade over time until the opto becomes useless. How long do you expect these boards to be in service?
 

Thread Starter

lindula

Joined Sep 23, 2016
90
If you had only the negative supply and a processor operating from that negative supply then you could do it without the isolation. I'm guessing that you processor is powered from the positive supply so you need the isolation or you have to use some kind of level shifting so you can talk to the device. Isolation is one way to do this but it is not the only way.

Is it a good idea? It depends on the circumstances. If everything is contained on a single board or boards in close proximity then it may not be necessary. It won't hurt in the absence of a better solution. One thing you might want to consider is that the CTR (Current Transfer Ratio) will degrade over time until the opto becomes useless. How long do you expect these boards to be in service?
Hello, I would guess about the board would be used for maybe 15-20 years.
Thank you
 
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