SparkFun Forums

[zacarias]

Hi all,

I bought this board relay.

It needs 5v on the VCC, but I would like to know that I do to connect with 9v of the other power supply. because the output of arduino its occupied by another gadget

http://www.sparkfun.com/products/11042

Could I power the board directly with 9v DC ?

Thanks to all, and sorry for my english

Zacarias.

[TCWORLD]

If you did that, you would most likely burn out the relay coil. You would have to regulate the supply with something like a 7805 regulator.

[Joeisi]

For easy reference: 7805

[jremington]

Yes, you can power the relay circuit with 9 VDC. It is very likely that the relay coil will tolerate 9V, at least for short periods.

If you wish to be completely safe, put a series resistor between the + side of the power supply and the relay coil, equal to about 4/5 of the resistance of the relay coil. That way, the resistor will drop 4 V and the relay, 5V. If the coil resistance isn't printed on the relay, you should measure it with an ohmmeter. However, the diode in parallel with the coil can disturb the reading, so make sure that the positive probe of the ohmmeter is connected to the + side of the coil (banded end of the diode). Or, you could measure the resistance in both directions and take the higher value.

[Philba]

Looking at the data sheet, there are 2 relays coils - 1W and 900 mW. I'll use the 1W coil data since the 900 mW one is only 10% different. The 5V coil is rated at 25 Ohms (200 mA). This means that at 9V it will draw 360 mA and dissipate 1.8 W. That seems pushing it, especially if it's a 9V battery - you'll be going through them pretty fast. Also, if not a battery be sure your supply can actually provide that kind of current. If you use a resistor, 20 Ohms in series with the supply will limit the current to 200 mA. You will need a 1 W resistor.

Probably a bad idea to consider powering this relay from an arduino anyway. Especially if it's running off of a wall wart - the regulator will be getting pretty hot.

By the way, there are relays that draw much lower current out there.