Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 30 |
Subtitle: EMI Suppression - Input Filters and Electromagnetic Compatibility |
Introductory Summary (Extended Section 30 Preview) |
In the previous sections, we explored the many components that make a barcode printer work - the printhead, the motors, the sensors, the memory, the communication interfaces, and the power systems. But a printer does not exist in a vacuum. It operates in a world filled with electromagnetic interference (EMI) - noise from other devices, radio transmitters, power lines, and even static electricity. At the same time, the printer itself is a source of EMI - its switching power supply, its fast digital signals, and its high-current motor drivers all generate noise that can interfere with other devices. This chapter is devoted entirely to EMI suppression and input filtering - the techniques that keep the printer's own noise from escaping and that keep external noise from affecting the printer. We will explain what EMI is, why it is a problem, and how it is suppressed. We will cover the input filter - the circuit that is placed at the AC power input. The input filter typically consists of a common-mode choke, differential-mode inductors, and X and Y capacitors. We will explore the common-mode choke - a transformer-like device that attenuates the common-mode noise. We will look at the X capacitors (across the line and neutral) and the Y capacitors (from line to ground and neutral to ground). We will examine the ferrite beads that are used on the DC lines and the signal lines. We will look at real-world designs from major companies: the use of a common-mode choke (e.g., the Schaffner FN series), the use of the X and Y capacitors (e.g., from KEMET or TDK), the use of the ferrite beads (e.g., from Murata), and the use of the EMI filters in the power supplies. We will also discuss the regulatory requirements - the FCC (US), the CE (Europe), and the CISPR standards. By the end, you will understand how the printer is kept quiet and immune, and you will appreciate the critical role of the EMI suppression in the printer's operation. |
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Chapter 1: The Problem - The Printer Is a Source of Noise |
A barcode printer is a source of electromagnetic noise. The switching power supply generates high-frequency noise. The printhead strobe pulses generate fast transients. The motor drivers generate high-current switching noise. The digital signals (clock, data, latch) generate high-speed edges. All of these signals radiate electromagnetic energy. This energy can interfere with the other devices - the radio, the television, the Wi-Fi, and the cell phone. The printer must be designed to limit the emissions. The printer must also be immune to the external noise - a nearby cell phone or a walkie-talkie should not cause the printer to malfunction. The electromagnetic compatibility (EMC) is the discipline that ensures the devices can coexist. |
Design Example: Radio Interference in an Office |
An office worker complained that the label printer was interfering with the radio. The printer was generating a buzzing noise on the AM radio. The problem was traced to the printer's switching power supply. The manufacturer added an EMI filter to the power supply, and the problem was solved. |
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Chapter 2: What Is EMI- An Unwanted Signal |
EMI (Electromagnetic Interference) is an unwanted signal that is radiated or conducted. The EMI is generated by a source, is coupled through a medium, and is received by a receptor. The source is the device that generates the noise (e.g., the switching power supply). The medium is the air (for the radiated emissions) or the power line (for the conducted emissions). The receptor is the device that is affected by the noise (e.g., the radio). The EMI is a regulated quantity - the emissions must be below the limits specified by the regulatory bodies. |
Design Example: Conducted Emissions in Sato Printers |
Sato's printer was tested for the conducted emissions. The conducted emissions are the noise that is transmitted back to the power line. The test measured the noise on the power line. The printer had to be modified to reduce the conducted emissions. The manufacturer added an input filter to the power supply. |
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Chapter 3: The Input Filter - A First Line of Defense |
The input filter is the circuit that is placed at the AC power input. The input filter attenuates the conducted emissions - the noise that is transmitted back to the power line. The input filter also attenuates the external noise that is coming from the power line. The input filter is a critical component of the EMC design. The input filter typically consists of a common-mode choke, the differential-mode inductors, and the X and Y capacitors. |
Design Example: Input Filter in Brother Printers |
Brother's printer uses an input filter on the AC power input. The filter is a module that is mounted on the PCB. The manufacturer chose the input filter to reduce the conducted emissions. |
Chapter 4: The Common-Mode Choke - A Transformer-Like Device |
The common-mode choke is a transformer-like device that attenuates the common-mode noise. The common-mode noise is the noise that is present on both the line and the neutral wires (the same polarity). The common-mode choke has two windings that are wound on the same core. The windings are connected in series with the line and the neutral wires. The common-mode choke has a high impedance for the common-mode signals and a low impedance for the differential-mode signals (the normal power current). The common-mode choke is a critical component of the input filter. |
Design Example: Choke in Zebra Printers |
Zebra's printer uses a common-mode choke on the AC input. The choke is a 10-millihenry choke from Schaffner. The manufacturer chose the 10-millihenry choke because it provides a high impedance at the switching frequency (65 kilohertz). |
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Chapter 5: The Differential-Mode Inductor - A Series Inductor |
The differential-mode inductor is a series inductor that attenuates the differential-mode noise. The differential-mode noise is the noise that is present between the line and the neutral wires (the opposite polarity). The differential-mode inductor is a single winding that is connected in series with the line or the neutral wire. The differential-mode inductor is a simple and effective component. |
Design Example: Differential Inductor in Sato Printers |
Sato's printer uses a differential-mode inductor on the AC input. The inductor is a 10-microhenry inductor. The manufacturer chose the 10-microhenry inductor to attenuate the high-frequency noise. |
Chapter 6: The X Capacitor - A Line-to-Line Capacitor |
The X capacitor is a capacitor that is connected across the line and the neutral wires (line-to-line). The X capacitor attenuates the differential-mode noise. The X capacitor is a safety capacitor - it must be designed to fail open, not short. The X capacitor is rated for the AC voltage (e.g., 250 volts AC). The X capacitor is a critical component of the input filter. |
Design Example: X Capacitor in Brother Printers |
Brother's printer uses a 0.1-microfarad X capacitor on the AC input. The manufacturer chose the 0.1-microfarad capacitor because it is a standard value. The manufacturer chose the X2 type, which is a safety-rated capacitor. |
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Chapter 7: The Y Capacitor - A Line-to-Ground Capacitor |
The Y capacitor is a capacitor that is connected from the line to the ground and from the neutral to the ground (line-to-ground). The Y capacitor attenuates the common-mode noise. The Y capacitor is a safety capacitor - it must be designed to fail short, not open. The Y capacitor is rated for the AC voltage (e.g., 250 volts AC) and has a high insulation resistance. The Y capacitor is a critical component of the input filter. |
Design Example: Y Capacitor in Zebra Printers |
Zebra's printer uses a 2.2-nanofarad Y capacitor on the AC input. The manufacturer chose the 2.2-nanofarad capacitor to attenuate the high-frequency noise. The manufacturer chose the Y2 type, which is a safety-rated capacitor. |
Chapter 8: The Ferrite Bead - A High-Frequency Attenuator |
The ferrite bead is a small, cylindrical component that is placed on a wire or a trace. The ferrite bead acts as a high-frequency resistor - it absorbs the high-frequency noise and converts it to heat. The ferrite bead is used on the DC power lines and the signal lines. The ferrite bead is a simple and effective component. |
Design Example: Ferrite Bead in Brother Printers |
Brother's printer uses a ferrite bead on the 24-volt DC rail. The bead is a 600-ohm bead from Murata. The manufacturer chose the 600-ohm bead to attenuate the high-frequency noise. |
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Chapter 9: The EMI Filter Module - An Integrated Solution |
The EMI filter module is an integrated solution that combines the common-mode choke, the differential inductors, and the X and Y capacitors in a single package. The EMI filter module is a convenient and reliable solution. The EMI filter module is typically a through-hole component that is mounted on the PCB. |
Design Example: EMI Filter in Sato Printers |
Sato's printer uses an EMI filter module from Schaffner. The module is a complete filter that is mounted on the PCB. The manufacturer chose the EMI filter module because it simplifies the design and is reliable. |
Chapter 10: The Shielded Cable - A Cabling Solution |
The shielded cable is a cable that has a metal shield (a braid or a foil) that surrounds the conductors. The shield is connected to the ground. The shield attenuates the radiated emissions. The shielded cable is used for the USB, the Ethernet, and the motor cables. The shielded cable is a critical component for the EMC. |
Design Example: Shielded Cable in Zebra Printers |
Zebra's printer uses a shielded USB cable. The shield is connected to the ground at both ends. The manufacturer chose the shielded cable to reduce the radiated emissions. |
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Chapter 11: The PCB Layout - A Critical Art |
The PCB layout is a critical aspect of the EMC. The high-frequency signals must be routed carefully. The traces must be short and wide. The ground plane must be solid. The power and the ground planes must be separated. The sensitive signals must be routed away from the noisy signals. The PCB layout is a critical art that requires the careful attention. |
Design Example: Layout in Brother Printers |
Brother's printer uses a 4-layer PCB for the EMC. The top layer is for the components and the traces. The second layer is a solid ground plane. The third layer is for the power. The bottom layer is for the signals. The manufacturer used the ground plane to reduce the emissions. |
Chapter 12: The Grounding - A Single-Point Ground |
The grounding is a critical aspect of the EMC. The ground is the reference point for all the signals. The ground must be a low-impedance path. The ground must be a single-point ground - all the ground connections must meet at a single point. The single-point ground prevents the ground loops. |
Design Example: Grounding in Sato Printers |
Sato's printer uses a single-point ground. The power supply ground, the digital ground, and the analog ground are connected at a single point. The manufacturer used the single-point ground to reduce the noise. |
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Chapter 13: The ESD Protection - A Safety Feature |
The ESD (Electrostatic Discharge) is a high-voltage pulse that can damage the electronics. The ESD protection is a TVS (Transient Voltage Suppressor) diode that clamps the voltage. The ESD protection is placed on the external connectors (the USB, the Ethernet, and the keypad). The ESD protection is a critical safety feature. |
Design Example: ESD in Brother Printers |
Brother's printer uses a TVS diode array on the USB connector. The array is a USBLC6-2SC6. The manufacturer chose the USBLC6 because it has a low capacitance and a fast response time. |
Chapter 14: The Regulations - A Legal Requirement |
The EMC regulations are the legal requirements that specify the limits for the emissions and the immunity. The regulations vary by the country. The most common regulations are the FCC (US), the CE (Europe), and the CISPR (international). The printer must be tested and certified to meet the regulations. |
Design Example: FCC in Zebra Printers |
Zebra's printer is certified to the FCC Class B. The manufacturer submitted the printer to a test lab. The printer passed the FCC test. |
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Chapter 15: The Testing - A Verification Process |
The EMC testing is the verification process that measures the emissions and the immunity. The testing is done in a shielded chamber. The chamber is an anechoic chamber that absorbs the reflections. The testing is a critical part of the product development. |
Design Example: Testing in Sato Printers |
Sato's printer was tested in a shielded chamber. The manufacturer measured the radiated emissions and the conducted emissions. The manufacturer also tested the immunity to the ESD, the EFT, and the surges. |
Chapter 16: The Radiated Emissions - A Measurement |
The radiated emissions are the electromagnetic waves that are radiated from the printer. The radiated emissions are measured by an antenna and a spectrum analyzer. The emissions must be below the limits specified by the regulations. |
Design Example: Radiated in Brother Printers |
Brother's printer was tested for the radiated emissions. The manufacturer measured the emissions from 30 megahertz to 1 gigahertz. The emissions were below the FCC Class B limit. |
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Chapter 17: The Conducted Emissions - A Measurement |
The conducted emissions are the noise that is transmitted back to the power line. The conducted emissions are measured by a line impedance stabilization network (LISN) and a spectrum analyzer. The emissions must be below the limits specified by the regulations. |
Design Example: Conducted in Zebra Printers |
Zebra's printer was tested for the conducted emissions. The manufacturer measured the emissions from 150 kilohertz to 30 megahertz. The emissions were below the FCC Class B limit. |
Chapter 18: The Immunity - A Robustness Test |
The immunity is the ability of the printer to operate in the presence of the external noise. The immunity is tested by applying the noise to the printer. The noise includes the ESD, the EFT (Electrical Fast Transient), the surges, and the RF fields. The printer must operate correctly during and after the tests. |
Design Example: ESD in Brother Printers |
Brother's printer was tested for the ESD immunity. The manufacturer applied an 8-kilovolt contact discharge and a 15-kilovolt air discharge. The printer continued to operate correctly. |
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Chapter 19: The System Integration - A Complete EMC Design |
We have now covered the EMI suppression and the input filtering. Let us put it all together. The input filter reduces the conducted emissions. The shielded cables reduce the radiated emissions. The PCB layout and the grounding reduce the noise. The ESD protection protects the printer. The EMC design is a complete system. |
Chapter 20: The Future of the EMC - Smarter and More Integrated |
The future of the EMC lies in the smarter and more integrated solutions. The future printers will have the integrated EMI filters and the smarter power supplies. The future printers will be quieter and more immune. The future printers will be easier to certify. |
Chapter 21: The System Integration - A Complete Design |
We have now covered the complete EMC design. The EMC design is a critical part of the printer's reliability. The EMC design ensures that the printer is quiet and immune. The EMC design is a critical enabler of the printer's operation. |
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Chapter 22: The End User - The Ultimate Beneficiary |
The end user is the ultimate beneficiary of the EMC design. The EMC design ensures that the printer does not interfere with the other devices. The EMC design ensures that the printer is not affected by the external noise. The EMC design is a critical enabler of the printer's reliability. |
Chapter 23: The Future - Smarter and More Reliable |
The future of the EMC lies in the smarter and more reliable solutions. The future printers will have a more robust and more integrated EMC design. The future printers will be more reliable and more user-friendly. |
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Detailed Summary - Tying It All Together |
We have now completed our comprehensive exploration of the EMI suppression and the input filtering - the techniques that keep the printer quiet and immune. We began by understanding the problem: the printer is a source of the electromagnetic noise, and the printer must be protected from the external noise. We learned that the EMC is the discipline that ensures the devices can coexist. |
We explored the input filter - the common-mode choke, the differential-mode inductor, the X capacitor, and the Y capacitor. We saw how these components attenuate the conducted emissions. We examined the ferrite beads that are used on the DC and the signal lines. We looked at the EMI filter modules that integrate the components in a single package. |
We discussed the cabling - the shielded cables that reduce the radiated emissions. We examined the PCB layout and the grounding techniques. We looked at the ESD protection that protects the printer from the static discharges. We examined the regulations - the FCC, the CE, and the CISPR. We discussed the testing - the radiated emissions, the conducted emissions, and the immunity tests. |
The overarching lesson is that the EMC is a critical part of the printer's design. A well-designed EMC system ensures that the printer is quiet and immune. A poorly designed system causes the interference and the malfunction. Understanding the EMC is essential for any engineer who wants to design a reliable and legal printer, and this chapter has provided that understanding from the basic principles of the input filter to the advanced techniques of the PCB layout and the shielded cables. |
End of Extended Section 30 |
cs@easiersoft.com