> RE: CS>MultiMeter Question
> From: David Bearrow
> Date: Wed, 7 May 2003 05:22:54
> I use an LM317. It provides smaller current than the 117.
You may have found one that works, but if you read the
specification, the minimum load current for the LM317 with 40V
differential is 3.5 to 10mA, and the LM117 is 3.5 to 5mA. The LM117
is better than the LM317, but neither device is guaranteed to work
outside the specifications.
See http://www.national.com/ds.cgi/LM/LM117.pdf
> I have been able to consistently get the current limited to 1.7ma
> with the LM317 using a 1K feedback resister at 38V supply.
The equation for setting the current is R = 1.25 / I
For 1.7mA, the resistor would be 1.25 / 1.7e-3 = 735 ohms. Since you
need a 1k resistor, this shows the device is not operating properly
and it is doubtful it is operating very well as a current regulator.
Have you plotted the current-vs-time curve? I'll bet it changes with
temperature.
The LM117/LM317 reduces the available voltage by 3V, so you are only
supplying 35V to the electrodes at power on. This will increase the
initial current compared to 27 volts from three 9V batteries, but
not enough to make much difference.
Unless you have a very large surface area for your electrodes. even
with a properly-functioning current regulator, you are probably
operating in the exponential portion of the curve as shown in Fig. 1
on my web page. This means your results are subject to the normal
process variations in purity of the distilled water, ion channel
formation, and hydrogen bubbles at the cathode reducing the surface
area.
I had all the same problems and found the only way to solve them is
to go to a higher voltage. The voltage that is needed could be
lethal if the circuit is not designed properly, and two people have
recently posted discussions on using the raw 110VAC without an
isolation tranformer. That is why I posted the 130V design.
> The TO-92 package is easy to work with as well, it looks like a
> small transistor. And the cost is excellent. Only 50 cents at
> Digikey. I can post my schematic and parts list on the web if
> anyone is interested.
The schematic is one component and one resistor, and is shown in the
datasheet along with the device specifications.
> I designed it based on the best advice I gleaned from Bob, Trem,
> and Jason on this list. It uses a small motor with a straw for
> constant stirring. If you mount the whole thing on a plastic mason
> jar lid with your screw type connectors pointing down it makes it
> a lot easier. Just put your silver wire on the connector. (I
> prefer silver ribbon, you get more surface area and increases
> capacitance between electrodes)
The ribbon increases the surface area, which lowers the current
density for the same supply current. I'll bet they are fun to
clean:)
Even with a dielectric constant of 81 due to the distilled water,
the capacitance between the electrodes is still negligible. It has
no bearing at DC.
No doubt you are making cs, since you are applying current. But many
people have posted discussions on the unrepeatability of their
process, and some even think the phase of the moon has some effect.
I think a lot of problems people have been having, aside from
variations in the purity of the distilled water, is the low voltage
normally used, and unreliable current regulation. Your circuit
certainly falls in this category.
Best Regards,
Mike Monett
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