William K.
User Rank
Unexpected interactions
William K.   9/19/2011 6:50:23 PM
In years past we needed to do similar things with PLC input modules that were driven by solid-state proximity-type limit switches. The fault was that the off-state current draw of the switch was enough to convince the sensitive input module that it had a valid input. 7500 ohm, 5-watt resistors across each input solved the problem.

But it was good detective work that found the problem with the transmitter shutoffs.

User Rank
similar problem
Thinking_J   9/20/2011 5:54:39 PM
I have resolved several situations with same root cause.. inputs with not enough protection from RF sources.


A CPLD IC (22v10) on a failsaft circuit. It's inputs would pick up enough RF energy from relay contacts (equ. spark gap transmitter) loaded with inductive load. Actually erased the device!.. required 1000pf caps across all inputs for this environment (couldn't put a snubber across the contacts - leaked enough to hold the smart magnetic contactor load "on" , even when contacts opened).. and we couldn't move the relay farther away from the electronics.

Always keep mechical relays / large inductive loads away from the rest of the electronics (if possible)... just asking for trouble when close to each other.


Battery powered device with GSM transceiver/processor monitoring an external device via a simple contact closure. Reduced the pull up on the contact closure in effort to reduce battery drain (resistor array - 100K) , contact was normally closed drawing current across the pullup. The external circuit - no problem. However, the other three inputs (membrane push buttons with flex cable) would normally be "open". These inputs would close when the GSM transciever transmitted. The RF energy (not that much) would be rectified by the silicon junction on the processor input and pull the input low (randomly).... net result: it "pushed" it's own buttons when transmitting (occasionally)!

Next time , will use ferrite beads on these inputs. This time , changed pull up resistor array to 10K.

Partner Zone
Latest Analysis
General Motors is putting an off-road twist on hydrogen fuel cell technology with an imposing new pickup demonstrator called the Chevrolet Colorado ZH2.
Fine powder printing of industry-standard metal and ceramic powders with a grain size of less than 10 microns is now available from industrial 3D printer maker ExOne for its Innovent printer.
At ARM TechCon 2016, CEO Simon Segars will discuss how he sees billions of devices scaling to trillions as IoT applications proliferate. We know it’s happening. How do we prepare?
The term “autopilot” is now at the heart of a growing debate between Tesla Motors Inc. and Germany’s Federal Motor Transport Authority.
A make-your-own Star Wars Sith Lightsaber hilt is heftier and better-looking than most others out there, according to its maker, Sean Charlesworth. You can 3D print it from free source files, and there's even a hardware kit available -- not free -- so you can build one just in time for Halloween.
Quick Poll
The Continuing Education Center offers engineers an entirely new way to get the education they need to formulate next-generation solutions.
Oct 10 - 14, Embedded System Design Techniques™: Getting Started Developing Professional Embedded Software
SEMESTERS: 1  |  2  |  3  |  4  |  5  |  6 |  7 | 8 | 9 | 10

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.
Next Course September 27-29:
Sponsored by 3M
Learn More   |   Login   |   Archived Classes
Twitter Feed
Design News Twitter Feed
Like Us on Facebook

Technology Marketplace

Copyright © 2016 UBM Canon, A UBM company, All rights reserved. Privacy Policy | Terms of Service