Jon Bertrand wrote:
>
>      Maybe I didn't understand the question but...
>
>      I'd be careful when using a cap to tap off of line voltage.
>
>      When I design anything I always think about stray bandwidth - do I
>      have more than I need?  If so, how are the various impedances in my
>      circuit going to respond to it?
>
>      Ideally, any supply that takes line voltage and steps it down would
>      limit the bandwidth to as low as your circuit could tolerate (it would
>      be a low pass filter).  By limiting the bandwidth you'd cut down the
>      RF, ESD, and transient-type problems in your circuit.
>
>      With a series cap as the voltage dropping element you'd have a high
>      pass filter - the higher the frequency - the easier it would conduct.
>      ESD, RF, Line spikes would all eat you alive unless you put a low-pass
>      filter after the cap.
>
>      It doesn't seem safe.
>
>      ... just my impression...
>
>      Jon Bertrand
>      jonb@cirris.com


As indicated in earlier msgs this subject:

1. You should include some series resistance in the circuit.  Use at
least 100 Ohms for 120VAC and 200 Ohms for 230VAC.  The resistor(s) must
have good surge current capabilities, i.e., do NOT use small thickfilm
chip resistors or metal film.  Use a combination of parallel chip
resistors (say 3 each 300 Ohm 1206s), carbon comp, or wirewound.

2. There are literally millions of capacitor power supplies in everyday
use.....

3. Transformers are only as good as their insulation, too.

4. Yes, it's too true that any connections made to the circuit are not
isolated.  Do not use capacitor power supplies unless all circuits are
self-contained.  You will find that consumer products that have external
connections for microphones, earphones, serial ports, and so on, are
ALWAYS powered by transformers.

Enough said.


--

Paul Mathews, consulting engineer
AEngineering Co.
optoeng@whidbey.com
non-contact sensing and optoelectronics specialists