multi voltage step down

For a project I have, there is 1x arduino, a transistor that needs 9v, and a relay with 110v AC. I have plenty of 12v power supplies from external hard drives and the like. This also has the benefit of being easily chained to the 110v AC in the same project box.

It looks easy enough to drop 12v to 9v (7806), and easy enough to drop 9v to 5v (7805). Should I be fine taking a line from my 9v into the regulation for my 5v, or is having multiple regulators and down steps in a chain a bad idea for any reason?

12v to 9v (7806),

7809

This is feasible. The power dissipated in the 7809 will be effected by the current drawn from the 7805.

LarryD:

12v to 9v (7806),

7809

This is feasible. The power dissipated in the 7809 will be effected by the current drawn from the 7805.

Right, 7809 sorry about the mistake. I am not entirely sure what you mean by the dissipation effected by the draw, though.

The dissipation is how hot it gets. The more current you use the hotter it will get. Using a chain of regulators spreads that power over the two regulators and so is a good thing. You want things to run as cool as possible.

Entirely depends on the current requirements...

Last night I used a tip41 (rated 6 amps) 14.5v on it's collector, 5v on it's base from a 5v 7805 regulator, the 7805 stayed cool andfrom the emitter I powered an Arduino and 2 shift registers..

Still transistor still got hot.... if the current draw is too high think about using a switching regulator or a large heatsink.

The power dissipation in a component is equal to the voltage drop across the component multiplied by the current flowing through the component...

So using voltage regulators in series spreads the total dissipation across the regulators, shared according to the voltage drop across each.

Although unlikely to be a consideration, using regulators in series would give better regulation with less ripple voltage on the output.