Back to Basics: Freewheeling diode and How to choose one

I was designing for some motor applications earlier in the week and had to select a freewheeling diode for the circuit. So, I thought it might be a good time to cover that here. Freewheeling or Flyback or antiparallel (there are more aliases as people call it whatever they want) diodes are normal diodes used uniquely in a circuit. It’s connected right across an inductive load like a motor.

So how does it help? Take the example shown in the image, there is a motor that is turned ON/OFF with a MOSFET. During the ON cycle, the current flows through the inductor/coil of the motor, and the motor rotates. Now let’s turn OFF the MOSFET, the current flow from the power source suddenly stops. From Inductor 101, we know that an inductor doesn’t like abrupt changes in current and it has stored energy( in its coils(magnetic fields) during the ON cycle. Since the circuit is open, it has no way to discharge that energy, which means there will be a large spike in voltage at the inductor node, potentially damaging the MOSFET.

To avoid this, we place a diode in the opposite direction across the inductor which opens a new path for the energized inductor to discharge on its ON. In normal operation, since the diode is reverse-biased, it doesn’t affect the circuit.

How do you select one for your design? Choose a Schottky diode as it is faster to react. Find the maximum current passing through the motor/inductor during normal operation, your diode’s average forward should be much higher than this value. I personally use times x2 as a safety margin(if I am not penny-pinching on BOM prices). The maximum reverse voltage rating of the diode should be again higher by a factor of 2 compared to the normal working voltage applied across your motor. That’s basically it. You can select a freewheeling diode keeping these in mind and it will work just fine.

Hope that was helpful.

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Cool Tech from CES 2024

It’s January and that means it’s time for CES launches. I am sure most of you would be tired of the CES coverage and overuse of the word “AI” over the last few days. When reading through many CES articles with the usual tech upgrades, I found a small gem that didn’t get much attention.

The company is called Eclypia which is into developing non-invasive reliable continuous glucose monitors. An accurate non-invasive glucose monitor is supposed to be one of the grails of medical tech problems many companies are trying to solve. The current state of the art is still pinprick blood draw or a prohibitorily expensive needle patch which needs to be replaced often. So folks with diabetes are still looking for solutions that can measure their glucose levels.

According to a white paper by Eclypia, the wearable device has infrared red lasers illuminating the skin. This excites the glucose molecules in the skin, which creates a thermal signal as they return to their normal state. This signal is amplified by a photoacoustic cell and then picked up by a microphone. So they have only tested this in a digital twin in an in-silico or a simulated computer clinical trial. They use this simulated clinical trial to create a synthetic dataset to fine-tune their deep-learning algorithms. I see that they have registered for a few in-person clinical trials online. The full results of it are not out yet and the paper claims a promising error rate of 18.4%. I wish them all the best for a larger clinical trial. It’s high time that someone finally cracks this problem once and for all.

What was your favourite CES tech this year?

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