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A diode is put in parallel with a relay coil (with opposite polarity) to prevent damage to other components when the relay is turned off.

Here's an example schematic I found online:

enter image description here

I'm planning on using a relay with a coil voltage of 5V and contact rating of 10A.

How do I determine the required specifications for the diode, such as voltage, current, and switching time?

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up vote 16 down vote accepted

First determine the coil current when the coil is on. This is the current that will flow through the diode when the coil is switched off. In your relay, the coil current is shown as 79.4 mA. Specify a diode for at least 79.4 mA current. In your case, a 1N4001 current rating far exceeds the requirement.

The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. A diode in your application having 50 volts would be more than adequate. Again 1N4001 will do the job.

Additionally, the 1N4007 (in single purchase quantities) costs the same but has 1000 volt rating.

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Nice tip about the 1N4007. – Samuel May 16 '14 at 6:12
For such a type of relay, even a 1N4148 would do the job (Vrrm = 100V, If = 200mA, Ifsm = 1A for 1 second). This diode might be faster and will most probably be cheaper too, although that's not important if used for low quantities of course. – GeertVc Jul 26 '15 at 8:56
  1. Voltage required is the nominal coil voltage, since that is what will be applied. Give it a factor of 2 for safety.

  2. Current requirement is the nominal coil current.

  3. Speed is probably not a consideration for relay coils, since they are not turned on/off very often, as compared to, for instance, a PWM motor drive.

In your case, a 1N4001 will probably work just fine.

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Things aren't always as simple as they seem, though in the case of relays its highly application dependant. While the diode provides a safe discharge path that preserves your switching transistor and power supply, it can cause a few issues in certain applications. Relays on closure can form a small weld at the contacts, and by placing the diode there you are essentially preventing the relay from opening with its full force. This can cause the contacts to 'stick' together slightly longer, and overall is bad for the relay. A trick I learned a few years ago to prevent that from happening was to put a zener diode in series (obviously in different direction) with the the regular diode, this allows you to control the maximum voltage and allows the relay's coil to discharge in a slightly better way. I recall some relay manufacturers had pretty good application notes on this, last one I saw was from Tyco but I couldn't find it again sadly.

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