Overcurrent Sensing

Question

I am designing an overcurrent protection circuit. I am sensing current with a LEM sensor across a sensing resistor of 16.5 ohm. The voltage drop for maximum current is 2 V. In the next stage I am using INA301A1 (CMVR 0 - 36 V, unipolar power supply 0 V / 3.3 V). This IC has a fixed gain of 20 V/V. Now for full scale drop of 2 V, my output will be saturated (40 V), so I need to I need to step down 40 V To 3.3 V.

What is the best way to do it?

I am thinking about using a potential divider between sense resistor stage and overcurrent IC stage but using a lower (10s of ohm) resistance divider may load my sense resistor and higher (kohms) might load input of IC.

Are these apprehensions plausible and what could be a possible solution?

Answer 1

What is the best way to do it?

The best way is to do what every engineer is doing in circuits like this - using as small shunt resistor as possible. 16 Ohm is not a "sensing resistor", it is unnecessary load. Current shunts typically in range of a few mOhm to 1 Ohm, which results in correct voltage drop in mV range (250 mV for INA301A1).

From your numbers the maximum current seems to be 120 mA, so 2 Ohm shunt will give you correct 250 mV voltage drop.

UPDATE

There was a misunderstanding corrected in the comments below. It turns out the OP does not measure voltage on shunt resistor in the actual high power circuit, but rather voltage drop on burden resistor of current transducer.

Taking this new information into account two solutions were suggested:

• adjust burden resistor to bring voltage swing into full-scale range of ADC input and use any OP Amp in comparator configuration for overcurrent signal generation;

• same as above but drop the overcurrent signal altogether and use MCU comparator input to generate interrupt on threshold. This would eliminate all external components except for one burden resistor.

All of the above assumes uni-directional current, implied by original use of INA301 chip. Bidirectional sensing requires more complex circuit, as discussed in @TonyEErocketscientist answer.

Answer 2

Rather than using a fixed gain INA, use a gain and offset DIff Amp OA design to scale your input to the desired output.

if 0 to 2V in and 0 to 3.3V out then non-inverting gain = 1.65

If -2V to +2V in and 0 to 3.3V out then gain = 0.825 and offset = +1.65V (=Vss/2)

I suspect you want to measure high current AC primary so you want to measure both polarities > +/- x.xx V so you compute dual thresholds y using Vdd/2 as a null reference.

But keep in mind radiated CM noise. Having 120 dB CM conducted noise rejection is wasted if the cable is only 50dB CMRR with a 500A impulse going thru a Rogowski coil, so choose your cable wisely. that is already 50dB above your current sensor secondary. Getting 100 dB attenuation of Injected H field current impulse near the cable may depend on doubled shielded twisted pair with a Pi filter CM Choke and orthogonal orientation. This is what I have had to use in the past with monitoring 10kA arc welders.

opinions and design specs?

For the OCP logic output, I would ask if real-time and polarity is important. Because, knowing the high Remanence Currents from saturation on power up due to the phase difference and resulting magnetization V difference between power input stop and restart, start current can be quite large if starting 180 phase shifted and make some Xfmr's "hum" for a long time until unsaturated or return to zero averarge magnetization current. Also Arcs can be polar so a single-sided OCP logic output does not use all the useful information available on different faults.