Connecting a 12V relay to Arduino

Step 5 Choosing your diode

The diode is needed cause the voltage will rise high if you suddenly change the voltage at the inductor. The formula for the voltage is:

U_L = - L * delta i/delta t

So theoretically if delta t equals zero U will be infinite.

But due to the minus in front you can add a diode in the "false direction" parallel to the relay. So the current can flow till its zero so the voltage is also zero.

Comments

[thecageybee]

Hi there.   Brilliant post.   Very informative, the formulas you have provided have been very helpful, as without them I'd of just been blindly putting components together.  Likely with disastrous results.

I've got a couple of questions for you though.

First the easy one.  Could you explain the formula in step 5.  What is "U_L", "L", "delta i" and "delta t"?  Where do I find their values in regards to the components I'm going to use.  Just not sure if it matters what rating diode I use, or if it's merely mathematical proof that I will need a diode and any diode will do.

Lastly, I'm going to be applying this for a project of my own, but want to make sure I can get the relay circuit working before I get an Arduino.  Can I simulate the arduino by applying a 5v positive to where the arduino would go, with the negative to ground in the circuit?

Again, many thanks for this guide, and thanks for any help you can give me.

Regards,
The Cageybee

[homunkoloss]

"U_L" (L is meant to be the index) is the voltage at the relays coil, Im writing U instead of V because thats how I learned it and how its written in europe. "L" is the inductance of your relays coil. "delta I" is the change of the current and "delta t" is how fast you apply the Voltage (Sorry cant explain this better). The problem is that the Voltage rises very high (probably about 100 to 300 V, but not harmfull because of the low current), this could destroy the transistor. So when choosing your diode you should use one that can stand high voltages.

[thecageybee]

Oopps!  'mybad.  Need to read the whole of the post.   Second question answer in step 7.

Appologises!  :-)
 The Cageybee