On Fri, 6 Dec 1996, Ed Maste wrote:

> In article <32A85C12.10AE@whidbey.com>,
> Paul Mathews <optoeng@whidbey.com> wrote:
>
> >I don't hesitate to design without transformer isolation.  Many of my
> >products use series capacitor power supplies, and any mention of this
> >always brings out dozens of responses of the "You're crazy if you don't
> >use a transformer!" variety.  I take comfort in the simple fact that the
> >vast majority of electrical products in the world don't use a
> >transformer.  However, it is too true that knowledge and care are
> >essential.  Electricity can kill.  I use isolation transformers, battery
> >operated scopes, differential probes, etc.  Take care.
>
> Could you start me off on the practical design of a series capacitor supply?
> I've got a 2 channel sequencer using a 16C84 which uses a resistor-zener
> supply from the 120V, 120V through a resistor for zero crossing and two
> triacs driven directly from PIC pins.  However, the current required by
> PIC and triac drive means that the resistor has to be low enough that it
> gets somewhat warm.  I can see the idea of using a capacitor there but
> don't know where to start in terms of cap. value or type etc.
>
> Thanks.
>

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'
(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.


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.


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)