Burned up my first chip!

[Tony Cooper]

Joe Boucher wrote:
> 
> I was playing around in the garage Sunday before THE GAME setting up a
> power supply from an old modem to test some of the circuits you guys and
> girls have come up with.  I was curious about a 7805 votage regulator I
> bought at Rat Shack.  The back of the card said the input voltage was 35
> volts.  I thought that probably meant max voltage so I hooked up a 12
> volt power supply to it.  I was getting 5 volts out of it when it
> started smelling funny and a small spot on the chip started bubbling.
> 
> Ooops.

I'm sorry Joe, but I just have to chuckle..... Reminds me of my first
'burning chip'...hehehehehee

Shouldn't laugh, specially as you have taken the plunge and connected
something up...Good on you.

Some information for you for your future projects.

The power disipation can be calculated as follows:-

(Vin-Vout)*Idrawn = Watts heat produced

Which translates to (12V in - 5V out ) * Current drawn (Lets say 0.5A)

Which is 3.5 Watts. The device (7805) has approximatly enough area in
free air to dissipate 0.2 Watt
, so for this application, a heatsink of maximum 14 degrees C per watt
(I don't like power devices getting
hotter than 50 degrees) will disipate enough heat.

Hope thisd helps (Hope this is accurate - I have been up all night
working and my vision is a bit fuzzy!!!)

Tony

-- 
Sent By Tony Cooper.
email: tony.cooper at virgin.net
Allow at least 10 working minutes for reply. ;)