Repost - sympatico hung when I tried to send. I don't think it went, but if so,
I
apologize for the duplicate post.
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CS>Re: Current limitation Please check my math
From: Dave Sawatzky
Date: Sun, 27 Apr 2003 20:17:53
Hi Mike Monett
Hi Dave,
Thanks for responding to my questions on the NO and NO3 results. I'm
sorry to hear of your neck injury - and I hope it heals soon.
[...]
> I am the guy that sent those samples to Ole Bob. The circuit I
> used is very much like the one you described, but when I designed
> it my primary concern was safety. I wanted to design a line
> rectified generator that was almost shock proof, so this is what I
> came up with.
> In the AC line I put a 3k ohm resister in both sides of the line
> before the bridge diodes, this limits the short circuit bridge
> current to 19ma. After the bridge is a 22 mfd capacitor with a 10k
> ohm resistor in both the positive and negative lines. (which
> agrees with your calculation of 20,216 ohms)
The calculation assumes an output voltage of 90VDC. Unfortunately,
the 3k resistors in series with the bridge gives very poor
regulation, so the output voltage is 90 volts only at one load
current.
> The purpose here is for greater shock protection in case of
> polarity reversal.
Another suicide circuit!
Unfortunately, you did not study the resulting shock current. At
120VAC input, the current is limited by the 3k resistor in series
with the 10k, plus the human body resistance. In wet conditions,
this can be 500 ohms or less. The current is then:
I = E / R
= 120 / 13,500
= 8.89 mA
This is well above the 6 mA that is considered the "freezing"
current. You could be frozen by this level of current and be unable
to disconnect yourself from the circuit.
At the very least, you would get a severe shock and might hurt
yourself in the involuntary reflex.
There are many references to the effect of current on the human
body, and there is a range in the published numbers. OSHA has a
badly written and ambiguous page, but they clearly state at 5 mA,
you can let go. Above this current, they state:
6-30 mA: Painful shock, muscular control is lost. This is called
the freezing current or "let-go" range.
http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html
There seems to be no way to isolate a DC supply from 120VAC to
eliminate the shock hazard without using an isolation transformer.
I would remove this circuit from service immediately before you hurt
yourself or someone else.
> After the current limiting resisters I added a 220k bleeder
> resister from hot to negative. An LED then connects to the anode
> rod. The 220k resister resister will reduce the open line voltage
> from 160 volts to 135 volts and will discharge the capacitor when
> unit is turned off. This resister can be any value between 10k and
> 220k or higher allowing you to change the open circuit output
> voltage to any level you choose. A 15k ohm resister will have open
> circuit voltage of 55 volts and a much lower shock hazard making
> it very safe to use.
I did a SPICE analysis of your circuit. Here is the schematic as I
understand from your description:
http://www3.sympatico.ca/add.automation/misc/cssch00.gif
I set the load resistor to 133k, which is approximately the number I
get for fresh distilled water. At little or no load, the DC output
of your circuit is 120V, and the voltage across the load is 97V:
The output takes about 1 second to stabilize due to the 3k resistors
in series with the bridge that have to charge the 20uF capacitor.
http://www3.sympatico.ca/add.automation/misc/csop00.gif
The first output is 24.5V and 3.24mA. This gives a load resistance of
R = E / I
= 24.5 / 3.24e-3
= 7561.7284 ohms
I had trouble getting the correct output at this load, and had to
increase the 3 k resistors to 3.3k.
Setting the output resistance to 7561 ohms gives a DC output of 93V
and 25V across the load:
http://www3.sympatico.ca/add.automation/misc/csop01.gif
The second value was at 5V and 5.65mA. This gives a load resistance
of
R = E / I
= 5 / 5.65e-3
= 884.95575 ohms
http://www3.sympatico.ca/add.automation/misc/cssch02.gif
However, the output was incorrect at this load value. The DC output
fell to 86V, and the voltage across the load was 3.64V:
http://www3.sympatico.ca/add.automation/misc/csop02.gif
Just on the chance there was a wiring error, I changed the output
series resisance to from 20k to 10k. This produce 66V for the DC
output and 5.39V across the load:
http://www3.sympatico.ca/add.automation/misc/csop03.gif
Normally, SPICE should give very accurate results for this type of
circuit. So there may be a wiring error, or an error in the measured
values.
In any event, the large resistors in series with the bridge make the
DC output dependent on the load current.
This means you will find it difficult to get repeatable results,
since the initial current depends on the impurities in the distilled
water and the temperature of the water.
I'd take a serious look at the shock hazard and lack of
repeatability, and retire this circuit immediately:)
[...]
> Dave Sawatzky
Best Regards,
Mike Monett
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