I'm working on a design project where some components (ie motors) need ~1A of current. But I've also got digital stuff on it that I don't want to exceed 200mA.

I've got only one source of power, from a 12V lead acid battery at 1.35A (ripped it off of an electric scooter).

I've got a 7805, connected directly to the battery to to drive the digital stuff. (The 7805 is getting kind of hot, I'm considering getting a 7809, and then connect the 7805 to it, is this a good idea?)

My question is, since the battery supplies this relatively high current, should I just connect it directly to the 7805 and drive the IC's? (I've tried it, and it works.)

My second questions is, if I do want to limit the current coming from the battery, what is a simple way to do it? ie. some chip I can just buy for the job.

Any suggestions my overall setup is also welcomed

Thanks.

 

Put a decent heatsink on the 7805, it should then be fine.

A battery doesn't 'supply' a current, the load DRAWS the current, so the load will take what it needs, no more!.

If you wanted to limit the current capability of the battery you could do so, a simple resistor is a crude and simple way - but you need to have a reason to want to do this, and I don't think you do!.

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Just what are you driving with a 7805? If you've got simple digital logic perhaps you should check it; most of the time simple stuff won't draw enough current to heat up a reg unless you're driving coils, leds, etc.

Resistors are a poor form of current limiting for motors, it makes them hard to start, run slower, and run at different speeds depending on loading. There are several schemes. A good simple idea might be a self-resetting circuit breaker, they don't drop much voltage until they open up. http://www.allelectronics.com carries some polymer fuses and flat battery pack fuses that both self reset.
http://www.allelectronics.com/cgi-bi...065&type=store
http://www.allelectronics.com/cgi-bi...00S&type=store

 

Here is a crop from LM317 datasheet...

Attached Images

317.gif (14.3 KB, 221 views)

 

Thanks a lot for the comments. I think I know what I have to do now.