Is my voltage regulator running too hot?

Hi

Bread-boarding an Arduino

(using this guide http://itp.nyu.edu/physcomp/Tutorials/ArduinoBreadboard)

AC/DC power supply output 9V/500mA

My meter tells me it's actually kicking out 15V!

Centre pin to pin 1 of a 7805 voltage regulator [*1] Barrel to pin 2 of 7805

10uF caps across power/GND on either side of voltage regulator

Meter across pins 2 and 3 (5V OUT and GND) of 7805 reads

+6.8V

The 7805 is uncomfortably hot. It doesn't burn to touch but within 2-3 seconds it feels like it will. That said it's been on a few minutes and it doesn't seem to be getting any hotter.

With another adaptor rated at 12V/500mA (but actually reading 18V on my meter!)

This has reversible polarity (confused about which way round)

One way gives an unstable 2.5-3V and the other gives about 8.8V from the voltage regulator which runs very hot

The weird thing is when I plug this adaptor into my Duemilanove I get 5.00V across pins 1 and 2 and 10.1V across pins 2 and 3 ... yet 16.1V across the centre pin and barrel connections from the 2.1MM power jack ...

.. God electronics is confuuuussssiiiinnggg!!! (but totally compulsive) :)

[*1 http://uk.farnell.com/jsp/search/productdetail.jsp?CMP=i-ddd7-00001003&sku=9666095 ]

if you apply ac or reversed polarity the 7805 will go poof in a very short amount of time and cause readings like that, the arduino has a diode to half protect it from that

Hi Osgeld

Yes I've desisted from randomly reversing polarities! :) It is a DC supply I'm using.

I went ahead with the adaptor that was spitting out the more manageable 15V and completed my breadboard Arduino which is sitting here blinking away (I had some ATMega328's from Coolcomponents.co.uk which were pre-bootloaded). Huzzah!

The 7805 is running hot but it's not burst into flames yet and it's been on a couple of hours! It also appears to have settled down at 5.6V.

Jim

Those unregulated DC power modules rate their voltage based on their maximum rated current. If the current drawn is less their voltage tends to raise as you have found. Also keep in mind that voltage regulators like used on the Arduino have over current and over temperature protection built into them, they just shut down until the limits pass. That said it's still not a good practice to run things hotter then needed. If you can keep your finger on the regulator of 3 seconds or more you are fine in my opinion. I ran for months with a 15vdc input to my Arduino clone that used a 7805 regulator. It ran very warm but was never a problem.

Lefty