Hi there,
simple question (I guess) : It seems that all the relays I can find are desinged to supply power from 5V DC to a highest AC voltage.
Isn't there any relay module from 5V DC(arduino) to 12V DC, not AC? Or should I use the same one?
The idea here is to use it to power up a 12V DC rearview car camera ( but for another purpose).
Thank you 
I think most (many?) relays give their output rating in VAC and VDC... I have one in my hand as we speak that says 12VDC in, and out is 125VAC or 28VDC, for example.
The output rating in VDC is always substantially lower than the VAC as far as I know.
The contacts of a relay are usually rated for the highest AC and DC voltages and currents they can safely switch.
For example, look at the relays on this board:
They say "10A 250VAC", "10A 125VAC", "10A 30VDC", and "10A 28VDC". This basically means you can switch up to 10 Amps when the supply voltage is up to 250 Volts AC or up to 30 Volts DC. Any equal or lower current with equal or lower voltage will also work. For example it can easily switch 5 Amps at 12 Volts DC because 5 < 10 and 12 < 30.
Why are 250VAC and 125VAC both specified when 250VAC covers both? Probably a standards requirement.
Why are 30VDC and 28VDC both specified when 30VDC covers both? Probably a standards requirement.
Hi,
Thank you both for your fast and meaningfull answers !