url: http://escribe.com/health/thesilverlist/m60250.html Re: CS>$$$ perpectives From: Ode Coyote Date: Mon, 16 Jun 2003 03:47:57
>> I use a comparator circuit as well. > With a couple of very large capacitors in the rectifier circuit > and an isolating cap on the trigger voltage feedback circuit, the > comparator takes as much as 40 seconds to trigger off at a dead > short below the trigger voltage. > No static spike is not going to get absorbed from the power supply > end and the water damps out static on the business end. > Like Trem says..bulletproof. > PS I don't use mosfets etc [TTL , I believe..virtually immune to > spikes blowing them out] > Ode CMOS is very sensitive to esd. TTL draws more power, but is pretty much immune. However, RFI problems can be tricky. The dielectric constant for water is around 80, so the capacitance between the probes is negligible. There will be little damping effect, and the probes will act as short antennas. Placing a dead short on the trigger voltage may not be telling you much. What counts is how much rfi appears at the probe input to the comparator. This is much more difficult to measure. However, your unit has very short connections between the comparator and the electrodes. This helps by reducing the capture area for esd and rfi events. Your pcb also has very short traces, so the stray inductance is low. This also reduces the effect of rfi, since the impedance at high frequencies is much lower than a system with long leads. So basically you have a little 7 inch antenna at the comparator input, which will not pick up much rfi at low frequencies. It will pick up cellular phones quite well, but the comparator is probably much too slow to respond. It might be an idea to check, though. A big electrolytic at the comparator input helps with low frequency events, but the series inductance limits the bypassing effectiveness to frequencies less than about 100KHz. This may be quite adequate in your system due to the very short traces. If the comparator were connected to long leads and noise proved to be a problem, it might be useful to add small resistors in series with the comparator input pins, say 1K, and small caps from the input pins to the nearest ground reference, probably the negative supply pin for the comparator. The 1K resistors would add negligible offset to the comparison voltage, and having equal values on both inputs would tend to cancel the offset. A small surface mount tantalum has pretty low series inductance into the megahertz region. A 1K in series with a 1uF cap has a corner frequency of 159 Hz and would provide 40dB attenuation above 100KHz. If cellular phone transmissions were a problem, additional 1nF SMD caps across the tantalums should eliminate the rfi. So it looks like everything is under control, but it's a good idea to check. A bad component or solder joint can create wierd symptoms that would be difficult to diagnose. The electric drill test is a crude but effective way to see if there is a sensitivity to noise. If it passes with the drill held near the electrodes, and phones don't trigger it, I'd say it is probably good enough. But you have to do the test when the run is almost finished. Doing it at the start of the run will tell you nothing, since the comparator inputs have such a large voltage difference. Best Regards, Mike Monett -- The silver-list is a moderated forum for discussion of colloidal silver. Instructions for unsubscribing may be found at: http://silverlist.org To post, address your message to: [email protected] Silver-list archive: http://escribe.com/health/thesilverlist/index.html List maintainer: Mike Devour <[email protected]>
