Voltage Regulator - Hot as Hell

Hi all, I'm trying to sort out a power supply for a project and the voltage regulator I'm using is getting really damn hot.

The voltage regulator in question is the L9705CV (5v 1A) (http://pdf1.alldatasheet.com/datasheet-pdf/view/22629/STMICROELECTRONICS/L7905CV.html) The plug I'm using with this is rated 7V 1A.

For now I'm just testing it on a breadboard with the fixed output regulator circuit in that datasheet and a LED wth resistor as the output. I've read the datasheet through and decided that a 22uf capacitor would be best for Ci, and 10uf for Co as I'm using electrolytic capacitors and it mentions using at least 10 times the capacitance if not using tantalum caps.

it's getting hot enough to smoke and burn my fingers when it's running, do I need a huge heatsink? Or is it more likely that I have done something wrong here?

The 9705 is a negative voltage regulator. Are you using it as such?

We need more information.

  1. What is the input voltage of the power supply? i.e., what are you regulating from.
  2. What is the current draw of your project?

Without that we cannot make any recommendations.

majenko: What I was attempting to regulate from was in the intro post: "The plug I'm using with this is rated 7V 1A."

What is the current draw of your project? "For now I'm just testing it on a breadboard with the fixed output regulator circuit in that datasheet and an LED wth resistor as the output" Using a 1k ohm resistor.

James C4S: Oh crap, I was mis-sold it as a positive voltage regulator.. I'm screwed then and this is likely why it was heating up so much. Can't believe I didn't spot it on the datasheet.

You want the 7805 then ;)

The voltage regulator in question is the L9705CV (5v 1A) (http://pdf1.alldatasheet.com/datasheet-pdf/view/22629/STMICROELECTRONICS/L7905CV.html)

The 9705 is a negative voltage regulator.

Just to get things straight - we are talking about a 7905 here aren't we?

Don

burn my fingers when it's running, do I need a huge heatsink?

.... no, you need not to touch it 8)

But seriously, what's a negative voltage regulator?

A negative voltage regulator regulates negative voltages.

A 7805 will regulate say +12v down to +5.

A 7905 will regulate, say -12v up to -5v.

It's used in dual-rail systems - specially for things like op-amps etc.

don: Yup indeed we are, typo there whoops, the datasheet is correct.

And it really makes sense it was overheating, feeding +7v to it, doh.

And in case anyone was wondering I grabbed a 7805 out of some old hardware I had lying around and no more heating problems and we have the correct voltage and current. This was the last peice of the puzzle for this project so I can get it all off of breadboard and onto perfboard.

Thanks for the help guys.