Circuit Breakers in Your PCB and Your Bill of Materials

December 12, 2018 Altium Designer

Welding torch burning a PCB

If you’re like me, you’ve probably caught some electronic components on fire in your early days of electronics design. I once shorted out to a breadboard and spent a couple hours trying to build a simple circuit before realizing what had happened. If only our breadboards had a small circuit breaker...

Including a circuit breaker in your PCB or in your overall electronic system is relatively a simple matter, and your customers will definitely appreciate the extra eye towards safety. Just like with your other components, you’ll need to include your circuit breaker in your bill of materials for your manufacturer and assembler.

 

Circuit Breakers in Your PCB

If your PCB contains sensitive elements that your need to protect from a short circuit current, you’ll need to include a circuit breaker in your PCB. There are many types of circuit breakers that can be included in your PCB, protecting your circuit from either AC or DC short circuit currents. If you are building a simple device on a pre-tracked board, these circuit breakers can be purchased at your local hardware store.

Sometimes, a fuse may be a better option, depending on how fast the tracks in your PCB will heat up compared to the fuse. If the sensitive traces and solder points on your PCB heat up faster than the fuse you install, then your traces and solder points are the real fuses. Either the solder will heat up and wick away, your traces will melt, any exposed copper will oxidize, or your traces might hold the high temperature as your board catches on fire.

Protecting a PCB from short circuit current requires some idea of the level of current that will propagate in the circuit once it shorts. There are a number of fuses and circuit breakers you can use that will mount to a larger PCB that can withstand on the order of 10 A of current. For example, the 1410-L110 series circuit breaker is a single-pole, press-to-reset, thermal circuit breaker with extremely fast switching. This circuit breaker easily attaches to your PCB as a through-hole component.

Other circuit breakers come in the double-pole variety and are typically used with circuits that require larger appliances. Something more heavy duty, like a Merlin Gerin circuit breaker, cannot be mounted on a PCB and must be mounted to the external packaging of your device. However, they can provide significant protection in the case of short circuit and prevent your board from failing or catching on fire, especially in a high power system.

Electrical failure in a power outlet

If only they used a circuit breaker

 

Selecting the Right Circuit Protection

There are four types of circuit breakers: thermal, magnetic, thermal-magnetic, and magnetic with a hydraulic delay. Each type of circuit breaker has a different current trip profile over time and distinct mechanical characteristics. Purely magnetic circuit breakers without hydraulic delay use a solenoid to break a current surge and are by far the fastest circuit breakers among the four options.

Some engineers will oversize their circuit protection to avoid accidental tripping, but they often specify a circuit breaker rated much higher than they should. The current rating on a circuit breaker tells you the maximum current that the circuit breaker can maintain at room temperature. For example, a 10 A circuit breaker will maintain a 10 A current without tripping. A typical 4A circuit breaker with a slow trip profile can tolerate a momentary 10 A current surge without nuisance tripping.

There are various terms used to describe the performance of circuit breakers. Terms like “ignition protection”, “water-proof”, and “dust-proof” are used to describe circuit breakers. However, these terms leave room for ambiguity unless specific industry standards are specified in the component documentation. You should always compare the specified standards for your circuit breaker with your intended application when selecting protection for your board. Make sure to include the relevant standards in your bill of materials.

Circuit breakers with plug-in style terminals are easy to install and replace. They can also be soldered, especially circuit breakers with through-hole terminals. Obviously, screw terminal connections are more secure and suited for applications with high current and in environments where vibration may become a problem. Not all screw terminal circuit breakers can be attached to your PCB through mounting holes, and you will need to examine these breakers before attempting to include them in your PCB.

Modular circuit breakers in an electrical cabinet

Heavy duty circuit protection in an industrial system

 

What to Include in Your Bill of Materials

If you’re trying to include surge protection in your PCB, you’ll need to include the important information for this component in your bill of materials. Design software that integrates your component libraries with your design and bill of materials tools ensures that you can always find the right components for your specific application.

Circuit breakers that make an appearance directly on your PCB should be included in your bill of materials that is sent to your PCB fabricator/assembler. Your fabricator needs to know voltage and current that trips your circuit breaker, as well as the important sourcing information for procuring these circuit breakers. Your fabricator should have all the information they need to test your board for safety.

If you are working with a circuit breaker that appears external to your board but is still required for safety, you’ll want to include this circuit breaker in your bill of materials for the assembler that completes your system. They will need the same information so that they can test the system for safety. If you find that your initial assessment was incorrect and you need to include a different circuit breaker, you’ll need to quickly select a different circuit breaker and provide this information to your assembler

Your PCB design software has a role to play in ensuring you pick the right circuit breaker for your board and helps you generate the information your fabricator/assembler require. Your design software should integrate directly with your component libraries and provide sourcing information for your components. You’ll be able to avoid time-consuming redesigns that can arise from including unavailable components in your PCB.

Unifying your schematic, layout, component data, and deliverable generation tools in a single interface gives you the power to build the best PCBs and easily bring them to production. The information you need for your bill of materials and other deliverables appears in a single program, and you can remain confident that your manufacturer can take your PCB to production successfully.

Altium gives you access to extensive component libraries and the best design tools for your next PCB. It also contains tools that take your design data and generate deliverables for your manufacturer. The tool, component libraries, and other design features in Altium let you create high quality layouts, generate a thorough bill of materials, and manage your components.

To learn more about how Altium can help you select the right components for your and bring your next device to production, talk to an Altium expert today.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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