We quickly discovered that the relay switching was inducing lots of noise throughout the system. Most notably, the backplane power was showing 6V-10V high-frequency noise on the 5V and 24V supplies, GND and frame GND, that was synchronized with the relay opening. The supplies (also made by our partner) were specified to limit transients to 10 percent over the supplied voltage. The noise was going right through our supplies and filters, and appearing on the outputs. With this much noise in the system it was a minor miracle that our module was operating at all.
We looked at the voltage across the relay and found that they were arcing badly every time they opened. The designer came up with a voltage snubber to suppress the arcing, and the problem went away. The supply noise was still there, but with smaller amplitude and shorter duration so that our supplies could handle it. We also discovered that if we put our module on an extended backplane (adding about 8 inches to the length), we could no longer duplicate the failure.
The designer read though the relay module installation instructions and found recommendations that “sensitive analog modules” should not be located near the relay module and that all inductive loads have snubbers installed as close to the load as possible to limit contact arcing and RF noise production. We decided that in a real-world installation, the noise was tolerable after installation, but as the relay contacts aged the transients got worse and eventually caused a failure in our modules.
We had observed the relays degrading during our investigation, and had to replace them twice when the contacts got welded together. We asked our partner to find out whether the customer had snubbers installed, and asked them to move the analog module away from the relay module. At this point, they told us that the customer had already moved it away and the failure was gone. We are now waiting for our partner to let us know whether the snubbers were there or whether they helped. I suspect that we will not hear any more about this.
This entry was submitted by John Elliott and edited by Rob Spiegel.
John Elliott graduated from UC Santa Cruz with a BA in Physics, where he fell in love with electronics and software. He spent 17 years as a test engineer working with computers, disk drives, optics, and instrumentation. After an eight-year detour as a software project manager, he is now managing a new product verification team for a small industrial automation company.
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