Bernhard Schweighofer wrote:
>
....<snip>
> Just some thougths:
>
> type:
> The capacitor should at least be built for the AC-voltage you are using.
> Important: The value for the maximum voltage for the capacitor
> ('Spannungsfestigkeit') has to be for 'AC' ! I think such
> capacitors are called 'Mains Voltage Capacitors'

For the low currents contemplated for miniature products, most film caps
with adequate voltage ratings will be OK, and most are 'self-healing'
for voltage overloads.  Just remember that DC voltage ratings must be at
least root 2 x higher than AC ratings.  Also, voltage transients do
happen, constantly, so design in a margin.


> (Netzspannungskondensatoren).
>
> value:
> Let's say the maximum average current of your circuit is Iav .
> The capacitor has to supply your circuit in one half-wave -->
> IC_average = 2*Iav.
> now the average Voltage of the capacitor during loading your circuit :
> UC_average = ( sqrt(2) * 120 / pi ) - U_Zener  (Hope that's right, haven't
> any reference handy now)
>
> and with Q=U*C=I*t -->
>
> C = I*t/U = IC_average * 1/(2*f) / UC_average
>
> Another thing:
> IMHO you should use a X-capacitor ('Schaltfester
> Netspannungs-Kondensator') Oh where's my english dictionary right now
> ???? :-(
> A X-capacitor is a capacitor which works with MAINS-voltage and you can
> short-circuit (also if fully loaded) without damaging it.
> If you use a 'normal' main-voltage capacitor then you should use a
> small series resistor (say 220 Ohm --> Imax = sqrt(2)*120 / 220 = 0.8 A)
> for protecting the capacitor.

Yes. The resistor(s) should be a type that doesn't emit flames or smoke
when failing.  Beware that thickfilm chip resistors have poor overload
characteristics.

>
> One addition:
> Because of the capacitor you get a phase shift between the AC-voltage and
> your load-current pulses , so your zero-detect circuit has to be
> connected directly with a resistor to the AC-voltage.
>

Good point.  You can still use the high-value-resistor method here, or
correct for the phase shift in software.

> bernhard.
>
> PS.: Hope all is correct and you can understand it ! For those
> words I wasn't sure about I wrote the german translation :-)
>
> Bernhard Schweighofer alias schweigi@sbox.tu-graz.ac.at
> (Student at Graz University of Technology, Austria)

--

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