Dropping voltage from 13V to 8.4V

Hi folks,

What I would like to do is quite simple. I want to take a 3-cell Lithium Polymer battery which has a nominal voltage of 11.1v (but when fully charged is well over 13v) and use it in an application requiring a 6- or 7-cell NiMH battery, that is, about 8V. This is simply because I want to reuse my R/C airplane batteries inside an R/C car. Why buy an expensive battery that weighs more, if I can build something cool that would be more weight efficient? Another reason is that I intend to mount some electronics on it which will be run off the lipo anyway.

Anyway, I'm having trouble figuring out what specific components I need to get this done. I know that I want a switching buck regulator of some sort. I would like it to be able to handle at least 20A. But I'm having some trouble parsing the datasheets. For example I'm looking at the LM27402 but I can't tell if I need to obtain some inductors and capacitors to connect to it as well.

edit: okay LM27402 is probably not something I can find anybody to sell to me easily.... probably isn't the right IC that I'm looking for either. What should I do then? Can I hook up a giant 4v diode to the battery? Do 4 volt diodes exist? I imagine it wouldn't be terribly efficient either, 4v at 20A is 80W.

use a 7809 its simple to connect and it will do the job with a minimum of components to keep weight down. Diagrams are easily available across the net. if condenser values are not written use 1uf.

jguy said:

use a 7809 its simple to connect and it will do the job . . . .

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at 20Amps? I don't think so.
I'm surprised that the application requires 20Amps but if using an IC regulator like 7809 or LM340 then it will require a bypass transistor to pass the specified current. But I agree that it will still be relatively simple and high current designs are given on all the manafacturers datasheets.

Is there a way to hook it up so the majority of the current going to the motor does not pass through the regulator? Like setting the regulator to 4V and attaching it's +Vout to the battery's negative terminal... Would that be a short circuit?

dc443 said:

Is there a way to hook it up so the majority of the current going to the motor does not pass through the regulator? Like setting the regulator to 4V and attaching it's +Vout to the battery's negative terminal... Would that be a short circuit?

Click to expand...

That doesn't seem workable the way you describe it, No. If you posted a circuit, then we could consider it.

Have you looked at any of the (very many) circuits on IC Manufacturer's websites for Regulator with a bypass power transistor yet? (these do exactly what you describe by "the majority of the current going to the motor does not pass through the regulator").
Is there any reason why they don't provide you with the simplest solution?

I have only taken an introductory course in electrical engineering, so I'm a little unsure about a couple of things. I will look into these bypass power transistors you speak of and see if I can figure out how it works. Thanks for helping me out!

Sorry, I didn't realise that it might not be simple to look things up. So I've just done a search for 7809.
Here's National Semiconductors' illustration of increasing the current capacity of one of their regulators.
**broken link removed**

and here's a more sophisticated regulator with current limiting:-


I suggest you familiarise yourself with voltage regulators by reading one of these DataBooks before building your regulator - its all very simple, component values are not normally critical, but you don't want to make any mistakes!

In my opinion, using a switch mode buck converter is the only reasonable way to handle the voltage conversion. You don't get buck converters with integrated switch for this currents, so external power transistors (as for the said LM27402) are required, a power inductor and capacitors anyway. Although not intended as a construction manual, the datasheets of these devices give you an overview about required circuit components. You can also start the Webench Power Designer at national.com, or use similar tools from other major manufacturers and get a suggested BOM for your switcher.

As a simple (possibly ignorant) question: Are you sure, that your R/C car's power stage, that most likely also operates in switching mode, can't handle 13V?

You bring up a good point, FvM. It seems to me that there is a good chance i can run a 3s lipo in one of these things, especially if I permit upgrades. I think at this point it would benefit me more to choose an appropriate model to run my batteries in rather than trying to modify my batteries, which seems like quite an undertaking for me already.

Thanks for the help, especially DXNewcastle! I will surely be back here to seek advice when I get around to working on my quadrotor idea again.