Subject: Current sinking circuit instability
Date: Fri, 17 Jan 2003 13:27:36 -0000
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Organization: Telepac II - Comunicacoes Interactivas, SA
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 described
An LM351 opamp output is connected to the Gate of a BUZ71 (50V,14A) MOSFET,
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 opamp.
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
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 value
(10A) that have a more linear characteristic at low values of Vgs.
Any thoughts much appreciated.