beautypg.com

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

Application information

background image

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

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