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I've connected up the coil of a relay to a transistor, which is in turn connected to an Arduino Nano. The circuit is a timed relay switching circuit. I know that when the current to an electromagnet is turned off, the collapsing magnetic field causes a reverse voltage to travel across the power connections. My question is: if I use a diode to protect the circuit from the voltage do I place the diode on the anode, or the cathode of the coil? Or does it not matter which? enter image description here

Mario Gianota
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  • @Sim son When I place the diode as you say and the coil generates a reverse voltage when it is switched off does the diode sink the current preventing it from traveling down the wires? – Mario Gianota Sep 11 '21 at 19:58
  • This has been covered in so many ways in so many answers on stack exchange so, do some searching and be prepared to have your answer closed. – Andy aka Sep 11 '21 at 20:00
  • @Andy Yes, thanks. It answered my question. – Mario Gianota Sep 11 '21 at 20:08
  • Well, it's going to get closed so save people the waste of time reading it and delete it yourself is my advice. – Andy aka Sep 11 '21 at 20:11
  • It won't allow me to delete the question. I guess someone else can close it. – Mario Gianota Sep 11 '21 at 20:13
  • It also would not drive 5V to the coil. Since Arduino output is 3.3V, it would output 3.3V minus about 0.7V for the Vbe drop, or about 2.6 volts to the relay, due to it being connected as emitter follower. The buttons are incorrectly connected too, the Arduino inputs are always grounded, whether or not the button is pushed. – Justme Sep 11 '21 at 20:28
  • The arrangement shown in the diagram won’t be effective- a1N4001 is rated to 50V, the coil will generate enough voltage to cause the diode to break down. You may get away with it since the diode will dissipate most of the energy but it’s far from ideal. The preferred solution is to connect the diode directly across the coil. – Frog Sep 11 '21 at 20:39

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You connect the diode between the coil's contacts: cathode at positive, anode at negative node (so the diode does not conduct in normal operation).

Sim Son
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