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

You can use a switcher chip, but that's little bit of a pain to build by hand, since the majority are surface mount, and you need a good ground plane (not really hard, just not as easy as a 7805 and a bridge rectifier). Basically, the problem is that a diode drop is .6 volts, so you need a minimum of 5.6 volts of input, and that's with a low drop-out regulator. That doesn't leave much room for your battery to die. A normal 7805 really wants 7V minimum to regulate reliably. And depending on the current load, 15V on a 7805 is a Bad Idea, because it's got to dump that 10V drop as heat. That's 10 watts at 1 amp, which is a damn lotta heat in a TO-220 case. Ideally, a 7805 should be run at 7V or 8V to keep heat to a minimum. If you do want to build a switcher, just about everybody has a switcher chip that will work about .2V to .4V above the regulated output voltage. Maxim is the first place I'd look. Get some double sided copper clad board for a ground place, use an Xacto or a Dremel tool to route traces. Some of the companies also offer free or very low cost eval boards. For instance, National Semi has a switcher eval that takes 4V to 30V and produces 3.3V out at 1 amp. They're $8. It has two caps on it that cost me $2 each, plus having boards turned, the switcher itself, and a few other incidental parts. It's cheaper for me to buy them than it is produce my own (I use them a lot in prototyping projects). When trying to get 5V from 6V, especially on batteries, linear regulators aren't really the way to go. You'll dump a lot of the battery's energy as heat, which is wasteful. Same for a Zener type regulator. You could use a a couple diodes in series, but your output voltage will sink as your battery voltage drops, since the drop across the diode is the same. --John -----Original Message----- From: cctalk-admin(a)classiccmp.org [mailto:cctalk-admin@classiccmp.org]On Behalf Of Ethan Dicks Sent: Friday, December 06, 2002 15:45 To: cctalk(a)classiccmp.org Subject: Driving a 7805 or how else to get +5VDC reg (was Re: My first good find!!!) 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 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. __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com