Automotive transients. Fuse or PPTC ? - Page 1
Forum section: electronics / beginners

author: Hydrochloride
author_rank: Contributor
timestamp: April 29, 2025, 09:14:28 am
content: Hi everyone,I have a question regarding the design of a product that will be directly connected to the lead-acid or LiFePO4 batteries of a truck. The battery has a nominal voltage of 24V, but due to the alternator, the voltage can remain at a constant 28V.I need to carry out transient tests in compliance with ISO 7637-2 and ISO 16750-2, similar to those shown in the reference web.monolithicpower → From Cold Crank to Load Dump: A Primer on Automotive TransientsI understand which TVS diodes or varistors are typically used to suppress voltage spikes,but I’m unsure how to determine the appropriate overcurrent rating for the fuse.The goal is to select a fuse that will open in the event of an internal short circuit on the board—potentially caused by transient pulses if the protection components fail. But has to remain closed in normal operation.My concern is that using a non-resettable fuse could lead to trips during pulses, leaving the equipment disabled even if there’s no actual product fault. I’m not sure how to properly size the fuse based on current for this kind of application.I’m considering is using a PPTC, like one from Fuzetec.Their technical expert assured me that overvoltage wouldn't be an issue for the PPTC and that it's a viable option. However, from an academic perspective, I’m still curious: how exactly is a fuse sized to withstand these transient pulses? Most resources I find online or in technical literature recommend using a fuse, but they rarely go into detail about how it's dimensioned for this kind of testing. They dimension the TVS or Varistor, but not the fuse.For reference : Load dump parametersUa = 28VUs = 174Vtr = 10mstd = 350msRi can be 2ohms or 8ohms. Depends on the truck alternator. Likely this sets the current, but not sure the approach to get the value to select a non-resettable fuse. You know → triptime, rated current to trip.Adding second refence image.From https:https://www.analog.com/en/resources/technical-articles/loaddump-protection-for-24v-automotive-applications.htmlSo I need to select the non-resettable fuse above the value of that operation ?(Obviously with my load parameters. Not the ones in the image reference)

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2025, 09:47:20 am
content: PPTC resistors take a long time to blow.What's the nominal load current?What's the maximum rating of the load?Is the load a voltage regulator, powering a lower voltage device?The most effective way to do this is to use crowbar (a high current SCR and zener diode) with a fast blow fuse. To avoid nuisance tripping, put a low pass filter on the crowbar and add a TVS diode with a higher clamping voltage, to provide protection protection against short spikes.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 29, 2025, 09:56:26 am
content: Polfuses are one of those things that sound great in theory but have limited use in real life and this gets a *lot* worse if you operate in an environment with wide operating temperatures (which is very much the case for automotive).All depends on your use case, budget etc. but when encountering a similar question a while back I ended up designing in a LM74800.

author: PGPG
author_rank: Frequent Contributor
timestamp: April 29, 2025, 10:33:08 am
content: I have never done something like this so only few thoughts.1. Don't assume that PTC if tripped than after the pulse will then be automatically reset. It is because in such state (high voltage drop at PTC) even small current can be enough to keep it enough hot to not reset. So even you use PTC you will have a way to ensure reset it by reducing the load current close to 0.2. I think I²t is more important parameter than Ipeak to find if during pulse fuse will trip or not.3. From transient tests you should be able to estimate I²t your fuse will get in your circuit.4. May be (never tried) enough big L at supply input can reduce input voltage peak to the level allowing to ignore it in total.5. May be semiconductor device (I don't know them) opening with high voltage and getting to conduct back when voltage drop down can help - for example relative high voltage varistor at input (so getting not a big energy during pulse) protecting that semiconductor 'opener' against overvoltage then such 'opener' and then low voltage transil protecting the rest. Varistor protects 'opener', 'opener' protects transil, transil protects the rest. May be spark arrester in paralel to varistor can protect varistor against to high energy dissipated in it. At the beginning varistor limits the volatage and shortly the spark arrester is ionized enough to switch on protecting varistor.

author: Hydrochloride
author_rank: Contributor
timestamp: April 29, 2025, 10:43:09 am
content: What's the nominal load current?1.5AWhat's the maximum rating of the load?Product will be dimension to sustain 36V DC constant. Even if max input voltage will be 28V.(from truck battery with alternator)Is the load a voltage regulator, powering a lower voltage device?Yes. Its a low voltage device. With its own power/voltage regulation for MCU and such.Whole system power path will be (energy given from left to right)Alternator → 24V battery truck → Service truck internal HW → Truck fuse box → Wires to my power my device.If there is a short in my device and the operator has put a fuse manually, then no issues, the short will be controlled. But if he/she forgets it, then I need to place a protection just in case.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2025, 09:52:36 pm
content: Use a crowbar circuit to blow the fuse, if the voltage gets above, say 32V.https://en.wikipedia.org/wiki/Crowbar_(circuit)And an ordinary TVS which will clamp at a slightly higher voltage.The TVS will be faster than the crowbar, so will protect against short, very high voltage spikes.