DPRG mailing list archives: [DPRG] (no subject)

Thu Mar 29 10:44:05 CDT 2007

Zac,

I'm a little confused about this circuit you gave a link to. He uses a P
channel MOSFET to turn on the load when the battery is the correct
polarity, but I think there's a problem with the logic. MOSFETs have an
inherent reverse bias diode that comes as a side effect of the
construction. When the MOSFET is reverse biased, wouldn't it just
conduct in the reverse direction with a diode drop?

Rick

-----Original Message-----
From: Zac Wheeler [mailto:zac at poor-robot.com] 
Sent: Monday, March 26, 2007 12:36 PM
To: Rick Bickle
Cc: Dprglist at Dprg.Org
Subject: Re: [DPRG] (no subject)

Rick Bickle wrote:
>
> Hello all,
>
> I've got an application where I want to put a number of 9v batteries 
> in parallel to increase the current capacity. I need to prevent the 
> higher voltage cells from discharging into lower voltage ones, but I 
> don't want to have the 0.7 volt diode drop. (Germanium diodes don't 
> have enough current capacity.) Doe anyone have a circuit to do the 
> same job as diode isolation without the voltage drop?
>
Just out of curiosity, how much current are you pulling from each 9V
that you can't get a Schottky diode (similar voltage drop to Ge diodes)
with that current capacity? 9V batteries have a reasonable amount of
internal resistance, and they don't last long when they get hot.

You could also look at a solution with a P-FET, although it may take
some modification to work well in this application:
http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm