T
Tom
- Jan 1, 1970
- 0
Hi all,
Ive been trying to design an accurate variable current source for some
time now and I am struggling with certain aspects.
I have developed a current source using two op amps, a number of
transistors which forms a variable current source.
I am looking for the following spec:
current from 0-3amps over a load range 0.1R - 10R
I have a supply voltage of 40v so achieving this spec is possible but
I am faced with a big problem with my current design, keeping the
current source accurate to within 20ma.
I will explain my circuit...
I have a current sence resistor on the ground supply rail, at 0.1R.
The voltage drop over this resistor is multiplied by the first op amp
to a useable level.
The output from this opamp goes to the negative input of the second
opamp, the positive input is sourced from a voltage reference
(variable resistor voltage divider) to set the required output
current.
This opamp output is then sent to the base of a transistor, which
drains the base of the main drive transistor (when the current load
gets too high).
Esentially the design does work, it does limit the current output but
at lower load levels the current increases.
I think the problem is due to the gain of the op amps, and transistors
but Im not sure how I can drive the op-amp in a manner that will cause
accurate current limiting.
This is of course a PSpice simulated circuit, as I dont want to try
anything for real until I can get a decent simulation.
I would appreciate any advice.
Cheers,
Tom
Ive been trying to design an accurate variable current source for some
time now and I am struggling with certain aspects.
I have developed a current source using two op amps, a number of
transistors which forms a variable current source.
I am looking for the following spec:
current from 0-3amps over a load range 0.1R - 10R
I have a supply voltage of 40v so achieving this spec is possible but
I am faced with a big problem with my current design, keeping the
current source accurate to within 20ma.
I will explain my circuit...
I have a current sence resistor on the ground supply rail, at 0.1R.
The voltage drop over this resistor is multiplied by the first op amp
to a useable level.
The output from this opamp goes to the negative input of the second
opamp, the positive input is sourced from a voltage reference
(variable resistor voltage divider) to set the required output
current.
This opamp output is then sent to the base of a transistor, which
drains the base of the main drive transistor (when the current load
gets too high).
Esentially the design does work, it does limit the current output but
at lower load levels the current increases.
I think the problem is due to the gain of the op amps, and transistors
but Im not sure how I can drive the op-amp in a manner that will cause
accurate current limiting.
This is of course a PSpice simulated circuit, as I dont want to try
anything for real until I can get a decent simulation.
I would appreciate any advice.
Cheers,
Tom