A resistor is an electronic component that limits the flow of current into or through a circuit to protect other components from damage. Resistors are a functional design element that support current flow management throughout the design.
Most electronic designs will use several resistors placed strategically to achieve their intended effect—which initially has almost nothing to do with electromagnetic compatibility (EMC). Even when EMC has entered the chat, it would be rare to add resistors to a design to help solve EMC issues.
So why are we talking about resistors on an EMC blog? Indeed, perhaps you saw the title of this post and thought, “I have never needed to consider resistance in EMC.” This is a very fair statement and very often the case. However, there are a few reasons to give resistors a little more consideration.
Resistors have reactive responses that can be leveraged, for free, to improve the emissions and immunity of your design.
Resistors can be used to simply temper communication energy and, hence, emissions. If you have plenty of functional margin, but you believe that functionality may be the source of emissions, then consider troubleshooting load or series resistance to confirm your hypothesis. You may also wish to take a second look at the frequency response of your resistors.
Once you have identified the type of failure in your product, it’s time to consider solutions, which may include incorporating or tuning your resistors.
Every design includes communications components, whether that communication is between ICs within a product or between modules in a system via cabling. And anywhere you have transmitters and receivers, there’s a risk of higher emissions.
If you find yourself with a product that works functionally but the emissions are too high, one option is to add or tune existing resistor values. This can enable you to meet EMC requirements while maintaining functionality of the product by staying within the range of acceptable resistance values.
Pull-up resistors are already part of your design in the reset line. You might have tuned these resistors to the value suggested in the IC Datasheet, and this is a very good starting point. However, don’t forget to look at the reset line pull-up resistance.
Reset lines can be very vulnerable to fast transients such as electrostatic discharge (ESD) strikes. If the pull-up resistor value is too high, transients such as ESD can pull it down momentarily and cause the whole system to reset. This is definitely not a problem you want to encounter for the first time at an EMC testing lab!
Lower resistance supports higher immunity, but there is a sweet spot. See if you can reduce the resistance value enough to provide a more solid pull-up without drawing too much current when the reset line actually needs to be pulled down.
Your circuit was working fine until it was subjected to immunity testing, and it worked fine again after the test was over. It’s possible that the RF used in the test kicked off an oscillation within the circuitry, and that instability went away when the RF was removed.
Resistors can be used to quell this instability and improve immunity. Low series resistance leverages a series of three resistors with low values, between 1-5 ohms, to manage stability factors. This consideration is most relevant for circuits with amplification components.
If you already have a hypothesis about the source of your EMC failure, then resistors can be useful for confirming it. Don’t be afraid to stick a trimmer in your design and tune your response. The trimmer allows you to adjust resistance by adding more resistance or adjusting the value of existing resistors.
Chip attenuators are resistor components that maintain characteristic impedance (50Ω and 75Ω) between a product and its external communication cables or internal traces on the PCB. If impedance is mismatched across the connection, performance will suffer and emissions will be higher.
Chip attenuators reduce the communication signal (and with it, emissions), add isolation and protection, and help manage power transmitted between systems to support maximum power transfer and optimal performance. They can be implemented by themselves, or you can start with an IC that already includes this resistor component in the design.
As you can see, while resistors are not typically an EMC troubleshooting item, they can be of use, and you might consider incorporating or tuning them depending on your type of failure. However, do be aware that resistors—like everything in circuit design—come with compromises.
Resistors dissipate energy and can reduce the efficiency of your design’s performance. When adding resistance to fix one issue, always consider the impact to other aspects of the design. As with any design, this trade-off may still offer a favorable alternative to fully redesigning your product.
The secret to making resistors work for you is knowing your circuit inside and out—and even better than that, knowing when and where it’s failing. As you troubleshoot, consider all the components and techniques at your disposal—even ones like resistors that you might not initially suspect.