Jon Titus
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Re: Part IV: Fundamentals of Digital Debugging
Jon Titus   1/19/2012 3:19:31 PM
Q: Re: power problems with logic circuits. I designed a CMOS (not MCU) circuit for a starlight camera control function, and while it worked perfectly on the bench, it didn't work when hooked up the the camera system. It took a memory storage scope to discover that the pan/tilt function, which was 120 VAC switched by electromechanical relays, was generating extreme voltage spikes that showed up in the logic as spurious signals. Fixing the problem required capacitor filters across every 120 V relay contact.

So if it works in the lab, but not the actual environment, start looking beyond the MCU.


A:  Yup.  But in the lab you might have found the problem if you simulated the tilt/pan loads and ran 110V to them.  Switching loads with relays almost always causes problems, either from RFI caused by the contact closure or by back EMF caused when the relay coil field "collapses."  To reduce contact switching, look for a solid-state relay with zero-crossing switching.  That arrangement will limit inrush current as the 110V AC line voltage is at zero when the SSR switches.

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