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From: "Graham Holloway"
Subject: Re: Current sinking circuit instability
Date: Fri, 17 Jan 2003 13:50:53 -0000
NNTP-Posting-Date: Fri, 17 Jan 2003 13:52:29 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"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.
Have you fitted a series gate resistor to prevent oscillation?
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