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Old 27th August 2012, 03:16 PM   #1
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Default variable constant current source?

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?
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Old 27th August 2012, 03:20 PM   #2
EUVL is offline EUVL  Europe
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See TL431 datasheet. Fig.32.
http://www.ti.com/lit/ds/symlink/tl431a.pdf


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Old 27th August 2012, 03:53 PM   #3
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I will take a read. I think I may have some of those? I will have to look
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Old 29th August 2012, 05:34 AM   #4
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This might be of interest to ya, mate!

Constant Current Source/Load (LM317) | The Paranoid Troll

In general, the LM317 IC is used like this:
Click the image to open in full size.


I did a quick Google search, based on my memory of seeing this in a data sheet once upon a time. I am sure that there are other resources about this out there.

-Charlie
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Old 29th August 2012, 06:48 AM   #5
EUVL is offline EUVL  Europe
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A more modern alternative to LM317 for low current:
LT3092 - 200mA 2-Terminal Programmable Current Source - Linear Technology

I also use LM317 or LT1083, etc., but usually for current above 500mA.


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Old 29th August 2012, 07:06 AM   #6
jcx is offline jcx  United States
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while regulators can be hacked to give CCS they often have limited bandwidth

purpose designed chip:

LT3092 - 200mA 2-Terminal Programmable Current Source - Linear Technology

can also make "Howland Current Pump" with op amps or power chip amps depending on requirements

http://www.ti.com/lit/an/snoa474/snoa474.pdf
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Old 29th August 2012, 07:20 AM   #7
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Quote:
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?
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.
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Old 29th August 2012, 07:51 AM   #8
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A depletion mode mosfet, like the DN2530 or DN2540, will make very good current sources up to about 100mA. See attached drawing.
pb
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File Type: jpg Adjustable-Current-Source.jpg (109.7 KB, 337 views)
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Old 29th August 2012, 12:44 PM   #9
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Quote:
Originally Posted by PaulBysouth View Post
A depletion mode mosfet, like the DN2530 or DN2540, will make very good current sources up to about 100mA. See attached drawing.
pb
I have plenty of DN2530's on hand. I like that solution. a little limited at only 25ma but that will work for most things. I will review the others, thank you everyone!
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Old 29th August 2012, 02:11 PM   #10
iko is offline iko  Canada
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Why not something simple like this. Almost any general purpose npn and power mosfet will do. Play with R1 and R2 to get the current you want.
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