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

Thu Mar 29 12:52:39 CDT 2007

Ah, you are correct - I should have read more closely instead of just
looking at the schematic.

This circuit though, is to protect the load against having the battery
inserted backwards. If I had several batteries and MOSFETS paralleled as
in this circuit, the MOSFETs would happily conduct in the reverse
direction (which is really the forward direction), draining my other
batteries into the weaker one.

Rick

-----Original Message-----
From: Eric Sumner [mailto:kd5bjo at gmail.com] 
Sent: Thursday, March 29, 2007 11:07 AM
To: Rick Bickle
Cc: Zac Wheeler; Dprglist at Dprg.Org
Subject: Re: [DPRG] (no subject)

> 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?

He's actually got aan explanation of that in the article.  The FET is in
the circuit "backwards" to fx that problem at the expense of it not
operating at maximum efficiency.

  -- Eric