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Subject: Re: TWL: Electrical Problem
From: Arild Jensen (elnav@XXX.XXX)
Date: Sat Feb 02 2002 - 17:42:20 EST
At 08:59 PM 02/01/2002 -0500, The Henry's wrote:
>There is a very sensitive, open-ended transistor circuit hanging out there
>just looking for a ground to make it switch. When you touch it, you ground
>the circuit which causes the lamp or mode or microprocessor to switch.
REPLY
I don't think so. Using a body's resistance to ground will not meet the
UL safety requirements.
I once had to design such a touch control circuit for Schick Electric.
We had to come up with a CSA/UL approvable design that was cheaper than a
regular switch, capable of controlling a hair dryer and inherently safe
for use in the bathroom.
We usd the blocking oscillator approach with a National Semiconductor chip.
I used the gold foil cosmetic decoration of the plastic housing as the
capacitive elements and designed the pattern so it wrapped around the
shell in such a manner that no matter how you picked up the hair dryer the
circuit would trigger and shut off as soon as you put it down, even in the
wet sink; which would of course be grounded.
The key was the body's capacitance and assuming the person was properly
grounded. standing on the floor.
By the same token, the double insulated case had to prevent electrical
shock shoudl the user be unlucky enough to be touching a wet tap.
Standing on a boat not connected to a the power grid represents a unique
situation not anticipated by the lamp designer.
Plugging the boat into the shore power re-establishes the "normal" and
expected relationship upon which the design is based.
So WRT gensets and inverters - your milage may vary < grin>
Cheers
Arild
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