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Archive for December 4th, 2013

Hall Effect Current Control PCB: Voltage Variations

The whole point of the Hall effect current sensor was to get a reasonably efficient linear LED driver that could control the LED current until the battery voltage matched the LED forward drop. Based on the preliminary firmware, it works pretty well.

With a setpoint of 160 mA, the current stabilizes around 150 mA due to the Arduino’s 0.4% PWM resolution. It steps back and forth between 150 and 190 mA as the loop bumps the PWM by one count; these scope shots came from the lower current passes.

At 8.4 V from the bench supply, the MOSFET sees about 2 V. The top trace is the drain voltage, the bottom is LED current at 50 mA/div:

VIN 8.4 V - VD ILED 50 mA-div

VIN 8.4 V – VD ILED 50 mA-div

At 7.4 V, close to the nominal voltage during most of the discharge curve, the drain sees about 1 V:

VIN 7.4 V - VD ILED 50 mA-div

VIN 7.4 V – VD ILED 50 mA-div

And at 6.4 V, even though the drain voltage hits zero, the current remains around 150 mA:

VIN 6.4 V - VD ILED 50 mA-div

VIN 6.4 V – VD ILED 50 mA-div

Admittedly, down there the loop doesn’t have much in the way of control authority, but I planned to turn the lights out at about that point, anyway.

The driver efficiency is 86% at 7.4 V and it’s pretty nearly 100% at 6.4 V.

Of course, the Hall effect circuitry and Arduino Pro Mini soak up another 40 mA or so, so (assuming a 10% duty cycle) the overall efficiency is down around 70%, but that’s including the debugging LEDs and suchlike, so some tweaking is in order.

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