url: http://escribe.com/health/thesilverlist/m61936.html
Re: CS>A Cheap, High-Compliance Constant Current Source
From: Dan Nave
Date: Wed, 13 Aug 2003 06:53:29
Hi Dan,
> Mike,
> Your constant current source looks very interesting.
> A few questions... What sort of voltage supply did you envision
> that would be giving 150 to 160 volts to use with this circuit?
> Rectified and filtered AC line voltage (120V, nominal) would work
> out to approximately 170 VDC. Would this be suitable?
Sure, but be very careful connecting to the AC line. You can use a
polarized plug with a wide blade, or a regular 3-wire plug with a
ground. But watch out for reversed connections on the socket. I find
them all the time.
Your calculation is correct. It should give 170VAC, but there are so
many tv's, pc's and other devices on the line that tend to flatten
the peak. If you look at it on a scope, you can definitely see the
peak has a ramp shape as the filter caps get charged. I also like to
add a 10 ohm surge limiter in series with the diode. This reduces
the voltage after the rectifier, so I only get about 164VDC.
Another alternative is to use two voltage doublers on a 24VAC bell
transformer. This will give about 130VDC. Here's a schematic:
http://www3.sympatico.ca/add.automation/misc/130vdc.htm
If you want to use this, increase the caps to about 10uF per mA that
you expect to operate. This will give about 1.5V p-p ripple with a
half-wave rectifier at 60Hz. I deliberately set the cap values low
in the schematic to minimize the short-circuit current in the event
of a fault.
In any case, be very careful with voltages and currents that can
kill. Never trust circuits that run at these levels. Make sure
inquisitive kids can't get to it while you are out. Arrange the
power switch so it discharges the caps when power is turned off,
like this:
http://www.geocities.com/mrmonett/shingles/120vac.gif
> Also, what did you mean by R5=2 X 47K ohms? Perhaps you stated the
> wattage of the various resistors but I didn't see it. I suppose we
> could calculate it...
Yes, I forgot to describe that. The TL413 requires a minimum of 1 mA
so I set the current to 1.5mA. With 150V source, this takes 100k,
and the power dissipation is 225mW.
A common 1/4W resistor could be used, but I don't like running
components at their rating, so I used two 47k in series to get about
100k.
> What is your electrode size and spacing and how long does it take
> you to reach current limit in your setup?
Each electrode is 48 inches of 12 ga wound in a flat spiral to fit
horizontally inside a Pyrex 2 litre measuring cup. The top electrode
is at the 48oz mark, and the bottom electrode is at the 16oz mark.
They are about 1.5 inches apart.
The cover is a plastic child's starter plate from WallMart with a
slot cut in one edge for the handle. I used 6-32 screws to keep the
lid aligned and center the electrodes in the Pyrex.
It's kind of springy. When I walk in the room and sit at the
computer, I can see the 4th digit on the cell voltage bounce around
due to small waves disturbing the Nernst diffusion layer. It settles
down after a while and the readings are stable to the 5th digit.
I am now running at 776 uA, which is about 67uA/sq.in. The initial
voltage across the cell is only about 12V, so the current limiting
is instantaneous. There's enough headroom to stack 10 cells in
series and still have plenty left over.
> Wasn't there some caveat about making some silver compound at the
> water/air interface when using these voltages? Did you paint or
> coat the electrodes at the interface area?
Nah. That's Bob's crazy idea. I still have the first electrodes I
used for many years. The bottoms measure about 0.027 inch and the
tops measure 0.080 inch. There is no necking whatsoever at the water
line as he claims. If there were, everyones's electrodes would
quickly wear and they would fall in the water. That has never
happened:)
I definitely do not recommend painting the electrodes at the
waterline. You have no idea what kind of chemical reactions might go
on during electrolysis.
But I do recommend painting the 6-32 screws that hold the electrodes
to the cover. Even with galvanizing, they tend to rust after a
while. Probably due to the high humidity and long brew times.
> Thanks,
> Dan
Best Regards,
Mike Monett
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