OPTO 22 SNAP-PAC-R1 User Manual page 84

TROUBLESHOOTING I/O MODULES: FREQUENTLY ASKED QUESTIONS
Q: I have an input module that seems to "flicker" when a voltage is applied to it.
What is happening here?
A: Opto 22's basic input modules contain full-wave rectifiers. In an AC application it is
possible to destroy one-half of the rectifier and still have the module function after a
fashion. The full-wave rectifier turns into a half-wave rectifier, causing the module to turn on
and off at the line frequency when an input is applied. This may be seen visibly as "flicker"
on the channel status LED.
Digital logic devices, having much faster detection times than the human eye, can have
serious problems with this failure mode. For example, a digital counter attached to a failed
module will count at the input frequency.
This failure occurs most commonly in applications where the input module is placed in
parallel with an inductive load. When the load turns off, the back-EMF goes right through
the module, destroying one-half of the bridge.
Placing a transorb or large R-C snubber in parallel with both the load and input module will
significantly reduce the frequency of, or eliminate entirely, this type of failure.
Q: I have an output module driving a load. The load turns on okay but never seems
to turn off, unless I remove power from the relay entirely. What might be
happening?
A: This problem may occur when using an output module with a high-impedance load,
such as a neon lamp or a small solenoid. Loads like these often have relatively large initial
currents but relatively small "hold-in" currents. The result is that the off-state leakage current
through the module is insufficient to cause the load to turn on to start with, but sufficient to
keep it on once started.
The solution to this problem is to place a power resistor, sized for 8–10 times the rated
maximum leakage current for the module, in parallel with the load. Make sure that this
resistor has a high enough power rating for the application.
Q: I have a new AC output driving a solenoid. It turns on okay once, but will not turn
on again. What is going on?
A: Some solenoids and some types of halogen lights incorporate a diode in series with the
coil or filament. This causes the light to behave as a half-wave rectifier. Opto 22 output
modules have a built-in R-C snubber circuit in parallel with the output. The capacitor in this
circuit charges up but cannot discharge through the series diode, causing a voltage to
appear across the output terminals. Because the module must see a zero voltage across the
terminals to come on, it can't turn on again in this situation.
The solution is to find a lamp or solenoid that does not have an internal diode; a potential
work-around would be to put a high-value resistor (several tens of KOhms) across the
output terminals of the module, to allow the capacitor to drain its charge.
Q: I have an AC output module in a circuit, wired in series (ANDed) with another
contact. When that other contact closes, my load turns on momentarily, even
though my output module is off. Is my module broken?
SNAP PAC R-Series Controller User's Guide