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variable constant current source?
variable constant current source?
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Old 29th August 2012, 03:52 PM   #11
CharlieLaub is offline CharlieLaub  United States
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variable constant current source?
Originally Posted by abraxalito View Post
I would have suggested LM317 like the others but I think down at 1mA it won't regulate properly. Also as jcx says the bandwidth is somewhat limited - watch out for the parasitic capacitances.

A solution that will encompass 1mA would be an opamp driving the gate of a MOSFET with a resistor to a power rail from its source. -ve input of opamp to source, +ve input of opamp you feed a voltage which defines the current you want. Choose the resistor to suit the current range you require. You need an opamp whose CM input voltage goes down to the -ve supply with an N-channel MOSFET - this makes a current sink. For sourcing, a P-chan MOSFET is called for and the opamp needs to allow CM voltages up to the +ve supply.
Oops, you are right about 1mA - I misread the post. But it seems that the 317 can get down to a couple of mA depending on the voltage drop. See below:
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Old 19th January 2014, 03:50 PM   #12
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Join Date: Jan 2011
Location: Quebec
Wink Useful 0-10A CURRENT SOURCE

I'm (new here ),and wish you a New Year! and bring new things like a 0-10A variable precision current source implemented with a XTR110 from Burr-Brown.Click the image to open in full size.
Click the image to open in full size.Click the image to open in full size.
I'll come up with a class A later done with-it
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File Type: bmp 0-10A source small.bmp (97.5 KB, 71 views)
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Old 19th January 2014, 05:19 PM   #13
DUG is offline DUG  Canada
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Join Date: Jan 2003
Location: mississauga ontario canada
Originally Posted by Zero Cool View Post
I would like to build a variable constant current source for experimenting with various stage topologies. I would like something that is variable from say 1ma to more than 10ma maybe as high as 100ma?

any suggestions?
What voltage swing do you need to accommodate?

Kind of important.

Some more:

Isolated? (floating)
Accurate? (or you could measure what you dial)
Equivalent source resistance? (Mohms)

Been there, done that.

Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
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Old 19th January 2014, 09:10 PM   #14
FoMoCo is offline FoMoCo  United States
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Join Date: Dec 2012
You can also do a Howland Current Pump with an op-amp and external power device. An advantage of this is that the input can be derived from a reference. (Even a DAC if you so please.) The Howland does have its disadvantages, such as needing twice the load voltage as a supply voltage and sometimes being hard to compensate.
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