# Relay power supply

The reason I am writing is I was curious about hooking up a higher external power supply, Can I hook up a 120v to the 4channel 5v relay module? or does it have to be 5v. I am trying to only use my arduino for i/0 not power. I am also looking at powering high wattage devices, over 1000w.

• Are you looking to switch 120VAC, or are you trying to use 120V to power the relay board (VCC)? The first is fine, the second is not. – W5VO Jul 21 '12 at 2:24
• @W5VO:I was about to try the second, but now I know I couldn't. Thanks. – user10768 Jul 21 '12 at 2:29
• @user10768 you can, just need to regulate it down. – Kortuk Jul 21 '12 at 4:14

There are two sides on a relay, the coil side for activating the relay, and the contact side, for switching.

Your relay is a 5 V relay, that's the activating side. If you look at the (messy) datasheet you'll see that it has a resistance of 55 Ω, for a 450 mW power. Some of that power is used to create the magnetic field needed to close the contact, but most is dissipated in the coil's resistance as heat. Power can be calculated as

$P = \dfrac{V^2}{R}$

and at 5 V that's indeed 450 mW. If you would power it with 120 V then

$P = \dfrac{(120 V)^2}{55 \Omega} = 260 W$

will most likely will make the relay explode. The datasheet says the maximum voltage is 110 % of the rated voltage, so that's 5.5 V maximum.

That activation power will switch any voltage and current as long as they're below the rated values. You don't need a higher activation voltage to switch 10 A than for 1 A, or for 120 V instead of 12 V.

The contacts are rated at 10 A, for DC a maximum voltage of 30 V is given for the form A contact (SPST). This is a form C contact (SPDT) however, and this doesn't specify a DC rating for a resistive load. For AC it's 240 V. (JeeShen's answer is not correct here.) But power is maximum 240 W, so at 120 V AC you can only switch a resistive load of 2 A.

• I found a better datasheet here. Note the AC rating is 800VA (I assume the "C" is an error there) for a resistive load. – Oli Glaser Jul 21 '12 at 14:18
• Thanks Oli. (their server sucks: > 2 minutes to load the PDF!). You're right, it's a form C contact, so 800 VA. The "C" will indeed be an error. – stevenvh Jul 21 '12 at 14:42

The 5V rated on the module is indicating the input of the module, that means you can connect your Arduino output pin straight (which is ~5V when turn on, ~0.25V when turn off) to the input pin of the module.

As of the output of the Relay able to switch, it's rated at DC30V 10A. Means you can supply VDC 30V, 10A max; so means any voltage beyond 30 VDC (e.g. 120V) will kill the relay. The max wattage the relay in the module can support is P = V*I = 300 W. You will need to find other module which support higher wattage rating.