4QD-TEC Electronics Circuits Reference Archive
Battery Current Sensor

In case you have not read it, Part 1 of Battery Current Sensor


The circuit values are shown in the first diagram.


The project has not to date been finished as there seems now to be little demand, so the following notes describe the prototype and the principles.


The original idea was to use a soft iron strip as a loop to complete the magnetic path around the conductor being sensed as below.


The circuit board that was made for this was designed for this soft iron loop, which bolts to the circuit board, just to the right of the sensor.

Circuit Board


This board is shoewn from the component side, as if the circuit board is transparent.

Ferrite core

Just before shelving this project, we did experiments using a ferrite core with a slot cut into it. A suitable Ferrite can be appropriately slotted with a diamond saw blade.


Choosing Ferrites can be a bit of a nightmare: the magnetic parameters are probably OK if you have a degree in magnetic circuitry! However, the chosen ferrite should have a low hysteresis B-H loop. The circuit is measuring effectively d.c. most of the time and hysteresis is not useful! The one we chose was Farnell number 180-009. It's a Philips Ferroxcube component, Philips number 432202097190, type TN23/14/7-4C65. The 4C65 bit is the grade of the Ferrite material. It's 23mm o/d, 14mm i/d and about 7mm wide, hence the 23/14/7 bit!

Magnetic Facts

The UGN 3503 has a sensitivity of 1.3mV per Gauss and a maximum measuring ability of ±900 Gauss. You should aim to work at around 200 to 300 Gauss to get good output signal with low drift.

According to the Allegro application notes, the Flux, in Gauss, produced by a straight conductor is equal to I/(4.PI.r) where I is the current and r is the distance in inches.

Using a toroid, you should get a flux of N x 6 Gauss per amp, where N is the number of turns. We were looking for a battery current limit in the region of 60-100 amps, so we would get 360-600 Gauss, a sensible range, with a single turn.

How it works

The Hall effect sensor type UGN 3503 is a linear sensor, whose output voltage is around half the supply voltage when there is no magnetic field. The output voltage swings positive or negative in ration to the strength and polarity of any applied magnetic field.

The UGN 3503 has a frequency response up to about 23kHz, so when using it so sense the field induced by the supply current to a PWM amplifier, its output wi;ll be a squarewave at the switching frequency. We want to average this voltage, so the sensor's output is fed through a low pass filter, R1, C1 to IC1.

In the original, IC1 was one half of an LM392. This is a very useful (if somewhat expensive) IC as it contains a comparator and an op-amp in a single package. Here, we are using the comparator. As soon as the average voltage from the sensor rises above the threshold set on Vr1, the output of ICs goes low, pulling down the output. An LED is present to show when the circuit is operating.

In the original circuit, this was used to ground an inhibit input. However, all 4QD controllers are driven by a demand voltage (from the throttle device) and this output can just as easily be used to short to ground the wiper of the throttle pot, holding the demand speed at whatever value is required to engage the battery current limit.

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