> Re: CS>Attaching 12 gage silver wire electrodes to container lid
> From: Jim
> Date: Wed, 7 May 2003 13:37:10
> I use 12 ga. wire, 12 inches long and put 2 of them connected to
> the anode. That gives me 24 inches. The rods don't have to be a
> continuous length as long as you have them connected properly.
> I don't use the LM117, I use a regulator that I made using the
> LM334Z with a 30 VDC power supply that I robbed from an unused
> printer that I have.. My current limiter limits the current to 3.4
> mA.
> I don't see why you feel that you need 40 inches of wire, I make
> 15 to 20 PPM CS in about 24 hours at 3.4 mA. If I were to use 20
> mA I would produce my CS much faster, and am not sure that is good
> to make it that fast.
> Jim
Hi Jim,
The LM334 is a great regulator below 10mA. It needs only 1V to
operate, so you get more delivered to the rods.
http://www.national.com/ds.cgi/LM/LM134.pdf
The key to your system is the large surface area of your rods and
the low operating current. Assuming equal anode and cathode area,
you are running at a current density of 3.4mA / 24 sq. in. = 0.141
mA/sq in, which is about the lowest I've seen posted.
With this low a current density, the initial voltage across the rods
will also be quite low. This means the LM334 is probably able to go
into current limiting right away, or very soon after power is
applied. So you probably get good repeatability.
Just because a current regulator is designed for 20 mA doesn't mean
it will deliver that current. It needs a low enough load resistance
so it doesn't go into saturation.
To run at 20mA and keep the same current density as you now have,
you would need 20 / 0.141 = 141 square inches, or 11.7 feet of 12
ga. wire for each electrode. That's a lot of wire!
You can run at a higher current density, but this means the initial
voltage will also be higher. At typical current density of 1 mA sq.
in, the initial voltage is usually well above the voltage provided
by a 7805, LM117, or LM334. (Of course, it also depends on the
quality of the dw.)
This means the system has to operate on the exponential portion of
the curve until the current builds up enough to reach current
limiting. This makes it dependent on the quality of the dw, hydrogen
bubbles at the cathode, ion channels, etc. So the repeatabilty
suffers.
If you want to use a higher current density, you need a higher
voltage from the regulator. But these current regulators cannot work
above 40V.
A series pass transistor can be used at higher voltages, but they
are vulnerable to failing shorted from ESD. A CLD can be used up to
100V, but they are also vulnerable to failing shorted due to ESD.
With high voltages, this could be lethal.
To summarize, you have a good system. But you can't increase the
current by a factor of 20/3.4 = 5.88 and expect it to work the same.
Best Regards,
Mike Monett
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