From: Spehro Pefhany
Subject: Re: Batt Charger 7905 discussion
X-Newsreader: Forte Agent 1.92/32.572
Date: Mon, 16 Dec 2002 01:23:16 GMT
NNTP-Posting-Date: Sun, 15 Dec 2002 20:23:16 EST
On Mon, 16 Dec 2002 00:56:33 +0000 (UTC), the renowned
>In sci.electronics.design Watson 'Atto Parsec' Name wrote:
>> I made up this constant current charger after some design on the web.
>> But I used a 7905 instead of a 7805, because I've got a lot of 'em
>> laying around from scrapped SMPSes. And I've seldom needed a negative
>> 5V supply. It works pretty good. I can switch between 50 or 150 mA.
>> But one question. Should the output lead to the battery have a 1N4002
>> in series to keep the battery's 6v from going back thru the regulator?
>> After a short while, the batts should charge up the cap in the wall wart
>> and no further current should flow. But when it charges up, it'll be
>> thru the regulator chip. In reverse.
>Read the datasheet of the 7905.
>I'm unsure exactly what will happen when you do this, I don't think you'd
>see any damage, but there might be a few milliamps of current
>drawn as the circuitry inside the 7905 gets powered.
>There is no reason that current will flow back through the 7905, though
Imagine the charger is unplugged and you attach the battery- the
filter capacitor of the power supply could be charged back through the
7905. This could be a problem if the battery voltage is high enough,
as the capacitance will certainly be more than the 100uF maximum
specified on the data sheet. I think the issue is reverse breakdown
of the E-B junction on the output device, so it shouldn't be a problem
if there are only a few cells.
A diode in *parallel* with the regulator would work fine, and wouldn't
have the drop of a series diode. A series one would probably be okay
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