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'Relay Control (Sprinkler)'
1998\07\04@225629 by paulb

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Hello there.

 Expensive options of solid state relays noted.  Minor limitation of
"self-excited" (the section following the photocoupler has to operate =
trigger itself from the line voltage) SSRs is that the triggering will
be delayed at low voltages; indeed the zero-crossing variety won't(!) at
a certain low voltage.

 Shall I tell a secret?  The commercial controllers, or at least the
one I have here, use (sensitive gate) Triacs with MT1 commoned to one
side of the supply which is also the logic ground so the gates are
driven directly.  It's the simplest and probably the most reliable way.

 Of course it means that they use a half-wave rectifier to supply the
logic circuits.  A resistor sinks most of the down-regulation power
(from 34V peak to 5V).

 Concept suggestion: use a 12V or 18V supply capacitor fed by resistor/
diode and regulated by a Zener.  Regulate a 5V rail referenced to the
positive of this with a 7905, and drive PNP transistors or P-FETs on the
positive rail to switch the Triacs (via resistors) on the 12V or 18V
negative rail.  The logic sees a regulated supply, but small
fluctuations on the Triac drive supply won't matter.

 Alternative; use a common negative rail, bias all the Triac gates on
from the +12V rail, but shunt them off with FETs.  Include pull-ups on
the FET gates to default the Triacs to "off".  This removes the need for
a Zener as the resistive divider is constant.

             R1              R2a
  AC24V ---/\/\/--|>|---+--/\/\/--+--- Triac gate
                        |         |
       Reservoir cap.---+        FET
                        |    R2b
       7812 to logic ---+--/\/\/--+--- Triac gate
                        |         |
                       etc.      FET

  R2=2*(no._channels)*R1
    =8V/Triac_trigger_current
  Reservoir cap = 1/20*R1 in Farad!   CV=It where I=10/2*R1
                 25V or 35V rating                t=1/50, V=2V ripple
            e.g. 470µF for R1=100 ohm

--
 Cheers,
       Paul B.

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