Burned up my first chip!

[Frederic Breitwieser]

>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.

Max input voltage on the LM78xx series is 35V, however if you don't want to
fry the unit, keep it under 28V for higher loads.  In fact, the higher
current you want to draw from it, the closer to about 15-16V you want to
go, that seems to be the "ideal" for that chip, based solely on the many,
many power supplies I've made where I've used one.

>I think I shorted two of the leads coming out of the chip with the
>voltmeter probes.  1) Did that do it? 2) Inputing 12 volts didn't do it,
>did it?  3) Did having no load do it?  4) Does it need a heat sink which
>is why the back side was nice and shiney?

Yes, if you are going to draw over 500ma, then a heatsink is a necessity,
linear power supply chips make a lot of heat, which is why they are
inherently inefficient devices.  Anyway, I'll save that lecture for another
time <grin>.

If you shorted the output and ground together, then yes, you'd draw a lot
of current, but a modem adaptor couldn't supply enough current to toast the
chip, assuming its a DC adaptor, and not an AC adaptor, like some of the
PPI, Hayes, and ZOOM modems.  Double check your adaptor type and
connections, for if you have AC or reversed the voltage, the chip will be
cooked :)

>for a lot less than Rat Shack.  But the hours are inconvenient and the
>owner is a jerk.  Can't have everything.

Well, tradeoffs for everything.  Just remember you are buying parts, not
proposing marriage <smile>


Frederic Breitwieser
Bridgeport, CT 06606
http://www.xephic.dynip.com/

1993 Supercharged Lincoln Continental
1989 4-Door Softtop Humvee (Hummer)
2000 Buick GTP (Mid-engine track car)

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