SNAP Digital Troubleshooting FAQ

Q: I have a SNAP digital module that has ceased to function. What may have caused this?

A: Loss of function in a SNAP digital module is normally caused by misapplication. As with Opto 22’s other digital I/O module families, there is no one way in which SNAP I/O modules fail. For one, input modules normally fail in a different way than outputs. Output modules are just a type of SSR, so the equivalent section from the SSR subFAQ also applies here. To summarize that, output modules normally fail open, or half-wave for AC, and the failure is normally caused by application of too much current, or degradation of the semiconductor switch by overvoltage. Please note that, because the module is separated into two parts by the optical isolator, it is possible to have an output module that behaves properly on the logic side, but is broken on the field side. The LED will in most cases continue to turn on and off as commanded by the driving logic device, even though the module has failed. The first step to troubleshooting a failed module is to check the fuse. If it is blown, simply replacing it should get the module working again. If the module continues to blow fuses, some analysis should be performed on the load to see if the module is really suitable to the application.

Input modules also typically fail on the field side, typically from overvoltage or overcurrent resulting from overvoltage. Opto 22’s SNAP AC and DC input modules contain a full wave rectifier (as opposed to the Standard and G4 modules, which contain a bridge rectifier), allowing hook-up without regard to polarity. Unlike output modules, the LED on an input module will normally not function as expected when the module has failed.

Failures of the field side of all types of Opto 22 digital I/O modules occur extremely rarely, and are typically the result of misapplication. The logic side failures of Opto 22’s I/O modules are also extremely rare. Usually, a failure of the logic side is attributable to using the module at the wrong logic voltage, or to miswiring.

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 results 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 is normally a problem 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 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 here 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 (AND’d) 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 is the case where 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 cannot turn it off until the voltage crosses zero again. There is no easy solution to this problem; typically a solution will involve either maintaining a minimal voltage across the module, pre-wetting it with a low voltage, or absorbing the energy from the half cycle turn-on with an R-C network in parallel with the load.

Q: Why are all four loads connected to my SNAP output module coming on when I’m activating only one of the channels?

A: The SNAP digital output is likely connected on the wrong side of the loads. Remember that all four channels share one fuse; thus one side of each output is common with one side of all the others. If the module is on the wrong side of the load, any one channel is effectively connected to all four. If the output is a SNK type, try switching to a SRC type. In AC applications, reverse the module wiring.

Q: I keep having failures on one channel of my SNAP output module. Why might this be happening?

A: Failures of only one of the four output channels would indicate a problem with the load being driven by that channel. Make sure that the load characteristics, including transient characteristics, do not exceed the capabilities of the module. Also be sure that the module is properly transient protected, using either an R-C snubber (AC) or a commutating diode (DC) across the load. Modules may also be protected through the use of transient voltage suppressors across the module itself.

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