Need help with resister value.

[trixdropd]

I have 9v dc. I am feeding a relay that needs 5vdc. The coil resistance is: 166.7and it has a current draw of 30ma. What resister do i need to get me where I need to be?

[slacker]

The current through the resistor will be the same as the current through the relay.
The relay is dropping 5 volts so the resistor needs to drop 4 volts.
So using Ohms law R = V/I = 4/0.03 = 133.33 Ohms

I've got no idea whether wiring a relay up like this actually works in practice though.

[trixdropd]

The current through the resistor will be the same as the current through the relay.
The relay is dropping 5 volts so the resistor needs to drop 4 volts.
So using Ohms law R = V/I = 4/0.03 = 133.33 Ohms

I've got no idea whether wiring a relay up like this actually works in practice though.

Thanks!! I'll breadboard it up and measure and see!!

[Ripthorn]

You could always stick a zener in there.  5.1V zeners are cheap and easy to find.

[trixdropd]

You could always stick a zener in there.  5.1V zeners are cheap and easy to find.

Will this be suitable? I know it's expensive, but I'm only using it as an example...

1n4733a

http://www.radioshack.com/product/index.jsp?productId=2049726