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How to Protect Against One of Your Biggest Enemies in Circuit Design

This brief tech blog is one of three that will be appearing on Planet Analog leading up to our “Ask the experts” chat session on Wednesday, September 24, 2014 at 10:00 a.m. PT (1:00 p.m. ET).

If you want to be a successful design engineer, you need to know your enemy. Who is one of the biggest enemies of your circuit design success? Over-current conditions.

Over-current conditions result when the current in the circuit exceeds the rated amperage capacity of the circuit or of the equipment connected to the circuit. Whether you’re designing automotive, LED lighting, or consumer electronics, over-current conditions are a threat to the reliability and durability of the technology you’re developing. When are these situations most likely to occur? When there’s a loose connection, ground fault, short circuit or surge power draw.

Now that you know your enemy, how do you protect your design against it? Over-current circuit protection can be accomplished with the use of either a traditional fuse or positive temperature coefficient (PTC) device.

Fuses
Fuses are current-sensitive devices purposely designed to serve as the weak link in an electrical circuit. They provide reliable protection for discrete components or complete circuits by melting under current overload conditions. PTCs
PTC devices are passive electronic components used in a wide variety of telecom, computer, consumer electronics, battery, and medical electronics product applications. In these particular applications, over-current events are common and automatic resettability is desired.

Fuse vs. PTC selection
Fuses and PTCs offer their own unique operating characteristics and benefits. Understanding the differences between the two technologies will simplify the selection process — depending on the application.

The most obvious difference is that PTCs are automatically resettable whereas traditional fuses need to be replaced after they are tripped. After most over-current events, a fuse will completely stop the flow of current — a result that may be very beneficial in critical applications. In contrast, PTCs significantly increase in resistance thereby creating the over-current protection, but still allow a trickle current to flow.

Because they reset automatically, many circuit designers choose PTCs in instances where over-current events are expected to occur by user created faults. Also, PTCs are useful when trying to maintain low warranty and service costs, constant system uptime and user transparency. They are often selected for use in circuits that are difficult to access or in remote locations, were fuse replacement would be challenging.

There are several other operating characteristics that should be considered when choosing between a PTC device and a fuse. For optimal safety and reliability, we recommend that you test and verify device performance before use within the end application.

Do you want to learn more about the importance of circuit protection (CP)? Have a question or two about choosing the right CP solutions that combat against over-current conditions in your design?

Join us for the Planet Analog Chat Session on Circuit Protection, September 24, at 1:00 p.m. ET (10:00 a.m. PT).

You can leave some early questions and observations on the chat board, which is now live. You will need to register on Planet Analog in order to ask questions and comment on the chat session.

Sign up now by clicking here to join or to leave an early message. Just click “Reply” or “Post Message” when you get to the site.

16 comments on “How to Protect Against One of Your Biggest Enemies in Circuit Design

  1. lucaethan1984
    September 19, 2014

    goo.com

  2. eafpres
    September 20, 2014

    Hi Timothy–I also noted this to Tim; I am very interested in PTCs or other auto-reseetting circuit protection.  An area for which I'm especially interested is the use of M2M communications to monitor remote sensors where such sensors are integrated “smart sensors” capabile of not only detecting an input, but automatically transmitting it.  By definition these remote nodes are fairly expnsive to go and do repair or replacement. In many cases these nodes may include energy harvesting to extend the field life to 10 years or more with no maintenance.  In such cases, protection that resets is the only way to go.

  3. Tim Patel
    September 20, 2014

    Absolutely — with smart sensors becoming more and more prevalent in electronics today, it's important to incorporate circuit protection that not only protects sensitive electronics, but also preserves functionality of the system.  Resettable protection is ideal for this case.

  4. Netcrawl
    September 21, 2014

    @easpres in M2M a remote sensor gathers data and send it wirelessly to a network, at that point the data is analyzed and and acted upon. Making a M2M communication work is a step-by-step process, the main elements involved are sensors, wireless network and a PC connected to the internet. 

  5. Netcrawl
    September 21, 2014

    @Tim I agree with you, without proper protection, those electronic system can fail without warning. Designing electronics for sensor technology is very challenging, wide temperatures wings must be anticipated, here humidity can range from desert lows and swamp highs. These are serious things, it could affect everything- from circuitry, electrical connections and insulation.

  6. Netcrawl
    September 21, 2014

    @easpres PTC devices act as automatically resetting fuses in manyy applications, at normal operatingg currents, a PTC has a low resistance value and when fault currents are present the PTC devices warm up increasing its resistance.

  7. eafpres
    September 21, 2014

    @Netcrawl–it is interesting to think about these nodes being smart, in the sense that they can do some data analysis and send less data out but more information.  Once you have that capability, the node can send out a health message every so often, and report on status of energy storage (batteries, caps), power sources (energy harvesting), and out of range events (over-current, noise spike, temperature out of range, etc.)  This then can help the users of the nodes predict which ones need maintenance.

    If you see a particular node gettig a PTC event a lot, someone can go check to see what is wrong before it completely fails.

  8. eafpres
    September 21, 2014

    @Tim–following on from my note to netcrawl, is there diagnositic information available from any integrated protection devices, or would the designer need to add in to the design a way to see when the PTC was needed? As noted, in a remote sensing case, it would be useful to see of a particular node was having recurring events causing the PTC to limit current.

  9. samicksha
    September 21, 2014

    I believe we can also try by alloying certain metals here.

  10. Tim Patel
    September 21, 2014

    @eafpres1 — Typically, a circuit designer would need to monitor the voltage drop across a PTC device to determine if that particular node was experiencing recurring fault events.  For multi-node systems, a PTC can be placed in series with each circuit to monitor the individual circuits.  However, each line protected by a PTC would exhibit a certain voltage drop across the PTC during normal operating conditions, so that must be accounted for.

  11. Tim Patel
    September 21, 2014

    @Netcrawl — Yes, you bring up a very valid point regarding the wide operating temperature range for certain applications.  Since PTCs are temperature-sensitive devices, one must carefully consider the operating temperature range of the end application.  This not only has an effect on the performance of the circuit protection components, but also can have significant effects on insulation.

  12. Tim Patel
    September 21, 2014

    @samicksha — Yes, some fuses do use metal alloy wire elements to operate within specific opening time limits under different current overload conditions.

  13. Myled
    September 23, 2014

    “If you want to be a successful design engineer, you need to know your enemy. Who is one of the biggest enemies of your circuit design success? Over-current conditions.”

    Timothy, that's true about over current. Now a days we used to make necessary arrangements for protecting the circuit from over current flow. 

  14. Myled
    September 23, 2014

    ” Yes, some fuses do use metal alloy wire elements to operate within specific opening time limits under different current overload conditions.”

    Tim, burning the fuse is the simple and easiest way of protecting the board or circuit from over current flow.

  15. geek
    September 27, 2014

    “burning the fuse is the simple and easiest way of protecting the board or circuit from over current flow.”

    @myanalog: That may be simple and easy but once the fuse blows away you can't reset it back immidiately. It takes time to bring the circuit to its original condition. This is why an alternate method may be required which can fix the circuit and also ensure that the performance isn't impacted for long.

  16. Myled
    September 28, 2014

    “That may be simple and easy but once the fuse blows away you can't reset it back immidiately. It takes time to bring the circuit to its original condition. This is why an alternate method may be required which can fix the circuit and also ensure that the performance isn't impacted for long.”

    Tzubair, you can keep the fuse outside the circuit board, so that it can be replaced without much effort.

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