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I am trying to harvest energy from a stepper motor by turning its shaft with my hand.

Since it is a 2 phase motor, I rectified both phases and connected them in parallel to get more current, then a capacitor stores and smooths the voltage and supplies a boost converter which outputs 2.7 V.

enter image description here

Sometimes the capacitor voltage is lower than 2.7 V but it can also be higher.

The boost I'm using is an MCP1640 which has a low input voltage (0.8 V startup voltage and 0.35 V after startup.) It is useful because the supply is a discharging capacitor.

It is ironic that I have to dissipate energy in an energy harvesting project.

In order to limit the voltage I use a 2.7 V/1 W through-hole Zener diode but it doesn't regulate the capacitor voltage as you can see on this screenshot:

Blue: capacitor voltage

Red: Boost output voltage

Before putting the diode in my circuit, I tested the reverse voltage with the diode check function of a multimeter and it displayed 3 V, so I guess the Zener is working correctly.

Why is the Zener not regulating the capacitor voltage?

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2 Answers 2

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You need a very stiff limiter/regulator with a sharp corner. A low voltage Zener isn't that. Consider a shunt regulator with feedback instead such as the TL431.

Or use a switching regulator that can boost and buck (e.g. a SEPIC or Cuk converter), for better efficiency. It is a harvesting application after all. Do consider though, that while you generate more power than you consume, the capacitor voltage will rise indefinitely. So at some point you would be forced to dissipate energy in a protective shunt anyway.

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You don't have a resistor before the Zener. A Zener regulator needs a resistance to drop voltage across. As the Zener conducts it draws current, the current causes a voltage drop across the resistor, that's how they regulate. You will get some amount of regulation due to the impedance of the generating circuit, but it will be too little for a Zener to regulate well.

schematic

simulate this circuit – Schematic created using CircuitLab

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