The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "Fritz Schlunder"
Subject: Re: Current sinking circuit instability
Date: Fri, 17 Jan 2003 10:11:03 -0700
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
X-Comments: This message was posted through Newsfeeds.com
X-Comments2: IMPORTANT: Newsfeeds.com does not condone, nor support, spam or any illegal or copyrighted postings.
X-Comments3: IMPORTANT: Under NO circumstances will postings containing illegal or copyrighted material through this service be tolerated!!
X-Report: Please report illegal or inappropriate use to
X-Abuse-Info: Please be sure to forward a copy of ALL headers, INCLUDING the body (DO NOT SEND ATTACHMENTS)
Organization: Newsfeeds.com http://www.newsfeeds.com 80,000+ UNCENSORED Newsgroups.
"Nick" wrote in message
> I have a simple current sinking circuit designed to test DC power supplies
> up to 5V, 10A. Not being much of an ASCII artist, the circuit is
> as follows:
> An LM351 opamp output is connected to the Gate of a BUZ71 (50V,14A)
> the Source of the FET is fedback to the -ve input of the opamp and a
> potential divider with POT provides an input to the +ve input to the
> There is also 0.1 ohm power resistor connected between the FET Source and
> ground (so a 200mV change on the opamp input sinks 200mA)
> The power supply under test is connected accross the FET Drain and the
> circuit ground.
> The supply required for the opamp and opamp input voltage is provided by a
> 9V battery so as not to load the EUT.
> The circuit appears to work ok, (ie when the EUT output is measured with a
> meter it is sinking the correct range of current) but when I looked on a
> scope I see +-200mV or so oscillations, as more current is drawn from the
> EUT, the instability worsens.
> The FET is working in its non-linear region (which could be the cause the
> problem?) but i am having trouble finding any devices of a suitable Id
> (10A) that have a more linear characteristic at low values of Vgs.
> Any thoughts much appreciated.
Winfield Hill's suggestions aught to solve your loop stability issues, but I
was just noticing something. 5v times 10amps equals 50Watts. With 0.1 ohms
in series at 10A the resistor will dissipate 10Watts. This means the other
40Watts will be dissipated in the MOSFET, but if we look at the BUZ71
datasheet (http://www.fairchildsemi.com/ds/BU/BUZ71.pdf) we notice that the
absolute maximum power dissipation rating at a case temperature of 25 deg. C
is 40W. In practice that 40W figure is a rather fanciful rating unless you
intend to use less than room temperature cooling methods such as peltiers or
phase change refrigeration. Assuming you are running this thing for more
than a couple seconds at a time I should imagine the device would be more
reliable using a somewhat larger MOSFET (with lower thermal resistance
junction to case values). That would also make your heatsinking task
dramatically more feasible (IE, possible)...
-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 80,000 Newsgroups - 16 Different Servers! =-----
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup