MAX471 current sensor

A high-side current sensor board: a 35 milliohm resistor in the supply lead, with the current on AT at 1 V an amp and the voltage on VT.

A small red board (GY-471): a screw terminal for the supply in and the load out, and a header with the current and the voltage as two analog outputs. Aliases: GY-471, MAX471 module, voltage and current sensor.

The chip has its own 35 milliohm sense resistor and an amplifier that turns the current through it into a current out, 500 µA for every amp; the board turns that into 1 V an amp on AT. It goes in the plus lead, between the supply and the load, and it runs from that supply: there is no separate power pin.

Pins

Pin	What it does
VIN	From the supply's plus, 3 to 36 V. One end of the sense resistor.
VOUT	To the load. The other end.
GND	Ground: the supply's minus and the Arduino's ground.
AT	The current, 1 V an amp, either direction.
VT	The voltage on VIN, divided by 5.

Properties

None: the board has one range, up to 3 A.

A real resistor in the circuit

VIN and VOUT are the two ends of the chip's 35 milliohm resistor, and the part is that resistor: the load's current really flows through it, and the readout on the board shows the same current the simulation computes. VT is a 30 kohm and 7.5 kohm divider, in the circuit as the two resistors it is.

What the model gets right

From the Maxim MAX471 datasheet and the board's parts:

The current. 500 µA out for each amp through the resistor, into the board's 2 kohm to ground: AT is 1 V an amp. On a 5 V Uno one converter step is 4.9 mA, and 1023 is 5 A, past the chip's 3 A.

Either way around. The output is the size of the current whichever way it flows; the chip's SIGN pin, which says the direction, is not on this board.

The ceiling. AT goes no higher than 1.5 V under VIN, so on a 3.3 V supply the current reads at most 1.8 A.

The supply. 3 to 36 V on VIN; under 3 V the chip is off and AT rests at 0 V.

The voltage. VT is VIN / 5, so a 25 V supply reads 5 V, the top of a 5 V Arduino's converter.

What it does not model

The chip's 2% error and its few microamps of output error at no load, the tolerance of the board's resistors (a measured board can read 5% low), supply rejection, the chip's 50 µA of its own, heating of the sense resistor, and anything past 3 A. Several board designs are sold; this is the red one with VIN, VOUT, GND, VT and AT.

Common mistakes

Putting it in the ground lead. It is a high-side sensor and runs from VIN: in the minus lead it has no supply and reads nothing.

Measuring under 3 V. The chip needs 3 V on VIN to run.

Forgetting the ground. AT and VT are measured from GND, so the board's GND, the supply's minus and the Arduino's ground must be one.

See it in action

Lamp power meter shows a 6 V lamp's volts, milliamps and milliwatts on an I2C LCD. Open it at /templates.