The Best Protection for your Circuit is NOT a Fuse!.....but a Resettable Fuse? EB#54

Sometimes when building circuits mistakes can happen which can be a reverse voltage, a voltage applied to the wrong pin, components failing due to their age or wrong usage or you simply accidentally short something. No matter the cause, the now appearing overcurrent is the root of all evil that needs to get stopped so that nothing explodes or burns down completely. To do that I think everyone is familiar with such small glass fuses, that you just put in series to your circuit and as soon as too much current is flowing the thin metal inside of it melts and thus interrupts the current flow. Only problem is that replacing such fuses can be cumbersome, it can get expensive if you are testing stuff and constantly destroy them and in case of enclosed electronics like your smartphone, there is simply no way to replace such a fuse. The solution to all these problems though are Resettable Fuses like these ones which I often feel like not many people know about. So in this video I will show you exactly how you can use them and with what limitations they come with because even though they are resettable and thus reusable, they are certainly not perfect. Let's get started! This video is sponsored by Mouser Electronics. Now truth be told I have been searching for fitting resettable fuses for a while on the internet and Mouser was the only seller that not only came with a big selection of them, but most of them were also in stock, I could easily filter for preferences and best of all they sell to private people like you and me and not just companies. I have been ordering my components from them since 2018 and I can tell you for sure that they have all the components you could ever need and you usually receive them within 3 or sometimes even only 2 days. So feel free to check out Mouser Electronics if I piqued your interest. Now first off; If you want to use resettable fuses then you always have the choice between the through hole ones; that I feel like are easier to use; and small SMD ones, which you probably already have seen on devices like the Raspberry Pi. Sometimes resettable fuses are also known as a Multifuse, Polyfuse or Polyswitch but if we want to be technically correct then they are actually called PPTC which stands for polymeric positive temperature coefficient device and............wait a minute PTCs are actually quite popular. What they do and what their name basically implies is that they increase their resistance when they get hotter. You can actually find their relative, the NTC aka negative temperature coefficient, on lots of 3D printers because they decrease their resistance when their temperature rises and thus they both can be used to measure temperature. But back to the resettable fuse which does in fact increase its resistance when temperature rises but it still features lots of differences to normal PTCs or NTCs. So as a practical example let's say I got a circuit that normally draws around 500mA at 12V like this RGB LED strip right here and I want to protect it from shorts so that the fitting power supply does not get damaged. Now to choose a suitable resettable fuse we only have to worry about 4 crucial properties and those are the maximum voltage and current as well as the hold current and trip current. And I think the maximum voltage and current is pretty self explanatory, meaning that if you exceed them you basically risk damaging your fuse. The hold and trip current is a tiny bit more complicated, but still pretty easy to understand. The hold current basically stands for the current that can flow continuously without having to worry about that the fuse will ever trip. And the trip current of course is the current that needs to flow so that the fuse will activate and thus interrupt the overcurrent, pretty simple. So with these 4 rough values in mind for my circuit, I went onto a search on Mouser Electronics and decided on this PPTC that can do 60V and 40A max and comes with a hold current of 0.5A and a trip current of 1A. And if you are asking yourself right now: “Why not use a fuse with slightly higher ratings for a bit of wiggle room?” then just wait a few more minutes for a little revelation. Because first off; it was time for a small experiment in which I simulated my “to be protected” circuit with a constant load to which I of course added my resettable fuse in series and powered it all with 12V. Now while slowly increasing the current flow through my fuse, I also directly measured and wrote down the voltage drop across it because those electrical values give us insight about the power the fuse wastes which directly determines its temperature and also what resistance it currently possess. So first big stop was the hold current of 500mA at which point only a small voltage drop across the fuse occurred that at some point also did not increase anymore meaning the temperature of the fuse was stable and thus it would never trip at this point. Next was the trip current of 1A and here the voltage drop did in fact increase over time until the fuse ultimately reached it high resistance state in which my constant load could no longer draw 1A and thus turned off. Now you might be thinking that these 26s it took for the fuse to activate are way too long but as a comparison let's see how a 1A rated glass fuse would react to 1A of current. And yes we could sit here forever because the glass fuse would never trip and even at 2A it still took way longer to activate once again in direct comparison to the resettable fuse. So yeah PPTCs are a bit slow; but traditional fuses are even slower. But getting back to topic because next; to truly investigate the trip occurrence I added a big power resistor as a load that would normally draw around 1.2A. And by the way these three components I added in parallel to the fuse are just a trip indicator which lights up as soon as the fuse enters its high resistance state. OK; so now with the resettable fuse in place the big current the resistor normally draws got quickly limited to only around 103mA. But wait a minute? Shouldn't a fuse cut down the flowing current to nothing like a normal glass fuse would do? Well; such PPTCs can not do that because they are basically just resistors whose resistance increases drastically at the trip current because of the rising temperature. That does not mean they are useless though because such a drastic current reduction can definitely prevent exploding capacitors, smoking transistors or even save battery packs when things go accidentally wrong. And speaking of battery packs; PPTCs also often gets used with them since they trip when they get too hot and battery packs definitely love to get hot when they are not handled correctly. But getting back to the trip event at which point a so called leakage current keeps flowing that basically keeps the fuse hot enough so that it stays in its high resistance state. I also did some additional measurements in this state to find out that no matter what input voltage gets applied, the resettable fuse basically creates a power loss of around 1.2W which is quite a bit more than what the datasheet claims. And speaking of datasheet; this is a good time to try out a PPTC with slightly higher ratings like this one that holds at 0.9A and triggers at 1.8A. I pretty much did the same tests and measurements as before and the differences are that the “bigger” fuse features less power losses during normal operation but waste more power when it got tripped. So the choice is up to you what is more important for you. Now the last thing we have to take a look at is the area between the hold and trip current and here the resettable fuse can trigger but does not have to. For example at 700mA the voltage drop and thus temperature only increases very slowly meaning eventually this fuse might trip if the ambient temperature is also high. But at 900mA the voltage and temperature increases way faster and thus the fuse tripped in my case after around 1 minute and 42 seconds. This typical time to trip diagram given by the datasheet can also help you out a lot regarding those questions. And by the way the PPTC also becomes rather quickly conductive again as soon as the current decreases; but after tripping once its resistance will take quite a long time until it will reach its initial value once again. And with that being said you should now be familiar with most of the basics when it comes to resettable fuses and yes; they are definitely a good fit for the LED strip example we started with and they are also a standard when it comes to many other protection tasks. Of course traditional fuses are a bit more efficient meaning they produce less power losses. But then again eFuses, which I presented you in a previous video, pretty much come with the same low power losses as traditional fuses. And even better, they do trip in a way more precise and fast way since they do not rely on a rising temperature that burns a metal or increases a resistance. But then again such an eFuse is a bit more complicated to set up, it is more expensive and also not resettable by itself. They can also only be used for a limited voltage and current range, while the resettable fuses can handle quite a bit more and traditional fuses are beasts in that regard. What I am trying to say here is that each one of these fuse types is tailored to a specific application and I feel like with this video coming to an end you should now have a very good idea when to use which one. I hope you enjoyed it and learned something new. If so consider supporting me through Patreon to keep the show going. As always don't forget to like, share, subscribe and hit the notification bell. Stay creative and I will see you next time.

Channel: GreatScott!

Views: 1,006,218

Keywords: fuse, resettable, pptc, efuse, glass, traditional, protect, protection, over, current, circuit, electronic, electronics, basic, basics, greatscott, greatscott!, tutorial, guide, beginner, make, project, diy, do, it, yourself, learn, ptc, ntc, positive, temperature, coefficient, device, negative, metal, resistor, resistance, maximum, voltage, explain, how, to, use, trip, hold, time, datasheet, diagram, power, loss, comparison, price, cheap, expensive, fast, precise, example, short, safety, safe, reverse, mistake, error, prevent

Id: sF0KOVWj9p8

Length: 12min 11sec (731 seconds)

Published: Sun Feb 26 2023