Current Resistor Selection Help

TC

Hello all,
I am working on a current sensor, and I am using Resistors to measure the current. I think I know what I need, but I would like to find out I am right with my math.

I am using Microchip’s PAC1720 Dual High-side current sense monitor. It has a sense voltage of 10mV, 20mV, 40mV, 80mV. And I am measuring 3 separate voltages. 3.3V, 5V, and 12V. Since the PAC1720 has 4 programmable sense voltages, I thought I could have 4 current ranges. Here is what I was thinking, please tell me if I am wrong

Using a .008 ohm, 1W resistor
.01V(10mV) / .008 ohm = 1.25A
.02V(20mV) / .008 ohm = 2.5A
.04V(40mV) / .008 ohm = 5A
.08V(80mV) / .008 ohm = 10A

Please let me know if I am wrong, because it does not seem right to me.

Thanks
TC

Duane C. Johnson

Hi TC

TC wrote: »

Hello all,
I am working on a current sensor, and I am using Resistors to measure the current. I think I know what I need, but I would like to find out I am right with my math.

I am using Microchip’s PAC1720 Dual High-side current sense monitor. It has a sense voltage of 10mV, 20mV, 40mV, 80mV. And I am measuring 3 separate voltages. 3.3V, 5V, and 12V. Since the PAC1720 has 4 programmable sense voltages, I thought I could have 4 current ranges. Here is what I was thinking, please tell me if I am wrong

Using a .008 ohm, 1W resistor
.01V(10mV) / .008 ohm = 1.25A
.02V(20mV) / .008 ohm = 2.5A
.04V(40mV) / .008 ohm = 5A
.08V(80mV) / .008 ohm = 10A

That seems correct to me.

It sounds like you can measure only 2 supplies in voltage and current.
So 2 of these will be needed to measure 3 supplies.
(Unless it can read its own VDD, but I didn't see that. Maybe it can do this.)

1W would be sufficient for the current shunts. However, I might suggest a 2W unit would be a better choice to lessen the thermal drift at high currents.
Use Kelvin Sensing to keep it accurate.
I.e. no high currents flowing in the sense lines.

Duane J

TC

Duane C. Johnson wrote: »

It sounds like you can measure only 2 supplies in voltage and current.
So 2 of these will be needed to measure 3 supplies.
(Unless it can read its own VDD, but I didn't see that. Maybe it can do this.)

1W would be sufficient for the current shunts. However, I might suggest a 2W unit would be a better choice to lessen the thermal drift at high currents.
Use Kelvin Sensing to keep it accurate.
I.e. no high currents flowing in the sense lines.

Duane J

Wonderful.

I was going to use 2 of them.

I was thinking of using a higher wattage, but I didnt know if it would matter.

I was wondering why some of the current resistors had 4 terminals. Now I know why, and I think I will use them.

Thanks
TC