# Resistor selected for Low-Side Sensing

I’m planning low-side sensing with the Allegro Two-Wire Hall-Effect Switch (A1190LUBTN-T) and I’d like to confirm my resistor selection for Rsense.

Current is 2-17mA and my ADC range is 0-3.3V.

V=I*R: 3.3V = (.017A) * (Rsense) gives Rsense = 194 ohms.

P=I*V: P = (.017 A) * (3.3 V) = .056 W

So, this 220 ohm 1/8 watt SIP-11 resistor array 4611X-101-221LF should be just about perfect?

Thanks!

• 200$\Omega$*17mA=3.74V, which is out of the ADC range. If you're just trying to detect the switch position, you can use a comparator instead of the ADC. – vofa Jan 22 '18 at 18:29
• @vofa -- (sorry i didn't mention upfront) I am just detecting switch position, so I don't need the entire dynamic range. I also need to use ADC on the Espressif ESP32 and not dissipate excess power on Rsense. Will that help specify a solution? – robg Jan 22 '18 at 19:39

## 1 Answer

The minimum supply for the A1190LUBTN-T is 3.0 volts so if you want to get a signal out that is as high as 3.3 volts to suit your ADC you need a supply that is at least 6.6 volts.

So, if you plan on using a 3.3 volt supply, your permissible maximum signal output level with respect to 0 volts is 0.3 volts.

At 17 mA this means a resistor of 17.6 ohms.

If you have a supply in the order of 7 volts or above then to obtain 3.3 volts across the shunt you need a resistor of 194 ohms (or less) as you calculated.

• My supply is 3.3V and as clarified above, I am just detecting switch position. Since I then don't need the entire dynamic range, is Rsense = 17.6 ohms what you'd apply for robust ON/OFF without excess power dissipation? – robg Jan 22 '18 at 19:52