EVM430-I2040S: Maximum shunt current

Hi abhishek,

Please find answers from our metering expert:

1. There’s two things to keep in mind here with a shunt: 1) the maximum voltage generated across the shunt by the maximum current 2) the size and power rating of the shunt. 

First, we want to keep the heat accumulation on the shunt as low as possible (lower the resistance the better), but we also want to take advantage of the whole input range of the SD24 to minimize noise. At the lowest gain of x1, the SD24 has an input range of around 900mV. At the highest gain of x16, this range is around 900mV / 16 = 56.25mV. Convert this to RMS voltage by dividing by sqrt(2), which equals 39.77mVRMS. Now, we know the max current and the max voltage across the shunt, so we can calculate the max shunt resistance. Since V = I*R, R_max_shunt = VRMS / IRMS. For a max RMS current of 30A, R_max_shunt equals 39.77mV / 30A = 1.3257mOhms. Assuming you don’t have any size restrictions on how large the shunt can be, I’d recommend to use a 1mOhm shunt resistance. If this value dissipates too much power, you can choose a lower resistance while keeping in mind that you could see more noise. 

With regards to the size and power rating of the shunt, we recommend that the power rating of the shunt is 8 times the actual power dissipation at max current. At 1mOhm, the power dissipation at max current is P = (I^2) * R = (30^2) * 1mOhm = 0.9W. Thus, you’ll need a shunt with a power rating of 0.9W * 8, which equals around 7.2W. To be safe, you can round up to 8W. Now, since the resistance (1mOhm) and the power rating (8W) are both known, you can find the proper shunt. For your reference, I found an exact match on Digikey. The P/N is CSS2H-3920R-1L00F, and it’s offered in a 3920 (1052 Metric) package. You can always use a smaller size for testing/development if you want to use the EVM430-I2040S.

2. Using the EVM430-I2040S, you can make the following connections. You can solder the wires to these through-holes. Make sure you are careful with the High Voltage too! 

• LIVE_IN: Line
• LIVE_OUT: V+ of the load
• NEUTRAL_OUT: V- of the load
• NEUTRAL_IN: Neutral

Please find the figure 4 of the TI Design Guide.

Any other question, please feel free to let me know.

Wei,