250V AC configurable current limiter or electronic fuse? - Page 1
Forum section: electronics / beginners

author: johnkeates
author_rank: Contributor
timestamp: March 12, 2022, 06:17:36 pm
content: I'm looking for a way to test SMPS repairs without constantly tripping a breaker or destroying single-use fuses all the time. Sometimes a short or overcurrent is easily spotted and measured, but I've had some really odd cases where it takes multiple repar-blow-reinstall-fuse cycles to get to the bottom of a problem and there has to be a better way to do this.Most of the power supplies are of the laptop variety and they don't pull all that much from the net, around 5A for a 'big' one, but about 2A is generally what they peak at. I've looked at 'normal' breakers that you'd install on a DIN-rail in a fuse board but those are of course intended for normal use and not for "I only want 5A 250V AC" types of scenarios. The goal here is to trip that breaker instead of tripping the breaker for an entire room or blowing the one-time fuse in the power supply during a test.So far, I've found expensive lab power supplies that can go up to 250V and have current limit configurations, but that's really overkill for what I need. I did try to simply do a "fuse in a box" combined with a power strip and that does work, but I'd still have to replace that fuse every time the measurements didn't detect any dynamic fault and it blows again.I've looked at ABB and Eaton since they are big around here but it's all 'normal' stuff that doesn't go as low as 5A and 2A...

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2022, 06:24:08 pm
content: a simple way of doing it it to put a light bulb in series

author: johnkeates
author_rank: Contributor
timestamp: March 12, 2022, 06:28:00 pm
content: Quote from: langwadt on March 12, 2022, 06:24:08 pmThat makes sense. Would that also prevent the in-SMPS fuse from blowing because the bulb (a resistive one, not a LED I presume) limits the current sufficiently?Edit: Or, if I'm applying Ohm's law correctly, if V = IR and I want 250=5*R that means R = 50? Does that also mean I need a 1250W bulb?

author: Terry Bites
author_rank: Super Contributor
timestamp: March 12, 2022, 07:03:23 pm
content: The genius that is glasslinger designed a good oneThere is a video about improving itBetter still

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2022, 07:04:32 pm
content: Quote from: johnkeates on March 12, 2022, 06:28:00 pmyes it needs to be an old school filament bulb, it might take some trial and error to find the right bulbThe reason to use a bulb instead of a resistor is that as long as filament is cold the resistance is very low, it's only when the current gets high enough to heat the filament the resistance increases limiting the current

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2022, 08:16:33 pm
content: It's something I've thought about making, but it's not easy because of the extreme power levels encountered, that is, relative to semiconductor devices, but really not all that much in general -- I mean it's just a 120V outlet or whatever, how bad could it be, right?Well, to get reliable switching, you first need input transient protection so you know it's not going to blow up on 1.5kV+ surges. Infrequent maybe, but nothing a small stack of MOSFETs will appreciate, and once they go, your load gets full power. That's not great. Or you massively crank up the silicon to handle such voltages... no thanks.Then you need enough switching capacity to handle the load. Will you actively limit current? If so, then for how long? It needs to be able to charge capacitors or whatever (inrush). It shouldn't stay on forever, dumping current into short circuits or arcs. If not limiting current, then how long can it stay on for, and what current should it break at? Mind that inrush is often over 100A, and residential mains outlets have a typical fault capacity of 2kA or so; there's no way you handle the full fault capacity, not without an actual bucket of silicon.At least there are better types -- SiC MOSFETs are available that can handle full surge voltage, and have higher power density besides. But they're still fairly pricey -- well worth for the capacity and speed boost, but not like pound-for-pound, and you will need a lot of them to get this kind of capacity.So that's basically why semiconductor breakers don't exist, at least not yet. There's research papers discussing this sort of thing, but it'll be a while before it's a viable product, let alone a drop-in breaker for your electrical panel, say.And there's the matter of dropping vs. switching. If it should limit current, then two issues arise:1. Limited current will prevent fuses from opening, as quickly as intended, or entirely.2. What to do with the power loss (dropped voltage * limiting current)?If using a bare transistor as a current sink or electronic load, all that power flows into the die, and there's not actually all that much silicon in a transistor. You need a hell of a lot of die area to absorb mains power (several kW) for any kind of time (several kW, for maybe a few cycles -- 33ms+?).The power doesn't need to be spent in the transistor, it can be switched into something else. Basically, make a buck-boost SMPS with a dump load on the side. The dump can be TVSs, MOVs, resistors, whatever. Obviously, those in turn need to be able to handle the power as well; TVS are more robust than MOSFETs, but not much cheaper per watt; MOVs handle more energy but aren't rated for continuous dissipation; and resistors can be arbitrarily large for not too much expense, but the clamping voltage is proportional to current so they may be less flexible (say if you want an adjustable current limit).So, I mean I'd love to make the product -- it's not going to be cheap though, and when the alternative is as dumb easy as a light bulb, and only slightly less convenient to use -- would there really be a market for it?Tim

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2022, 08:49:40 pm
content: Quote from: T3sl4co1l on March 12, 2022, 08:16:33 pmwouldn't avalanche rated FETs be ok with surges?Don't remember all the details but long time ago we did an electronic fuse on a board with bunch of triacs for the actual loads.The triacs couldn't handle a half cycle into a short so just turning off the triacs wouldn't be enough, but the "fuse" which wasback-to-back mosfets controlled by an mcu measuring the current could turn off "instantly"

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2022, 11:27:18 pm
content: Quote from: langwadt on March 12, 2022, 08:49:40 pmSame problem but worse -- the power dissipation capacity is very limited compared to how much you need, plus avalanche damages them (hot carrier injection into gate oxide, something like that).QuoteHah, neat.Hmm wait, why TRIACs at all, if the MOSFETs have enough switching capacity? Or, if they're being used to short out the TRIACs to get them to turn off, same thing but worse (can't turn off if Rds(on) * Ipk > V_TM). Not sure.Well, maybe that's arranged other than I think, or maybe it wasn't...all that well designed after all?, but... There's another common circuit like that, which is: current sense, into latch, into just a (mechanical) relay. This has slow response (~10ms) so still allows fault current through, but only maybe a cycle worth, and by adding a modest amount of series resistance, that can be limited to reasonable values, avoiding a waste of fuses, say.Which, compared to the kind of stuff I described earlier -- while the above is not fancy by any stretch, it's interesting that it's a quite useful and practical circuit, given how easy it is to build, and how much extra work the other kind is.Tim

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2022, 11:55:31 pm
content: Quote from: T3sl4co1l on March 12, 2022, 11:27:18 pmhow much energy are we talking about?Quote from: T3sl4co1l on March 12, 2022, 11:27:18 pmif generally true then the would be no point in avalanche ratings ...Quote from: T3sl4co1l on March 12, 2022, 11:27:18 pma single set of back-to-back FETs controlling the power before the triacs, just like a fuse but much fasterwithout it all the triacs would have to be big enough to handle a mains cycle into a short

author: bdunham7
author_rank: Super Contributor
timestamp: March 13, 2022, 12:37:29 am
content: Quote from: T3sl4co1l on March 12, 2022, 08:16:33 pmIMO, the trick is to reduce that fault current since it doesn't need to be that high. A 5% full load voltage drop (generally deemed acceptable) on a 120VAC 15A circuit implies a 0.4R source impedance and a 300A fault current. And if you were testing devices that had lower power requirements, you could have an even higher source impedance. For a current limiting device for DUTs up to 4 or 5A, you could start with a 1R power resistor in series. After that, the simplest implementation might just be a triac that can take 120A for a cycle or two. Or even an appropriate relay and a slow-blow fuse backing you up in case things go sideways.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2022, 12:46:09 am
content: Quote from: langwadt on March 12, 2022, 11:55:31 pmWell, what's fault current? Say 160V peak at a modest 50A limit is 8kW, and for just one cycle that's 133J square pulse, or 85J sine pulse. Transient thermal impedance will get you a small advantage here (maybe 2x, for a single cycle duration, for a typical TO-247 part?), but mainly you just need a lot of 'em (like 20x). Which will get you quite a low Rds(on) in normal operation at least, but yeah, what a pain!QuoteNot completely; it's alright once in a while, maybe for load switches for example. It's definitely not something you can do in repetitive operation, not to much level anyway -- some parts do give repetitive ratings and they're quite small (low mJ). And I don't know how many cycles that's for (1e6?).Another thing, note that protected switches never rely on it. Although they probably can't anyway because of tight integration of the control circuit (avalanche --> free charge carriers --> control circuit goes loopy), so that might not be so much for reliability after all...Tim

author: bdunham7
author_rank: Super Contributor
timestamp: March 13, 2022, 12:51:04 am
content: Quote from: johnkeates on March 12, 2022, 06:17:36 pmI'd suggest a small panel-mount breaker in series with an MDL series fuse of a slightly higher capacity. Perhaps a 4A breaker and a MDL 5A fuse would be a good combo.https://www.mouser.com/datasheet/2/358/typ_T9_311-1275876.pdfhttps://www.mouser.com/datasheet/2/87/eaton-mdl-time-delay-glass-tube-fuses-data-sheet-1608815.pdf

author: langwadt
author_rank: Super Contributor
timestamp: March 13, 2022, 12:55:34 am
content: Quote from: T3sl4co1l on March 13, 2022, 12:46:09 ambut why would you need to handle fault current for a cycle? you can turn off a fet much faster than that

author: themadhippy
author_rank: Super Contributor
timestamp: March 13, 2022, 01:02:46 am
content: How about a motor overload relayQuote2A mcbs certainly exist, for examplehttps://uk.rs-online.com/web/p/mcbs/5354100

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2022, 02:17:57 am
content: Quote from: langwadt on March 13, 2022, 12:55:34 amI said in a current limiting situation...Tim

author: jonpaul
author_rank: Super Contributor
timestamp: March 13, 2022, 03:11:09 am
content: Variac

author: geggi1
author_rank: Frequent Contributor
timestamp: March 13, 2022, 09:44:39 am
content: Look into ajustable motor protective devices like the scneider electric GV2 type.You can get these with a premade enclosure.If you read the manual there should be s description on to run it with two phases.

author: johnkeates
author_rank: Contributor
timestamp: March 13, 2022, 06:30:14 pm
content: Thanks for the replies everyone!Quote from: bdunham7 on March 13, 2022, 12:51:04 amQuote from: langwadt on March 12, 2022, 07:04:32 pmQuote from: themadhippy on March 13, 2022, 01:02:46 amQuote from: geggi1 on March 13, 2022, 09:44:39 amLooking at the simplest options, a Wolfram (or Tungsten?) filament lamp, panel-mount breaker or MCB seem to be the easiest to start with. I was looking at the vendor websites for MCBs as that was my first search, but I guess I had the wrong series because I could never find ones that go low enough (always 16A minimum), but knowing that they at least exist make it a whole lot easier to go look for them. They don't seem to be available locally anywhere, but I suppose it could be considered a specialty item.Say I have a T5A 250V PCB-mount fuse, and I know that it should never pull 5A from the net (unless something is shorted/wrong), I could just get a 4A breaker (panel mount) or MCB (on a DIN-rail in a box) and as long as it trips fast enough I can debug a circuit without killing it and without a more complex or more expensive protection setup, correct? (I tried finding a classic lamp but it doesn't seem to be available in most consumer shops and the industrial suppliers are closed on the weekend here)

author: nightfire
author_rank: Frequent Contributor
timestamp: March 13, 2022, 07:19:33 pm
content: IN my last job, I was looking for some small DIN-mount circuit breakers to protect some power measuring devices in an industrial environment.Long story short, those can be had for as low as 2A rated current with standard characteristic B and C.If you want to have some faster tripping curves, there is the Z-Characteristic that has a lower tripping point than B.Example on short glance:ABB 2CDS251001R0318Series: S201-Z3EAN: 4016779530361But keep in mind that depending on the current the breaker of the wall outlet might also be affected, and when the current is high enough, both breakers will ultimately trip...

author: johnkeates
author_rank: Contributor
timestamp: March 14, 2022, 03:46:38 pm
content: Quote from: nightfire on March 13, 2022, 07:19:33 pmThe wall outlets are on 16A B's so that should probably be a big enough gap that a ~3A Z or B trips. I had a few cases where the PCB-mount fuse blew, and both the circuit for the wall tripped and the main breaker for the entire phase tripped as well. In such a case a Z might trip fast enough to prevent the rest from tripping as well.I'm thinking about a small enclosure with a C14, a DIN-rail and a schuko socket + breaker, perhaps multiple breakers and then a switch to select which limit you want. I do know that technically putting breakers behind breakers has some electrical code limitations like they need to always be going to lower ratings in the downstream direction, and the wiring has to be twice what the breaker normally protects etc. but in this case as long as the wiring is the same solid copper and the same diameter as what we have in the wall, everything after the socket is just a luxury imo.

author: bdunham7
author_rank: Super Contributor
timestamp: March 14, 2022, 07:15:45 pm
content: Quote from: johnkeates on March 14, 2022, 03:46:38 pmTo prevent your wall or panel breaker from tripping first, instead of trying to use a fast breaker to beat them to the punch you might concentrate on reducing the fault current below the instantaneous trip current (magnetic threshold) of your system breakers. The problem with the former approach is that allowing peak current to exceed the magnetic threshold for even a very small time--perhaps less than 1ms--may trigger the trip in the larger breaker even if it takes longer--16-20ms--to actually finish clearing. This is how you end up tripping multiple breakers. If you take the latter approach, then the larger breaker will only respond thermally and as long as the trip is not completed before the small breaker clears, it will stay closed and cool off.A typical HACR-rated breaker might have a magnetic trip of 10X the nominal rating. The common (in the US) Square D QO 20A model has an instantaneous trip range of 6.5-11X nominal, but I think limiting fault current to 8X would make a trip very rare. In your case I'd suggest 2 1R power resistors in series, which would limit your fault current to less than 120A but only result in a 8-volt or 3.3% voltage drop at a 4 amp load. Your wall and panel breakers might differ, but you may be able to find a datasheet and chart like this for them: