Sat Apr 26, 2003, 12:00:03 am
> CS>RE: Current limitation
> From: edkas...@pacbell
> Date: Fri, 25 Apr 2003 12:12:33
> presently I am using 110VAC with a bridge rectifier to 54 VDC. I
> would like to set a current limit.
> Would the optimum setting be 2.3 ma ??
> My present set up
> two silver wires each 7 inch wetted area spaced 6 inches apart
> with a mechanical plastic stirrer in between, set 1-2 inches in
> the water
> with 1 1/2 gallons distilled water (old battery charger glass
> container)
> All advice and comments appreciated.
> Ed Kasper
Ed, this is not a good idea. You have a good chance of killing
yourself.
First of all, if you put 110VAC into a bridge rectifier, the output
voltage across the load is not 54VDC.
The output is a full-wave rectified sine wave that goes from zero to
the peak value of the sine wave, which is about 160V.
See "Bridge Rectifier" for a picture with waveforms:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectbr.html
With 110VAC input, the peak voltage across the load is
sqrt(2) * 110 = 155.56349 Volts.
The average voltage across the load (that you would read with a DC
voltmeter) is
Vav = 2*Vp/pi
= 2*155.56349/pi
= 99.034794 Volts, not 54V
See "Voltage Conversion Factors" for the exact formulas:
http://mathforum.org/library/drmath/view/56347.html
If you use the line voltage without an isolation transformer, one
rod will go from zero to +156V peak, and the other will go from zero
to -156V peak.
Here is an animation of a bridge rectifier showing the voltage
across the rods. (The green arrows show conventional current flow,
which is from positive to negative. This is the opposite direction
from the flow of electrons.)
http://engineering.dartmouth.edu/other/3Dcircuits/animations/bridgeR.html
If you touched either rod to a water pipe, sparks would fly as the
bridge rectifier burned out and the rod welded to the pipe.
Hopefully, the breaker would pop before any more damage was done.
If you happen to touch either rod, then touch a water pipe (or some
other grounded object), you could be electrocuted. Especially in the
case of an accidental spill when you are madly reaching for
something. If your wife or neighbour tried to grab you to save you,
they could get electrocuted also.
If a child wandered in while you were absent, the first thing they
would want to do is touch the pretty wires or dip their hands in the
nice water.
As far as trying to regulate the current, the maximum voltage
differential across a LM117 is 40V. Applying 156 volts will destroy
it.
I would strongly recommend you abandon this approach before you kill
yourself or someone else.
If you really want to use current limiting, get a
transformer-isolated power supply that delivers 30 to 40 volts dc,
then try your LM117.
BTW, a lot of the formulas I see on the web to set the value of the
limiting resistor are incorrect.
The reference voltage for the LM117 is 1.25V. Say you want to set
the current to 1 mA:
From Ohm's Law:
R = E / I
= 1.25 / 0.001
= 1250 ohms.
The nearest standard value is 1,200 ohms.
Best of Luck!
Mike Monett
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