Optimum arrangement for circuit protection components? - Page 1
Forum section: electronics / beginners

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 21, 2016, 09:32:55 pm
content: I have some components to protect the power supply on a circuit of mine: a PTC fuse for over-current, a polarity-protection diode, and a TVS diode for transients. I'm not sure what the optimum arrangement of these components would be.At the moment, I have them like this (F1, D1, D2 in the diagram):The fuse is first in series, then I have the TVS in parallel, then the 1N4007 in series.But, looking at the available selection of fuses and their voltage ratings, the thought occurred that a different arrangement might be better. Maybe the TVS first, then the 1N4007, then the fuse? Or 1N4007 > TVS > fuse? Or...Ithinkit might best to have 1N4007 > TVS > fuse. The 1N4007 shouldn't be bothered by over-voltage, as it's rated for 1000V; the TVS, being unidirectional, should be behind the diode, and will clamp to ~40V, so if the fuse is rated for 60V, that's fine; and, even though the 1N4007 is rated for 1A, the fuse will ensure that in normal operation, it won't see more than 200mA.Thoughts, anyone?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2016, 10:30:02 pm
content: You want the fuse first, but don't expect it to clear a fault. Fuses open after a LONG time, and lots of amps/charge/power.You also want the reverse diode first, so that reverse connection results in zero current, rather than a blown fuse. Note that the TVS conducts (Vf ~ 0.7V) in the reverse direction! (Unless you use a -CA model, which is bidirectional -- but that should be shown with a different symbol, because that one letter makes a lot of difference!)Remember that fuses prevent fire hazards. The likely failure condition would be: surge toasts TVS; TVS fails shorted; DC power flows into fault condition; fuse opens. If the DC source doesn't deliver fault current (as is the case from a current-limited PSU), the fuse will never open, and becomes redundant.The type of fuse is important, too. Self-resetting (polymer, PTC) fuses have a low on/off ratio (i.e., they have relatively high ESR when "on", and continue to dissipate fairly high power when "off"), and take quite long to operate (~100ms). Fast-blow (conventional, one-time) fuses can go in ~ms, but this is still too long to protect all but the most robust semiconductors. In other words, if you want to have the TVS blow the fuse (rather than the other way around), you need to do some work, making sure it can handle that much power. Often, a crowbar circuit is used, which has a lower voltage drop and can therefore deliver more fault current for the same power rating.Fusing at 12V is relatively easy; standard SCRs can be used to crowbar automotive circuits (~1kA worst case fault current).Fusing residential mains is a bit more tricky, as the fault current is similar but the voltage is 10-20 times higher (120/240V). Fusing industrial mains can get quite messy indeed, with lively voltages (400-480VAC) and impressive fault current capacities (100kA).Tim

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 22, 2016, 02:34:10 pm
content: Quote from: T3sl4co1l on July 21, 2016, 10:30:02 pmHmm, yeah, now I read the datasheets, it's longer than I assumed - in the order of seconds. For example, a Bourns MF-R020 I was looking at is just over 2s @ 1A, and approx. 10s @ 400mA.Quote from: T3sl4co1l on July 21, 2016, 10:30:02 pmDoh! Of course, in my current arrangement the TVS will conduct with power hooked up in reverse polarity, blowing the fuse!I knew I had it wrong.So, the arrangement I want is: 1N4007 > fuse > TVS?Quote from: T3sl4co1l on July 21, 2016, 10:30:02 pmSo, what you're saying is that, in an over-voltage situation where the TVS starts conducting, because a PTC fuse will take some time to trip, the TVS will be subject to a relatively long surge, which it may not be able to handle, and thus could actually be the first thing to blow. On the other hand, if the TVS is beefy enough to weather the storm, the fuse will trip first.I'm not familiar with crowbar circuits, but it looks like it's more complex and uses more components, so I probably wouldn't have room for that on the board. I'd like to keep things cheap and simple.Here's a couple more questions:Is there any point me using an LM2931 now I have a discrete reverse-polarity diode and the TVS? I initially chose it because of its built-in reverse-polarity and load dump protection, but I guess this is redundant now I have external counter-measures (because I want some 'safe' 12V as well, not just for 5V). Would I now be just as well-off using a cheap LM7805/LM78L05?Why do radial through-hole PTC fuse packages have such a large lead stand-off height from the board? Compared to, say a similar-sized ceramic capacitor. It's going to be like a skyscraper compared to everything else on my board, and might have to fold it over. I guess the manufacturers want you to space it away from the board that far, as that's why they put crimps in the leads at an appropriate point - but I can't imagine why.

author: tszaboo
author_rank: Super Contributor
timestamp: July 22, 2016, 02:40:59 pm
content: Quote from: HwAoRrDk on July 22, 2016, 02:34:10 pmOn the other hand, if you have an ESD event, the diode might be gone when the TVS starts clamping the voltage.I've also seem huge diodes blown up when reverse polarized. It was a badly designed crowbar. Not pretty.I would use bidirectional TVS and put the diode after the fuse and the TVS.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2016, 11:55:42 pm
content: Quote from: HwAoRrDk on July 22, 2016, 02:34:10 pmYup! Meanwhile, the TVS is burning, say, 26V at 400mA+ (>10W) for those seconds.TVSs usually fail shorted (until you reach their own, ahem, fusing current...), so the TVS can still manage to protect the circuit, sacrificing itself in the process. The fuse later clears the short, preventing a fire hazard.It's a good system, but for almost everything, it's a "your circuit board is toast anyway" situation. Sure, you can repair it, but it's not usually worth doing.QuoteYes. Or...Quote from: NANDBlog on July 22, 2016, 02:40:59 pmEither way has valid reasons!If you're expecting a suitably dangerous amount of negative transient/surge, the series diode can get toasted. It does allow a higher reverse rating, though -- if youreallyneeded to handle a big reverse surge, the nice thing is, you can use a 1N4007 (which hardly drops any more Vf) to get a 1kV reverse rating, then put on basically any value MOV (say, 50V to 300V), and have that reverse surge handled better than anything.You can employ a similar strategy in the forward direction, but you have the added difficulty of limiting the circuit's load voltage (which the LDO does to a point, but if you need considerably more than 30V, well...). If you can get a huge compliance range, though, you can do away with pricey TVSs and use a big dumb MOV there -- and gulp down mondo surges like it's nothing!Probably not something you'd do for most any automotive application, but it would be an excellent approach for high-reliability and mission-critical (aerospace, military) gear.QuoteWell, an LDO will get you a lower minimum input voltage -- maybe good for low-battery or cranking conditions. If you don't need that, a regular 7805 isn't bad.QuoteRemember how they have to heat up to "open"?SMT polyfuses are dubious components. Current rating depends on how they're wired up -- you want to avoid connecting to them with heavy copper pours, because you can easily double the trip current by dissipating heat. (And that's on top of the already-poor tolerance on the trip current, which is all the more reason to remember -- they're for fire protection, not protecting the attached device!)You also want to avoid placing them near super-hot components (resistors, heatsinks?), for the same reason -- if the trip point is when the active PTC-stuff reaches ~150C, that means your trip current also goes to zero, as ambient goes to 150C! If you need an 85C maximum ambient, you need twice the (trip at 25C ambient) rating just to keep the thing running normally!They're also rather unreliable, so don't depend on them for anything critical. It's a CYA component -- as a fuse should only ever be.If you need limiting, put in a current-limiting or protecting circuit -- and expect to pay for it as well (reliability isn't free!).Tim

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 23, 2016, 06:42:38 pm
content: Quote from: T3sl4co1l on July 22, 2016, 11:55:42 pmI think I could probably live with a 7V minimum input, as well as a lower 30V max input if I specify a lower-clamping TVS (you may have noticed I already did that in the revised circuit diagram I posted earlier - I realised 41V clamping voltage on a P6KE30A was higher than the 40V max on the LM2931!). The pin-out of an L78L05 is the same as an LM2931 anyway, so I can always experiment with both and see.Although, annoyingly, their TO-92 packages are opposite orientations.Quote from: T3sl4co1l on July 22, 2016, 11:55:42 pmAh, I see, that makes sense. To isolate them as much as possible from external heat sources, to avoid biasing the tripping point. There'll be no copper pour involved in it's connections, and the only adjacent components will be the 1N4007 and TVS, so I'm probably good with regard to external heat sinking/sourcing.Thanks for all the help!