>-----Original Message-----
>From: Martin McCormick [mailto:martin@DC.CIS.OKSTATE.EDU]
>Sent: 26 May 2004 16:58
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [EE:] Obtaining 5 Volts from live wire
>
>
>llile@SALTONUSA.COM writes:
>>There is a common circuit used to drop mains voltage to reasonable
>>levels, that works generally like this:
>>
>>A capacitor is connected to the AC mains on one end and to a
>full wave
>>bridge rectifier on the other.  It can be called a dropping
>capacitor.
>>The output of the full wave bridge can be connected to a
>zener diode or
>>some other suitable regulator.  The circuit will not work correctly
>>with a single diode, only a full wave rectifier will do.
>
>        This is a clever circuit all right but it kind of
>scares me. It's proper operation as opposed to smoke-venting
>mode depends upon the reactive resistance of a 1uf capacitor
>at 120 volts at 60 or 50 HZ.  It is possible for something
>like a brush motor or switching power supply to dump energy
>back in to the line at all sorts of frequencies.  Not only
>that, but the wave form of all that trash could be anything.
>
>        Do these circuits get fried very often by sustained
>radio frequency-like hash on mains wiring?  The circuit is
>basically a high-pass filter being operated at the extreme
>edge of its roll-off.
>
>        When one gets in to radio frequencies, that 1uf
>capacitor will begin to pass most of the signal appearing at the input.
>
>        If there is any energy there, it's going to heat stuff
>up until something pops.  It is probably that flame-proof
>resistor, but I am surprised more of these types of voltage
>dropping circuits don't vaporize more often.

Properly designed, these circuit shouldn't rely purely on the reactance of
the cap for the very reasons you mention.  Usualy part of the drop will be
shared with a resistor to ensure some impedance at high frequencies.


Mike

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