App note: Magnetic sensors’ and reed switches’ capacitive Loads

An application note from Littelfuse on peak current considerations when reed switches and magnetic sensors are handling capacitive loads. Link here (PDF)

When there is significant capacitance in a reed sensor, reed relay or reed switch circuit, the peak current and energy switched by the reed contacts should be considered. However, if the capacitance is less than 100 nF at 5 V or 0.1 nF at 150 V, and the cable length is less than 10 meters, the capacitance will not significantly effect switching life.

If a capacitor is placed in parallel across the reed contacts, the peak current will be determined by the load voltage, the contact resistance, the wiring resistance, the ESR of the capacitor, and the inductance of the circuitry. Because the resistance and inductance in the circuit path are likely small, the peak current can be amperes or tens of amperes, exceeding the maximum switching current of the reed switch, reed relay or reed sensor. Even if the maximum switching current is not exceeded, switching life may be reduced.

A capacitor not directly across the reed contacts may still generate a high current spike when the reed contact is closed. Depending on the circuit arrangement, the peak discharge current may occur when the capacitor is charged or discharged. Components other than capacitors can have significant capacitance, including long cables, MOVs (Metal Oxide Varistors), and MOSFET gates.

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