Need help selecting PPTC fuse - Page 1
Forum section: electronics / projects

author: elepo
author_rank: Contributor
timestamp: February 03, 2022, 08:12:35 pm
content: Hi.I have a circuit that controls 2-3 doors using solenoid locks. The doors are usually 5-10 meters away from the control board.I've realized that the way these locks are installed and wired is quite careless and short circuits areverypossible. So I've decided to add a PPTC fuse on the board for each lock.What gives me a difficult time choosing a fuse is the fact that the current consumption of the locks is not constant. Once activated they initially draw ~1.2A, and after a few seconds the current draw drops to a constant <200mA.Another thing is that due to the long wiring, and because they may (and most likely will) choose weak wires, a short circuit might actuallynotdraw that much current. The circuit voltage is 12v and it's quite possible that a short circuit in a 20 meter wiring would draw as low as 2.5 Amps. So I need the trip current to be as low as possible.Now I'm thinking about using 750mA fuses. They can pass 1.5A for a few seconds without tripping. And anything above 2A for more than a few seconds will trip them.But I'm not sure if the 750mA fuse will get damaged or not. Because these locks are going to be activated maybe more than a hundred times a day. Which means the 750mA fuse will have to pass 1.2A more than a hundred times a day, each time for a few seconds.Will this degrade the fuse over time?Should I choose something bigger like 900mA?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2022, 08:54:23 pm
content: Wrong component for the job -- fuses are there to protect the wiring, and have pretty gross ratings. Note also that PTCs take quite long to clear: 10-100ms even at rated fault current. They can be used to protect circuitry at low voltages (where transistors with high current ratings are affordable, and fuse or wiring resistance dominates), but aren't good for much more than regular wiring protection beyond that (say over 12V and some amperes).Is this AC, by the way? Either there's some kind of controller and DC operation, or it's AC and the lower current comes once the armature settles in place. Hmm, that shouldn't take some seconds though. Or maybe it's AC but controlled anyway, that'd be neat.If you can't control the size of the wiring (or the neatness, so as to avoid its being cross-wired or shorted in the first place, heh), there's really not much you can do. A 2.5A load sounds perfectly reasonable for the load, well, maybe somewhere in the 1.2-2.5A range would be an acceptable threshold, but yeah, that's much too narrow for a fuse to decide.If it's a controlled current (which it sounds like is the case), choosing appropriate control and rating would seem the best option. An electronic solenoid driver can measure load current, and perform active limiting or faulting as the case may be. Depending on how much you're willing to spend for the controller (and, if you can find any in stock anywhere right now..), versus do yourself (e.g. on a microcontroller, with suitable algorithm to handle it -- and assuming software doesn't crash and it gets stuck on or off, as the case may be..!).Can also program for open and short-circuit detection; open is simply the output drops to nil under some small bias current, and short can be measuring the small voltage drop (from the same small bias current) and comparing it to acceptable DCR, basically using the driver as an ohmmeter with a threshold to say whether it's just wiring (don't turn on) or looks reasonably solenoid-y (good to go). Some driver chips have this functionality too, or at least open detection; not sure about short detection.Mind that some EMC protection is probably a good idea too; give consideration to ESD (are these wires routed near anywhere people might be walking, zapping things, etc.?), maybe surge (probably only if the wires become shorted to GND, or other nearby wiring; should be okay with an isolated supply, and isolated comms if applicable), and maybe cross-wiring to other things (not just +24V/switch/GND getting cross wired to GND or each other, but could 240V H/N get applied to the poor thing too?). There are solutions for all these possibilities -- but the more dramatic ones (like 240V cross-wire tolerance) aren't going to be cheap or compact to implement, so a decision on what's reasonable is probably needed.As for PTC fuses, they tend to degrade with operation, yes. Rmin increases with cycling, and Ihold decreases. Note that Ihold also varies with ambient temp, since it's just temp actuated anyways; it's also sensitive to board layout (heatsinking), less so for THT parts but SMT chips for sure. So if it's getting hot inside the control box, that's yet another contraindication for them. I'm not sure that they degrade over time, under nominal loads without clearing operation; but in any case your margins are much too close to use such a device and it's likely that, on hot days or after some time in use, it goes from never clearing the fault, to inconsistent operation or outright never opening the lock. Definitely an electronic solution is best.Tim

author: nctnico
author_rank: Super Contributor
timestamp: February 03, 2022, 09:04:27 pm
content: My rule of thumb when selecting PTC fuses: use one rated for twice the maximum current the load draws. When the ambient temperature goes up, the threshold current goes down considerably.