Driving a 7805 or how else to get +5VDC reg (was Re: My first good find!!!)

From: Ross Archer <archer_at_topnow.com>
Date: Fri Dec 6 17:43:01 2002

Ethan Dicks wrote:
> > [ACE PSU]
> > ...a 7805 1A linear regulator which needs around
> > 8V to 15V to run properly.
> So I was thinking about this recently... I have this 6V battery
> pack that is intended to clamp to the back of a ZIP drive, allowing
> it to be used away from power, say with a laptop. I've wanted
> to use it for a portable source of juice for hobby projects (it
> has, among other features, a built-in 110V mains plug and recharger,
> making it handy to recharge), but I haven't expected to be able to
> feed a 7805 at +6VDC and get +5VDC out the other side reliably.
> How can I take +6VDC of battery power and get +5VDC regulated power
> from it? If it matters, the currents involved will be under 1000mA,
> but probably over 200mA.
> -ethan
> P.S. - I don't mind the concept of a DC-DC converter, but I'm not
> skilled enough to design one, just skilled enough to build it.

You don't have to build one. They are available as sealed
"modules" with pins for gazinta and gazouta; very easy to
use. An added advantage, when you're running off battery
power, is very high efficiency for maximum operating time
off your batteries. Just be sure to figure out what your
battery is putting out when it's just about to be exhausted,
and try to get a converter that can operate on this input
voltage to get all the capacity out. Check out Jameco,
Digi-Key's website to see if there's anything that fits the

I have a huge bin full of the venerable old 7805s. They
work and they're simple, but they're also terribly
inefficient and tend to run quite hot unless extensively
heat-sinked, especially towards the upper end of the voltage
input range and/or close to rated current flowing to the 5
volt load.

I fondly remember a home-made KIM-1 power supply made with a
7812 and 7805. Despite a relatively huge heat-sinking area,
the damn thing was running nearly hot enough to cause
burns. I think the input voltage was around 8-10 volts
somewhere. After the "MacDonald's Coffee-in-Lap" lawsuit, I
wonder if I coulda sued TI for burning my fingers. ;)

-- Ross

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Received on Fri Dec 06 2002 - 17:43:01 GMT

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