Maker Pro
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Multichannel constant current box

poor mystic

Apr 8, 2011
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So, a reliable constant current source that doesn't change over a day, and a really nice current meter to set your source by. Does that sound about right?
 

poor mystic

Apr 8, 2011
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The project begins to swim into the "possible" zone.
Something tempts me toward the Wheatstone Bridge approach.

PS mind you I can't see why, on further thought.
 
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poor mystic

Apr 8, 2011
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I have another question.
What is the range of currents that you need the equipment to be able to supply?
I'd like to work out a range of equivalent resistances.
 

poor mystic

Apr 8, 2011
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There's a suggestion that you might need up to 20V to drive your current at times, so I believe that for maximum control in that case we'd want a nicely-regulated 40V supply with series resistors which could be switched in and out using mercury relays if allowed in Illinois.

However a current range factor of 1000 suggests that lower voltages might sometimes be more appropriate, even down to sub-volt levels.

To drive 200 microamps with 20V implies eqivalent resistance of 100K, which is a reasonable resistance to use for control purposes.
To drive 20 nano amps with the same potential means 100megohms, and that's NOT a reasonably workable resistance.
Does the low current require a low voltage?
 

enahs

Jun 15, 2011
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10V is the maximum I can conceive we would need. Truthfully the voltage is not all that critical as long as it is fairly constant and measurable.

As for what we are allowed to do, being a government institution we can get away with stuff. While it is different, the electrodes these boxes would be supply are filled with Mercury anyway.
 

poor mystic

Apr 8, 2011
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The thing about the voltages is that you have such a wide range of desired currents.

Because of the wide current range, I think it makes better sense to have more than 1 voltage available to supply your current. The availability of several voltages at low supply impedance would allow a single set of resistors, switched betweeen supplies, to set current over the required range with more even precision.

The design approach I am taking here is to provide a set of 10 resistors switched in-and-out of circuit by mercury-wetted relay contacts. By selecting which resistors limit supply, current resolution of 1 part in 1024 can be obtained. Resolution can be increased (within limits) by increasing the number of switched resistors.

Just for fun, I have an inquiry with a supplier requesting 100-of prices for mercury wetted relays to switch 1mA at low voltages. Let's see what comes back!

whaddya say, daddles?
 

daddles

Jun 10, 2011
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Frankly, enahs has been pretty reticent about what the actual application is, so it's hard to make a judgment. The thing I find the most confusing is that apparently a working and proven design exists, so why spend effort on coming up with a new design? If you're hard-up for money, there are no doubt many U of I students who could be capable of building what you want -- not to mention lots of engineering talent in the Chicago area.

If you're a government lab, which one is it?
 

poor mystic

Apr 8, 2011
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we're all ears

PS I have discovered that the idea of using 10 bits of precision is unworkable; the maximum precision attainable with discrete components is 5 bits.
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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To drive 200 microamps with 20V implies eqivalent resistance of 100K, which is a reasonable resistance to use for control purposes.
To drive 20 nano amps with the same potential means 100megohms, and that's NOT a reasonably workable resistance.
Does the low current require a low voltage?

Of course, a perfect current source has an infinite internal resistance...

The problem with 20nA is more along the lines of the input current of the op-amp used to set the current (or by whatever means) is likely to be an appreciable fraction of the current under measure.

I'm also not sure why you think that an accuracy of 3% is the best you can get with discrete components. Perhaps I'm missing something.

But, more importantly (and this is directed generally, not at Poor Mystic in particular), this isn't something that's so potentially dangerous that we need to demand that his credentials be vetted and his bona fides established. For a rail gun... maybe. For a constant current source though?
 
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