user57340: I've built this simple Arduino controlled led-array. The Arduino sets a 5v pin on a relay to connect the high current power supply to a led array. 
However, just before the led array is being fully turned on I'm getting this fast flash of the led array. I really would like to have the led working in an "on/off" manner.

I remember that working with some motors one have to use capcitator in parallel because of resistence drop. Is it also the case with led array? And if it is why so, and why a capcitator would help?
Thank you very much.

user2967: As Wendall says in his comment, you probably have an issue with the power-on state of your logic pins before the Arduino finishes booting up.
In addition to that, you should not drive a relay directly from an Arduino logic pin, for a couple of reasons:

It can't provide enough current to drive a relay coil.
The relay emits a strong surge of reverse current when you switch it off. This will likely destroy the logic pin you are using to drive the relay, or it might even destroy the whole Arduino board.

For these reasons you should drive the relay using a transistor, and you should put a properly sized diode wired across the relay coil backwards from the flow of current when the relay is energized. The diode bleeds off the reverse current from the relay coil.

57340: <p>I've built this simple Arduino controlled led-array. The Arduino sets a 5v pin on a relay to connect the high current power supply to a led array. 
However, just before the led array is being fully turned on I'm getting this fast flash of the led array. I really would like to have the led working in an "on/off" manner.</p>

<p><a href="https://i.stack.imgur.com/lbGAG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lbGAG.png" alt="Scheme"></a></p>

<p>I remember that working with some motors one have to use capcitator in parallel because of resistence drop. Is it also the case with led array? And if it is why so, and why a capcitator would help?</p>

<p>Thank you very much.</p>
 2967: <p>As Wendall says in his comment, you probably have an issue with the power-on state of your logic pins before the Arduino finishes booting up.</p>

<p>In addition to that, you should <strong>not</strong> drive a relay directly from an Arduino logic pin, for a couple of reasons:</p>

<ol>
<li>It can't provide enough current to drive a relay coil.</li>
<li>The relay emits a strong surge of reverse current when you switch it off. This will likely destroy the logic pin you are using to drive the relay, or it might even destroy the whole Arduino board.</li>
</ol>

<p>For these reasons you should drive the relay using a transistor, and you should put a properly sized diode wired across the relay coil backwards from the flow of current when the relay is energized. The diode bleeds off the reverse current from the relay coil.</p>
