arduino power control

Thread Starter

Nathan Hale

Joined Oct 28, 2011
159
Hi folks, I was reading about the circuit below and i am having a hard time understanding what the author was trying to say in the description about the diode. Could you please try to explain what he is trying to say about the diode? How exactly is the diode helping us out by being in the circuit ?
The author said.....

Why Diode?
The relay coil cannot change it’s current instantly, diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors

Thanks a lot for your replies.

Image source: https://igniteinnovateideas.wordpress.com/2016/04/27/control-ac-light-using-arduino/


 
The inductor stores energy. V(t)=L * di/dt is the voltage across an inductor. When the transistor turns off, that's a large change of I(t), hence a voltage exists across the inductor that decays. We want that decaying voltage to be impressed across the resistance of the inductor. That's what the diode does, it creates a circuit with the voltage across the inductor, the resistance of the inductor and the diode and diode drop.

Otherwise the voltage might damage the driving transistor.

There are two rules you learn:
#1: The voltage across a capacitor can't change instantaneously.
#2: The current through an inductor can't change instantaneously.
 
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