Rockwell Automation 1791 APPL. INFO HALF-WAVE RECTIFYIN - Manual

Using HalfĆwave Rectifying Load Devices (Single
Diode) with 1791 ac Output Modules
Application Information
Some ac output modules use zero-crossing technology to allow output turn-on
when the voltage is less than some specified level (typically 40–50V). This type
of circuit greatly reduces the stress on contacts and other devices that are
sensitive to current surges.
When using an output module that includes zero-crossing technology with a
device that includes a single diode in series with the load, place a shunt resistor
across the load. The resistor is required to allow proper output turn-on.
L1
Load
L2
C1
39K Ω
Shunt Resistor required
Single rectifying diode
in series with load.
Control
ZeroĆCrossing
OptoĆisolation
Load
L2
Shunt Resistor NOT required
L2
Shunt Resistor NOT required
Simplified Circuit Diagram
1. When L2 is positive with respect to L1,
capacitor C1 is charged.
2. When the polarity switches, the diode
prevents C1 from discharging.
3. With C1 charged (possibly up to line voltage),
the zeroĆcrossing optoĆtriac sees that voltage,
and won't turn on.
4. The shunt resistor allows a bleedĆoff of the
voltage on the capacitor, allowing the circuit to
function properly.
No rectifying diode.
Full wave rectifier.
Load