An inexpensive current limiter for flashing and initial testing of ESCs – Mk II

There is a risk of damage when flashing ESCs. It accrues from the fact that ESCs have a three-legged H bridge and if a high and low FET are turned on simultaneously, damaging currents may flow. In fact, this can be an issue if the FETs are on together for just microseconds on each PWM cycle. Loading the wrong hex module is a recipe for disaster, it may turn on FETs in an unexpected way.

So, for safety, the ESC should be powered from a current limited power supply during flashing and initial motor testing.

In a process of continuing development, this article describes a variation on the inexpensive current limiter for flashing and initial testing of ESCs – Mk I.

This version uses a Polyswitch as the current limiting device. Polyswitches are a resistor which is very low in value until a threshold current is exceeded and the heat increases the resistance greatly. They can be though of as a self resetting fuse, though it takes a moment for them to cool down. Their response time depends on current, they are not as fast as the incandescent lamp used in the Mk I version.

CurrentLimiter02Above, the prototype.

Circuit is essentially the same, a 3A 72V Polyswitch was used as the current limiter, and in shunt with it is a Schottky diode to allow regenerative current to flow from the ESC back into the battery (so cathode towards the battery +) and also in shunt, a 2k2 resistor in series with a red LED to indicate when the Polyswitch has been triggered.

The current rating is sufficient to power a quadcopter FC, Rx, accessories and ESCs, and to allow a low speed test of one motor to be sure that current is safe enough to try directly on a LiPo battery.

I usually use an electronic regulated supply on the bench, but this little device can be taken into the field and flash updates done from a notebook computer and safely tested with the current limiter, the normal Lipo battery and a servo tester.