OPTO 22 SNAP-PAC-R1 User Manual page 79

CHAPTER 4: MAINTENANCE AND TROUBLESHOOTING
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?
A: This situation arises when the output module does not see a voltage across its terminals. This
occurs when another contact is wired in series with the module, between the module and the "hot"
side of the supply. Because the module is totally isolated from field to logic, the field (or contact) side
of the module must power itself from the attached line. By nature of the pilot circuit in the module,
the switching semiconductor is in an indeterminate state when there is no voltage across the
module. It may turn on for up to a half cycle if it is suddenly "wet." The pilot circuit will realize that
the module is on, but it cannot turn it off until the voltage crosses zero again.
73
SNAP PAC R-Series Controller User's Guide 73