# Amp sense across a motor or mosfet no sense resistor

I want to use a Cypress POSC4 product to do an amp sense without a sense resistor.
Using the on board op amp to amplify the volt drop and the ADC to measure the voltage.

The sense is to prevent the load from over current
Circuit
+12v - PnP Mosfet(RDSon .0024 ohm) - Brushed DC Motor (2 ohm) - ground
Mosfet is used a switch 12v off and 0v on
I will ignore the Start up current until the motor has time to spin up with load then measure the voltage drop across the either the motor or the mosfet.

The motor runs until it loads to 3 amps and then the motor is turned off
The motor is pushing a ram when the ram contacts and immovable object the motor amps goes up

The motor is rated to 5.5 amps for 15 seconds before it gets hot enough to damage it

The sense circuit will be through a 10k resistor with enough (capacitance to ground) to absorb back emf to clean up the signal

Is there any reason this approach won’t work?

It seems pretty sketchy. If you measure voltage across the motor, you have at least two problems. First, the voltages will be well beyond the range of the op amp, which should never be higher than Vcc of the chip. You can scale this using a resistor divider, but there are risks of exceeding the max input of the chip during transient conditions. It would need protections such as a TVS and possibly a low-pass filter (RC-type).

Second, and more fundamentally, because the motor is highly inductive, the impedance is not just resistive, but also reactive, and is therefore not a reliable way of measuring current.

If you use the MOSFET as your “current sense” resistor, that poses other problems. The first problem is that you are using a P-Channel MOSFET on the high side of the motor. This is a problem because not only will the voltage be above the maximum allowable voltage of the input of op amp, but also the low side of the MOSFET will not be referenced to the same potential (ground). In this configuration, you would be measuring a differential voltage with a large offset, which is not possible without additional external circuitry. If you changed the design and swapped the P-channel MOSFET with an N-channel and placed the FET on the low side, then, you have a common ground reference to measure the signal. You would still need to protect it using similar techniques as described above.

The second problem with using a MOSFET is that the Rds-on of a MOSFET can vary from part to part and as conditions change. It would not give a very accurate reading, as its value can more than double under some conditions. If you don’t mind very poor accuracy, this method is, at least, feasible.

You would get much better results using a current sense resistor or a current sensor, such as one of these:

Current Sensors