Hello,
I want to use some relays to switch a high power circuit.
I have read multiple times that using the Arduino 5V pin to power the coils can be dangerous because they can draw too much current, and I should use an external power supply.
So, is it safe to use a relay shield like this one ? I guess that I won´t be able to connect a power supply since the shield already connects Vcc with the 5V pin.
Similarly, there are some separate relay modules like this one, that has Vcc and JDVcc connected by default without any jumper wire.
I am working on a project where I won´t be able to have any power supply or batteries powering the coils, so I wanted to know how safe is it to use any of the relay modules mentioned above.
The relays support up to 30V DC 2A. Is that "high" enough?
Yes, I will be working in that range
OK, you are experiencing some confusion here.
The point is that you do not power a serious project vie "Vin" or the "barrel jack" on the Arduino UNO(/ Mega 2560/ Leonardo/ Nano/ Pro Mini.
Both the Arduino and the relays require 5 V power, so you provide exactly that - regulated 5 V - to the "5V" power input pin. The only problem specifically with the UNO and Mega 2560 is that if the PC is connected to the USB port while you are powering it through the "5V" pin, it may feed a slightly higher voltage to the PC's USB system and cause it to shut down, so you need to disconnect the 5 V while connected to USB.
Thank for the response Paul.
I might have not explained myself clearly.
I think that my question is just if it is safe to power the relays just using my Arduino board, or do I need an external power supply as it is described in this discussion.
How dangerous is it not to use an external power supply for powering the relays?
It won't work. At least not when you gravely need it.
You need an external power supply.
So if I´m using the relay shield linked in the OP, in which pin do I connect the power supply?
Check out #4. 
Just to clarify, I´m talking about the power supply powering just the relay, not the Arduino board, as you can see in the diagram below.
The current consumption of the coils is 35 mA per coil (so 140 mA total). You can power your project from a PC or an external 5V supply (on the 5V pin). Do not try to power from the external supply and have USB power (from PC) connected at the same time.
I'm not sure about transient currents.
What do you want to control? Depending on answer, you can consider a 'pure' electronic solution.
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