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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I feel like splitting this into a separate thread since the question is well-encapsulated:
I have seen various topologies for constant-current sources. Is there any one better than another (op-amp, transistor, combination), or are they very similar with regards to stability?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I'm a great fan of the L200 regulator, which can be configured as a programmable constant current source, or as a programmable combined constant current and constant voltage regulator:
I always make sure I have a few L200 and LM317T to hand!
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
In a precise constant current source, you want to make sure 100% of your output current flows through the sense resistor, so use a MOSFET outupt transisitor. Then make sure the sense voltage stays equal to your reference voltage, so have a a stable sense resistor, stable reference voltage, a large loop gain, and no evil thermo-electric voltages being developed at any of the junctions. Once you've got all of those, it's a piece of p$$.
HOWEVER, given that we can guess what you're using this constant current source for, it's not the way to go for best stability. Your RTD will be excited by a drifty CCS, and then measured by an independnt drifty ADC. Far better to combine the excitation and the sense into the same circuit, then any drift in the reference will cancel out.
Think Wheatstone bridge, where you are comparing an RTD leg with a temperature stable resistor, and all that nasty drifty amp/ADC stuff is just measuring the small difference between these two, with an order of magnitude improvement in stability. I don't have the circuit to hand, but I'm sure you can do the 3-wire trick in a Wheatstone configuration.
If you don't go to a Wheatstone brdige, but insist on using a seperate current source and voltmeter, then at the very least, reference your current source from the voltmeter reference, to eliminate one cause of drift. Have a look at Patrick's LiPo protection thread for hints as to how to move a reference in absolute voltage if you find that the current source and ADC references appear to need to be at different absolute voltages.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780
op-amp with negative feedback driving a mosfet
you really need feedback to ensure temperature doesn't dictate how your circuit behaves
in the linear appnotes there are nice references to constant current sources, especially in the field of optoelectronic and fiber optic current sources
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constant current source
