# Voltage controlled high current source problem with low value sense resistor

S

#### Steve

Jan 1, 1970
0
I've put together a voltage controlled current source for use at high
currents (5 Amps) based on the desgin at
http://www.daycounter.com/Circuits/Current-Servo/Current-Servo.phtml
It uses a single supply op amp (OPA340 or OPA703) driving a low gate
threshold logic level MOSFET (RFP30N06LE). Load connects between
drain and power source. Current sense resistor from MOSFET source to
ground with that junction feeding back through 10 K to inverting
input. Voltage control applied to non-inverting input.

Because of the high current and the desire to minimize power
dissipation in the sense resistor and voltage drop across it, I am
using a 0.1 ohm sense resistor. This is resulting in the current not
following the V=(current / 0.1 ohms) equation. I observe 0.725 A
(expect 1 A) at 0.1 control voltage and 3.5 A (expect 5 A) at 0.5 V.
These large errors are even present at low currents. Things are
better for higher valued sense resistors. For example, output current
is only 5% low when a 1 ohm current sense resistor is used. Is some
resistance not being compensated for or ?

S

#### Steve

Jan 1, 1970
0
No, it is stable as viewed on a scope.

J

#### Joerg

Jan 1, 1970
0
Steve said:
I've put together a voltage controlled current source for use at high
currents (5 Amps) based on the desgin at
http://www.daycounter.com/Circuits/Current-Servo/Current-Servo.phtml
It uses a single supply op amp (OPA340 or OPA703) driving a low gate
threshold logic level MOSFET (RFP30N06LE). Load connects between
drain and power source. Current sense resistor from MOSFET source to
ground with that junction feeding back through 10 K to inverting
input. Voltage control applied to non-inverting input.

Because of the high current and the desire to minimize power
dissipation in the sense resistor and voltage drop across it, I am
using a 0.1 ohm sense resistor. This is resulting in the current not
following the V=(current / 0.1 ohms) equation. I observe 0.725 A
(expect 1 A) at 0.1 control voltage and 3.5 A (expect 5 A) at 0.5 V.
These large errors are even present at low currents. Things are
better for higher valued sense resistors. For example, output current
is only 5% low when a 1 ohm current sense resistor is used. Is some
resistance not being compensated for or ?

Check for oscillation first, as Jim wrote. With the cap from OUT to IN-
it shouldn't happen but depends on your load. Which opamp did you use?
Does its input CM range include the negative rail? What's the offset spec?

Also, mind that cell phone signals and such can recitify in many opamp
inputs, so if you have long wires there you might at least want to
consider a cap from IN+ to ground. Hoping it's a ground plane ...

S

#### Steve

Jan 1, 1970
0
On Wed, 02 Dec 2009 17:39:19 -0700, Jim Thompson

Thanks. Looks like it probably is unaccounted connection resistance.
Rearranging things has eliminated most of the error. I'll solder it
all up with heavy gage wire and try again.

I have had issues in other projects where I've used alligator clip
jumper wires whose thin wire is simply folded over and held against
the clip by the clip's crimp.

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