Application of linear and digital ICs in the industrial environment has its own set of nasty problems. Circuits may work flawlessly on the bench, but be useless in real-world industrial applications. This is because ICs are highly susceptible to EMI and EMP.
We have all heard of EMI (Electro-Magnetic Interference) and most industrial circuits are designed and tested to EMI standards. Generally when we hear of EMP (Electro-Magnetic Pulse), it is in relation to a destructive EMP weapon that can render all electronics DEAD! However, in this case, the EMP that I am referring to is that of electrical contact noise that causes interference from both radiated and conducted noise. This discussion covers some very simple and basic means of dealing with EMP in the industrial environment –this is something that I learned by experience rather than in engineering school…
The next article; “Noise Hardening for the Industrial Enviornment, Part 2” discusses noise hardening techniques for ICs and circuit boards.
The primary source of electrical noise
Mechanical relay contacts that control the coils of relays and power contactors are the primary source of industrial EMP. I have observed wide-spectrum RF noise bursts resulting from electrical contact bounce that run in the order of 200W peak. To make matters worse, every electrical circuit with external wiring has a resonant frequency and the EMP noise pulse will cause severe high-voltage ringing at this frequency (standing waves). If such wiring is connected to a circuit board it introduces “conducted noise” into the circuit board electronics.
Correct noise at the source –a MUST!
All contactor coil driver contacts and must have R-C snubbers (Quencharc) across the driver contacts rather than the contactor coil. While connecting it across the coil may be the most convenient, it is only a partial solution as there is yet significant wiring inductance between the contacts and the coil. The R-C snubber attenuates noise in two ways: first, the 47 to 150Ω resistor provides an RF load for the resonant circuit, and second, the series 0.1 to 0.47uf capacitor absorbs the coil current for a sufficient period of time to allow the contacts to open far enough to obtain adequate arc voltage that extinguishes the arc. This is much like the Kettering automotive ignition system that requires a capacitor across the points for proper operation. Snubbers across contactor power contacts may also be beneficial depending upon the nature of the load –however, such noise is less likely to get back into your electronics.
Why are contactor coils perhaps the worst? First, the contactor coil has a very high inrush current due to low inductance that is caused by the open magnetic circuit. When the magnetic circuit closes, the coil inductance increases by orders of magnitude so the holding current drops greatly. Second, relay contacts bounce as they pick up –the bounce generates an arc that generates a wide spectrum of high power RF noise.
Since Quencharc devices seem unreasonably expensive, you may fabricate your own from a 2W carbon comp resistor and an AC rated capacitor such as one with polypropylene film. An alternative to the obsolete carbon comp resistor is the Ohmite OX or OY series ceramic composition resistor. Note that this resistor must have both a high pulse energy rating and very low inductance –carbon film resistors are subject to failure and sub-standard performance.
Many (if not most) electronic circuits are packaged in metal (shielded) enclosures. While this is helpful against radiated noise, it does nothing to attenuate conducted noise.
Shielded wiring may also help prevent both radiated and conducted noise from getting to the electronics provided that the shield is grounded at the location of the sensitive circuit. However, if the shield is grounded at both ends, the shield may conduct ground loop current (another source of noise) and do strange things. Electrical wiring is often a witch’s brew of complex, difficult to analyze resonant circuits.
The addition of ferrites to the above shielded wiring (cable) greatly reduces standing waves on the cable shield. The proper location of the ferrite is close to the electronics. Testing labs have manufacturer’s kits of assorted ferrites that can be evaluated while the unit is under test. Ferrite cores come in a number of mechanical configurations: round cores that slip over cables, split cores that clamp onto cables and rectangular for ribbon cables. The type of magnetic material is called a “soft ferrite.” Such not only adds series inductance, but is lossy so that it attenuates standing waves.