What is an X Rated Capacitor?

An X-rated capacitor, also known as a line-Filter capacitor or across-the-line capacitor, is a type of capacitor specifically designed and rated for use in power supply line filtering applications. These capacitors are meant to be connected directly across the AC mains power line to filter out electromagnetic interference (EMI) and radio frequency interference (RFI).

X-rated capacitors have several key characteristics:

• High voltage rating (typically 250V, 275V, or 300V AC)
• Low capacitance value (usually in the nanofarad range)
• Designed to fail open circuit
• UL and IEC safety certified

The “X” in the name refers to the capacitor’s safety rating according to IEC 60384-14 and UL 60384-14 standards. These standards define several safety classes for capacitors used in line-filtering applications:

Class	Peak Voltage Rating (V)	Permissible Peak Pulse Voltage (V)
X1	2500 to 4000	3000 to 4000
X2	1000 to 2500	1200 to 3000
X3	125 to 1000	1200 to 3000
X4	150 to 1000	1200 to 3000

X2 class capacitors rated for 275V AC are the most commonly used type in transformerless power supplies connected to 120V/240V AC mains power. The X1 class offers the highest safety margin but the capacitors are physically larger.

How X Capacitors Work

X-rated capacitors serve as electromagnetic compatibility (EMC) filters to suppress high-frequency noise from coupling onto the AC mains wiring. They are connected between the live and neutral wires, effectively shunting high frequency current back to its source rather than allowing it to propagate.

The simplified schematic symbol for an X-rated capacitor is:

      | |
--||--| |--
      | |

In reality, a complete EMI filter consists of two X capacitors (line-to-line and neutral-to-ground) and a common mode choke:

           +----|>|----+              
  Line ----|            |---o +DC    
           |            |      
           |    choke   |
           |            |                
           |            |---o -DC    
 Neutral --|            |
           +----|>|----+              
                 ||
                 ||
               Ground

The choke presents a high impedance to common-mode noise while the X capacitors provide a low-impedance path to shunt differential mode noise.

Failure Modes and Safety Considerations

X-rated capacitors are designed with an internal fuse structure that causes them to fail open if subjected to overvoltage or if they develop an internal short circuit. This prevents the capacitor from exploding or catching fire. Still, proper selection of voltage rating and continuous voltage rating is critical for safe operation.

When an X capacitor fails, it will often emit an audible pop and/or emit smoke as the internal fuse element vaporizes. The power supply will likely stop working. In rare cases, the capacitor fuse may fail to open and then the capacitor can overheat.

Some key things to check when selecting an X-rated capacitor:

• Voltage rating must exceed the peak AC mains voltage (120V AC has 170V peak, 240V AC has 340V peak)
• Capacitance value should be as low as possible while still providing sufficient noise filtering (typical values are 0.1uF to 0.47uF)
• Pay attention to the operating temperature range and derate the voltage accordingly if used at high ambient temperatures
• Flammability rating of the plastic case should be UL94 V-0 or better

Never use a non-X-rated capacitor, such as a general purpose film or ceramic type, in an across-the-line application! Standard capacitors are not designed to fail safely if shorted out.

Specifying an X Capacitor

Let’s walk through the process of specifying an appropriate X capacitor for a 15W, universal input (120V/240V) transformerless power supply.

Voltage Rating
For universal mains input, the capacitor must be rated for a minimum of 240V AC. Choosing the next highest standard rating gives us 275V AC.

Capacitance
The capacitance value should be chosen to provide sufficient differential mode filtering according to EMC standards while minimizing leakage current. 0.22uF is a typical choice. Going smaller than 0.1uF may not provide enough filtering while going larger than 0.47uF will result in excessive leakage current.

Rated Current
X capacitors have maximum ratings for continuous current. The capacitor current consists of the reactive current due to the AC mains frequency and the high frequency noise currents. For this low power example, the reactive current will be very small, on the order of a milliamp:

I = 2 * pi * f * C * V
= 2 * pi * 60Hz * 0.22uF * 240V
= 1.9 mA

So a standard 100mA rated capacitor will suffice. For higher power applications, make sure to calculate the worst case reactive current and select a suitably rated capacitor.

Package and Pitch
X capacitors come in both radial and box packages. For our example, we’ll choose a 15mm pitch radial through-hole part as it is easy to insert. Surface mount parts as small as 1812 are available for space-constrained designs.

Putting it all together, one suitable part would be:
KEMET R463I22205001M
275V AC, 0.22uF, 100mA radial X2 capacitor

Circuit Design Considerations

Some best practices to follow when incorporating an X capacitor into your design:

• Locate the capacitor close to the AC input connector
• Place the capacitor after the input fuse
• Include a bleeder resistor (typically 1Mohm) in parallel with the capacitor to safely discharge it when power is removed
• Keep high voltage traces short and away from low voltage circuitry
• Ensure adequate pad spacing on the PCB – 6mm between pads is typical for 275V parts
• If using a metal enclosure, connect the earth ground pin on the power inlet directly to the enclosure

Following these guidelines will help ensure your X capacitor performs safely and reliably.

Troubleshooting

If you suspect your X capacitor has failed, some simple tests can help confirm:

1. Visual inspection – Look for any signs of overheating or burnt plastic smell. A failed capacitor will often have a charred look.

2. Continuity test – With power removed and the capacitor safely discharged, use a multimeter to measure continuity between the two leads. A good capacitor will show open circuit (infinite resistance). A failed capacitor that didn’t open will show a short circuit (close to 0 ohms).

3. Capacitance test – Use a multimeter or dedicated capacitance meter to measure the capacitance. It should be within 10% of the marked value. A failed capacitor will often read much lower than expected.

If the X capacitor has indeed failed, be sure to replace it with one that has equal or better ratings. Do not substitute an under-rated part!

FAQ

Q: Can I use a Y capacitor instead of an X capacitor?

A: No, Y capacitors are meant to be connected between line/neutral and ground and have much lower capacitance values. They are not a substitute for X capacitors.

Q: Why do X capacitors come in so many different materials like film, paper, and ceramic?

A: The most common X capacitors are metalized polypropylene film type as they offer high stability, low loss, and self-healing properties. Paper capacitors were used in the past but are less common now. Ceramic capacitors are sometimes used but are limited in capacitance and have poor surge handling compared to film.

Q: Are X capacitors polarized?

A: No, X capacitors are non-polarized so the two leads are interchangeable. Polarized capacitors like aluminum electrolytics cannot be used for X applications.

Q: How often do X capacitors need to be replaced?

A: X capacitors do not have a set lifespan. As long as they are operated within voltage and temperature ratings and are not physically damaged, they can last indefinitely. Still, in high reliability applications, proactive replacement every 5-10 years is sometimes done.

Q: What happens if I use an X capacitor with too high of a voltage rating?

A: As long as the capacitance value is within limits, using an X capacitor rated for higher than necessary voltage is fine and actually gives more design margin. The only downsides are potentially larger physical size and higher cost compared to a lower voltage rated part.

In conclusion, X-rated capacitors are a critical safety component in transformerless power supplies. By understanding their characteristics and following best design practices, you can confidently use them in your next power supply design. Always select parts that are appropriately rated for your application, and don’t forget about the little but important details like bleeder resistors and proper PCB layout. With careful design, your X capacitor should provide years of reliable filtering.