Oh goddammit... - Page 1
Forum section: electronics / general

author: c4757p
author_rank: Super Contributor
timestamp: November 05, 2013, 11:19:39 pm
content: Hmm, this regulator's not working quite right. Let's see what's wrong with it.Oh, the current limit's kicking in really early in the cycle. Well brain, what's the stupidest idea you can come up with? (Brain: "Derrrrp.... uh... short out the sense resistor and see what happens?") OK! Here I go!Ow! Shit!(My best guess is that I miscalculated the inductance by an order of magnitude or so. Looked like saturation, but that core doesn't saturate south of six amps. Whatever it is, I'm sick of Mr. 34063, I'm doing something else tonight!)

author: dr.diesel
author_rank: Super Contributor
timestamp: November 05, 2013, 11:22:06 pm
content: Stuff the magic smoke back in and just glue the lid back on, good to go!

author: c4757p
author_rank: Super Contributor
timestamp: November 05, 2013, 11:31:22 pm
content: It just occurred to me that the 34063's tendency to self-destruct like that makes it a pretty poor candidate for use in a kit, which was its intended purpose. LM2575 it is, then. It's a bit more expensive, but works better and is more robust.

author: dr.diesel
author_rank: Super Contributor
timestamp: November 05, 2013, 11:33:52 pm
content: Which kit is this for? Don't you have 2-3 kits running now, starting a business are you

author: c4757p
author_rank: Super Contributor
timestamp: November 05, 2013, 11:37:14 pm
content: The only other one I was starting was an electronic load a while back, which quickly became uneconomical due to the need for an engine block-sized heat sink. This is for the function generator. I need ±15V rails and a +5V rail, all relatively beefy. The plan was to use any generic 18-24VDC adapter, with a linear reg down to 15V, an inverting switcher to -15V, and to-be-determined down to +5V (still not exactly sure how much load there will be on that rail).

author: kxenos
author_rank: Frequent Contributor
timestamp: November 05, 2013, 11:40:55 pm
content: In my workshop, we could easily with the seamless teamwork that we are known for, explode the IC, both caps and pretty much set half the lab on fire. I have a colleague that can explode even the coil core for christ's sake!

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 05, 2013, 11:48:18 pm
content: Design your DC load kit to mount a standard Intel heatsink. Everyone has one of those pieces of crap laying around, or can get one for <$10

author: c4757p
author_rank: Super Contributor
timestamp: November 05, 2013, 11:51:43 pm
content: Quote from: kxenos on November 05, 2013, 11:40:55 pmThis core is designed for pretty hefty applications. I'm sure that if I were using it near its design power, I'd find a way to do that too.Quote from: Phaedrus on November 05, 2013, 11:48:18 pmYeah, that's a good idea. Much better than "short out the sense resistor", which is probably the best idea I've had in the past hour.But I've pretty much completely lost interest in that one. Loads are all the hell over the place and way too simple. Hell, when I need a load I grab a fistful of resistance wire I ripped from a toaster.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 06, 2013, 12:31:16 am
content: You need a tube of superglue, then do a rain dance and pray.--------- Or just frisbee it out the window.

author: Corporate666
author_rank: Unknown
timestamp: November 06, 2013, 12:34:42 am
content: Damn, you harcore!That is some pretty adept finagling of parts on (what looks like?) a giant copper sheet.Well done - until it went south, of course

author: Dave
author_rank: Super Contributor
timestamp: November 06, 2013, 12:44:26 am
content: Quote from: lowimpedance on November 06, 2013, 12:31:16 amI sayWIDLARIZE IT!

author: c4757p
author_rank: Super Contributor
timestamp: November 06, 2013, 12:47:17 am
content: Quote from: Corporate666 on November 06, 2013, 12:34:42 amCopper-clad FR4. One of the things I wanted to measure was the output noise, so I needed a layout at least as good as whatever I could throw on a PCB.Quote from: Dave on November 06, 2013, 12:44:26 amSounds like a good plan.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 06, 2013, 01:25:32 am
content: Quote from: c4757p on November 05, 2013, 11:51:43 pmGive me a 1500W/140A electronic load with <0.2% accuracy. That's a kit I would buy.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 06, 2013, 01:29:56 am
content: Quote from: c4757p on November 05, 2013, 11:31:22 pmIt didn't self destruct... I am sure you helped it alongbut the LM2575 is a good part... really easy to get working, so in a kit it's possibly the best choice..

author: Stonent
author_rank: Super Contributor
timestamp: November 06, 2013, 08:02:38 am
content: If treated poorly, signal diodes become LEDs. I learned that at about age 11.

author: madshaman
author_rank: Frequent Contributor
timestamp: November 06, 2013, 08:20:55 am
content: All I want to say is: nice destruction!!! :-)

author: owiecc
author_rank: Frequent Contributor
timestamp: November 06, 2013, 10:15:28 am
content: Quote from: Stonent on November 06, 2013, 08:02:38 amA friend had a potentiometer turned to white LED. Power dissipated was just enough to make it shine but not high enough to burn the damn thing.

author: sprocket
author_rank: Regular Contributor
timestamp: November 06, 2013, 12:22:43 pm
content: Hmm there must be a marked for flakjackets, kevlar helmets and ballistic eyewear designed for electronics geeks. With schrapnel wizzing by and what not. O0

author: c4757p
author_rank: Super Contributor
timestamp: November 06, 2013, 12:34:11 pm
content: Quote from: Stonent on November 06, 2013, 08:02:38 amIt was a nice yellow-gold

author: dr_p
author_rank: Frequent Contributor
timestamp: November 06, 2013, 01:06:48 pm
content: some time ago I finished my design then noticed an electrolytic has getting hot. Also slightly bulged, then a bit more, then seriously bulged, then OH SHIT.Reverse polarity...

author: ciccio
author_rank: Frequent Contributor
timestamp: November 06, 2013, 01:18:35 pm
content: It was too nicely assembled.The IC decided to puff the magic smoke so you could post a photo and everyone in the forum could be jealous ans feel unprofessional...

author: c4757p
author_rank: Super Contributor
timestamp: November 06, 2013, 02:05:49 pm
content: Don't worry, when I get the 2575 finished I will post a picture of that and you can feel better about yourself againIts pinout isn't quite as conducive to a nice layout that looks like the schematic.Hmm.... having a bit of trouble getting more than about 150 mA out of it. It's like I suddenly forgot how to build switchers...

author: c4757p
author_rank: Super Contributor
timestamp: November 06, 2013, 03:31:42 pm
content: Ready to feel better about your circuit prototyping technique? Here's my LM2575 circuitTurns out the bloody inductor was saturating after all... at800mA! (I confused myself by misreading the scope scale on my current probing setup and didn't see what was really going on.) Turns out I've got a bag of dud toroids - some work just fine at high power, and others clearly saturate if you sneeze on them. (Explains a few other switcher mishaps I've had, too....) I didn't want to fart around with that, so I grabbed a 100uH 3A Murata out of the bin. Works fine now. Forgot I had those inductors...Clearly, you get what you pay for.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 06, 2013, 08:14:46 pm
content: Quote from: c4757p on November 06, 2013, 03:31:42 pmI found that some toroids are iron while others are ferrite.

author: c4757p
author_rank: Super Contributor
timestamp: November 06, 2013, 08:17:54 pm
content: These are almost certainly all ferrite. I am not sure what can go wrong in the production of a ferrite core to reduce its saturation flux to about one tenth - I know just over "nothing" about magnetics - but they all appear to be the same type. Some work, some don't. I may break open one of the bad ones to see if there's something obviously amiss.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 06, 2013, 08:26:44 pm
content: If the toroids are from acheapinexpensive source from the far east, then the seller may have mixed (less expensive) cores with lower permeability into the bunch, after having given the odd ones a new coat of paint.Try measuring the inductance of a few of the bad ones, and see if they all measure the same for identical number of turns. If they do, then they are fakes/repainted. If they are all over the place, then they are just lousy junk.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 06, 2013, 08:27:06 pm
content: Quote from: c4757p on November 06, 2013, 08:17:54 pmMaybe if you cut it open you'll find it's a cheap chinese made toroid with a plastic core, and just a few mm thick layer of ferrite on the outsideWouldn't that be a shockeredit: ElectroIrradiator beat me to the same thought by a second lol

author: dr.diesel
author_rank: Super Contributor
timestamp: November 06, 2013, 08:32:52 pm
content: Quote from: codeboy2k on November 06, 2013, 08:27:06 pmSeriously, has anyone found plastic core knock-offs floating around? I sure hope not, but wouldn't surprise me

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 06, 2013, 08:33:44 pm
content: To the implied question: Yes, it is possible to have two cores of similar dimensions having identical permeability yet different saturation flux, if that is what you are measuring. Same conclusion: Fakes/production rejects/cheap materials.

author: madires
author_rank: Super Contributor
timestamp: November 06, 2013, 08:43:18 pm
content: Ferrite cores allow high inductances with a few turns of wire but saturate quite fast. Ironpowder cores are the reverse and the built in distributed air gap helps to store more energy. So ironpowder toroids are the save bet.

author: westfw
author_rank: Super Contributor
timestamp: November 07, 2013, 12:57:14 am
content: I'm no inductor expert, but the coil in your initial picture has very few turns of wire compared to any 34063 circuit I've ever seen (or compared to your working version, for example.)How is saturation for cores specified, anyway? It's a magnetic flux characteristic, so surely "6A" would be a value for the finished inductor, rather than the core itself. You have "about 7" turns, and saturated at current about 7x lower than you expected; are you sure you don't need an I*n term in the flux calculation?

author: c4757p
author_rank: Super Contributor
timestamp: November 07, 2013, 01:02:09 am
content: Yeah, "6A" was just from experience with similar inductors made on those cores. I tested them somewhere around 300uH/ten turns when I bought them. These appear to be very high permeability; those seven turns give about 200uH. I don't have a specification on the saturation (these were just sold basically as junk from some electronics shop) but it would be specified as a flux density in gauss or teslas.

author: madires
author_rank: Super Contributor
timestamp: November 07, 2013, 01:30:03 pm
content: Quote from: c4757p on November 07, 2013, 01:02:09 amThose are ferrite cores without an air gap, great for filters and transformers but not for storing energy.

author: envisionelec
author_rank: Frequent Contributor
timestamp: November 07, 2013, 02:50:49 pm
content: Quote from: c4757p on November 05, 2013, 11:19:39 pmToo few turns for the material type, powdered iron which has high permeability, but is too lossy at those frequencies. The numbers aren't at arm's length for me anymore (not doing SMPS as much these days), but I can tell a dud attempt when I see one.I have a similar core here that is absolutely good for six amps and is being used in a commercial SMPS. It's the wrong type toroid in the application (powdered iron). It works but is unreliable, blowing the controller up with regularity. The better material type is 77 or F. Use a pot core for easier winding. At low current, you can maintain a reasonable expectation of flux containment.

author: c4757p
author_rank: Super Contributor
timestamp: November 07, 2013, 03:01:23 pm
content: Somehow I'm not surprised I was using entirely the wrong type of core. This is what happens when a magnetics novice buys his own cores...

author: envisionelec
author_rank: Frequent Contributor
timestamp: November 07, 2013, 03:06:09 pm
content: Quote from: c4757p on November 07, 2013, 03:01:23 pmIt's OK. I didn't know anything when I got started either. It's why I built a Lexan blast-containment shield when I nearly lost an eye working on a 2kW DC-DC converter. Wrong material type. Check out cwsbytemark.com - or just give them a call with your requirements. They're happy to help you succeed. I can count on one hand the number of engineers that can build a reasonably well-functioning SMPS the first time.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 07, 2013, 03:19:25 pm
content: c4757p, check Unitrode'smagnetics design handbook, as well as theirtraining material. I read some of it a long time ago, but it was truly enlightening...

author: c4757p
author_rank: Super Contributor
timestamp: November 07, 2013, 03:56:05 pm
content: Quote from: rsjsouza on November 07, 2013, 03:19:25 pmThat looks like some really good reading. Thank you!I will definitely be spending some time with this.In the meantime, can somebody check an inductor choice or two?Murata 33151C. This is a higher-current version of the one I'm testing with, so I suspect it would work. (The one I'm testing with only gives 40mA margin over the maximum current) I don't see any info on the core material but it lists "switching regulators" as an application.Bourns SRR1210-151M. Listed as for "input/output of DC/DC converters" - does that mean just filtering? It looks like the ones I've seen quite frequently as the primary inductor in DC/DC converters. Most of Bourns' SMD inductors appear to list that. The core is specified as "ferrite DR/RI" - no idea what DR/RI is and Google doesn't seem to know either. I know we've established that iron powder is superior to ferrite for switching regulator applications, but there seem to be a lot of inductors intended for DC/DC that are some sort or another of ferrite.Application: inverting buck-boost 15-24V in, -18V out @ 300mA, 1.36A peak, 52kHz frequency.Edit: Just double-checked my numbers and I have barely enough inductance, so I changed to 150uH inductors. That loses me a bit of max current, but still leaves at least 300mA margin (and the 300mA output current has margin as well, so probably 500mA or more above the real maximum).

author: c4757p
author_rank: Super Contributor
timestamp: November 07, 2013, 04:15:49 pm
content: By the way, I am 100% a convert to the church of LM2575. Screw this MC34063 shit! I just tried to blow up a sacrificial 2575 to see how robust they are and had one hell of a time doing it. Saturating inductor, shorted output, you name it, it can take it and comes right back for more. (Not only that, but I'm not even sure these are genuine...) Does anybody know of a similar (and similarly cheap) part that works in boost converters?

author: tszaboo
author_rank: Super Contributor
timestamp: November 07, 2013, 07:59:55 pm
content: So your dummy load is working after all. For a second.

author: BravoV
author_rank: Super Contributor
timestamp: November 08, 2013, 01:26:10 am
content: Quote from: c4757p on November 07, 2013, 01:02:09 amNot an expert though, but I do play a lot with salvaged inductors and switching ps circuits, high permeability core ferrite is usually not suitable for switcher and will saturate at low current. Other way is to break the core into 2 or 3 pieces, and glue it together using high temp epoxy to create gaps, the core saturation will increase by multiple folds.Suggesting you to build this, using dirt cheap jelly bean parts and very simple ->Inductor saturation testerdesigned by Jay_Diddy_B.

author: c4757p
author_rank: Super Contributor
timestamp: November 08, 2013, 01:32:23 am
content: Quote from: AcHmed99 on November 07, 2013, 06:05:25 pmNice. I prefer an external switch as well.Quote from: BravoV on November 08, 2013, 01:26:10 amThat is a very good idea.QuoteOh, I haven't seen that.I might build that this weekend.Quote from: NANDBlog on November 07, 2013, 07:59:55 pm

author: BravoV
author_rank: Super Contributor
timestamp: November 08, 2013, 01:36:19 am
content: Quote from: c4757p on November 08, 2013, 01:32:23 amShare it at that thread once you finished building it, please.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 08, 2013, 01:49:35 am
content: Designed a 20W LED driver off a TPS40210. Never built it, but it was a very good fit for my design, aside from high-ish Vref.

author: c4757p
author_rank: Super Contributor
timestamp: November 08, 2013, 03:19:07 am
content: And the award for most gratuitous use of PCB space goes to.....(I've got a stack of these big enough that I really can't be bothered to cut them...)

author: BravoV
author_rank: Super Contributor
timestamp: November 08, 2013, 03:29:48 am
content: I can see you're merging the power lines for the driver and for the inductor. Mine is separated, so I can hook it to "adjustable" voltage bench psu for the inductor ramp up source, its quite handy especially when you encountered a crazy huge inductor with really high saturating point (cranking up the voltage) or those very tiny one (cranking down the voltage).

author: c4757p
author_rank: Super Contributor
timestamp: November 08, 2013, 03:42:31 am
content: Good point. OK, easy enoughEdit - changed the higher current diode to MBR1535CT (15A) and added an RC snubber to keep ringing to a minimum.

author: BravoV
author_rank: Super Contributor
timestamp: November 08, 2013, 03:46:42 am
content: Cool !By cranking up/down the supply voltage for the inductor, its basically saves you from adjusting the pulse width and/or pulse period, hence, less adjustments at the scope side too, especially you're using analog scope, hope you get the idea.Just be careful not to crank up the voltage too high beyond the mosfets or diodes specification.

author: c4757p
author_rank: Super Contributor
timestamp: November 08, 2013, 06:47:18 pm
content: Just built it and it works great! Thank you for suggesting that. I'll post pictures to the other thread later.

author: Gall
author_rank: Frequent Contributor
timestamp: November 08, 2013, 08:49:33 pm
content: Hm... A ferrite inductor without an air gap? No surprise it got saturated.

author: c4757p
author_rank: Super Contributor
timestamp: November 08, 2013, 08:52:17 pm
content: Well, since I am pretty much a complete magnetics novice, it was to me.

author: c4757p
author_rank: Super Contributor
timestamp: November 08, 2013, 09:28:49 pm
content: Well, I think I know why some worked and some didn't. I took one that wasn't working properly (not this one - it was 1mH), unwound it, cracked it into four pieces, glued them back together and rewound it. The results: 1mH, saturated at 150mA (which makes sense considering that the 200uH one saturated around 800mA). After modification, the inductance went down to 32uH (yep, a whole 31-fold loss), and the saturation current went up to 5.8A (39-fold increase).I bet the "good" ones were actually slightly damaged in shipping!I'll post pictures of the tester and scope captures over at the other thread in a few minutes.

author: Gall
author_rank: Frequent Contributor
timestamp: November 08, 2013, 09:32:17 pm
content: I had to build a 100W 3kV flyback at one time. This was quite a challenge, I learned very much about these beasts. In a flyback the inductor has to store all the energy of one working cycle. Ferromagnetic materials like ferrite act like a conductor for magnetic field, they cannot store much. Most of the energy is stored in air gaps of the core. That's like an electric wire that just carries electric current without storing energy, while a piece of insulator across the wire (a capacitor) does. Unlike a wire, ferrite core still may hold a small amount of energy but usually not enough for high loads. That's why it's easy to saturate it. A small air gap is usually not needed in buck converters, while most boosts and flybacks cannot go without it. Most ferrite cores made of two parts have a "natural" gap that is enough for most purposes. High-power ferrites are made with an artifical gap. One can add a gap just by placing some thin paper between the parts of the core.Using a big ferrite core, it is possible to light (and even overload) a 240V 40W light bulb from just three turns of wire. The primary winding was pumped from 12V with IRF840. I did that in one of my experiments. I used an improper diode for secondary rectifier so it got really hot. (I decided to save time and sacrifice an "incorrect" diode instead of spending time finding the "correct" one).

author: Gall
author_rank: Frequent Contributor
timestamp: November 08, 2013, 09:33:31 pm
content: Quote from: c4757p on November 08, 2013, 09:28:49 pmYes, this IS the air gap!

author: BravoV
author_rank: Super Contributor
timestamp: November 09, 2013, 01:12:52 am
content: Glad to hear it works as expected.Quote from: c4757p on November 05, 2013, 11:19:39 pmSo no more Mr. 34063 hatred ?C'mon c4757p, you are the authority for this MC-34063 chip here in this forum, please don't hate & abandon it.

author: c4757p
author_rank: Super Contributor
timestamp: November 09, 2013, 01:56:04 am
content: MC34063 can screw itself and die in a fire.OK, OK, it does have one advantage: it's just about the cheapest damn DC-DC regulator you can find. It's not particularlygood, but it ischeap. I will definitely continue to use it, but nowhere near a kit.(Yeah, it was my fault it blew up, but that was established from the beginning! LM2575 does not become a DC-to-projectile converter when the inductor saturates.)

author: wraper
author_rank: Unknown
timestamp: November 09, 2013, 02:11:37 am
content: Saying that MC34063 is shit is at least unfair. You blow MC34063 because left it without current protection by shorting the current sense resistor. LM2575 have internal current limiting so of course you couldn't blow it because it was impossible to disable current limiting. Therefore this is not telling anything about reliability.

author: c4757p
author_rank: Super Contributor
timestamp: November 09, 2013, 02:22:09 am
content: Quote from: wraper on November 09, 2013, 02:11:37 amThat's my point. I'm kind of kidding about it being shit - as BravoV remembers, I was quite a fan of it. I don't like the idea of using it in a kit though - all it takes is a simple short to make it put your eyes out. The LM2575 has inherent protection against many things that is impossible to disable. I tried pretty hard to blow one up - the only way I finally managed to wreck it was to reverse polarize VCC, and it still didn't do anything too dramatic, even with the PSU current limit at 5A. (MC34063's tantrum happened so fast it could have done it on the energy in the decoupling cap.) Does that make it superior? Not necessarily. But it does put it higher on my list of chips to use with beginners.

author: wraper
author_rank: Unknown
timestamp: November 09, 2013, 02:34:05 am
content: Did you try the same stress test on MC34063 without omitting current sense resistor to be fair? As the main difference is that in LM2575 this resistor is inside the IC. Well you could include two extra MC34063 to the kit and it would still be cheaper than LM2575

author: c4757p
author_rank: Super Contributor
timestamp: November 09, 2013, 02:40:04 am
content: It would take it just fine without shorting the sense resistor, but that's the whole point! Properly implemented, it'll stand up to just about anything, butproperly implemented, it wouldn't have to! For use in the kit I want something that can be completely screwed up and still reasonably safe.I would just include two, but the problem is how violently it dies. That generally seems to be a problem with power devices in DIP format, they become guns when abused. The cost savings aren't really worth it for this specific application. It takes alotof power to make pieces come flying off a TO-220 or DPAK.

author: c4757p
author_rank: Super Contributor
timestamp: November 09, 2013, 02:46:13 am
content: OK, I justhadto take a screenshot.F*king dyslexia...

author: wraper
author_rank: Unknown
timestamp: November 09, 2013, 02:57:20 am
content: If LM2575 output transistor gets short I'm pretty sure it would fire the same bullet or at least emit magic smoke from the hole. That's just how power devices die with unlimited input current.

author: c4757p
author_rank: Super Contributor
timestamp: November 09, 2013, 03:01:48 am
content: The current isn't unlimited, though. The 34063 blew up almost instantaneously, even with a relatively weak power supply it could have done it just by eating the contents of all the power supply caps. I did short the LM2575 output transistor directly to ground for quite some time, with the power supply set to a limit of 5A. It didn't blow up, it overheated slowly enough for the overtemp cutoff to kick on. This thing is likely going to be powered by something much weaker than 5A.

author: wraper
author_rank: Unknown
timestamp: November 09, 2013, 03:13:31 am
content: Not exactly LM2575 but i have replaced LM2577 which was dead the same way as this MC34063. With big hole in the middle. Psu was sure <3A on 25v rail where it sat.

author: c4757p
author_rank: Super Contributor
timestamp: November 09, 2013, 03:15:35 am
content: Oh, I'm sure it's possible to catastrophically kill them. It's just more difficult/unlikely. There are a lot of ways to make electronics dangerous, I'm not going for "completely covered in bubblewrap". Just a bit safer for really not much more cost.

author: wraper
author_rank: Unknown
timestamp: November 09, 2013, 03:36:35 am
content: Regarding to beginners, reverse polarized electrolytics will make more sound effects and fly further. One friend of mine, being beginner managed to catch blown cap into his ear.

author: Bored@Work
author_rank: Super Contributor
timestamp: November 09, 2013, 07:33:57 am
content: Quote from: c4757p on November 09, 2013, 02:22:09 amDo not run with sharp objects.

author: madires
author_rank: Super Contributor
timestamp: November 09, 2013, 01:55:23 pm
content: Quote from: wraper on November 09, 2013, 03:36:35 amAlways wear ear protection at the shooting range :-) I've managed to make a LM2576 "pop" by reversed voltage some while ago.

author: Flávio V
author_rank: Regular Contributor
timestamp: November 09, 2013, 10:45:59 pm
content: I destroyed a LM2576 by shorting the input capacitors(2 3300uF 35V) multiple times, i still can't figure out why it happened.

author: c4757p
author_rank: Super Contributor
timestamp: November 09, 2013, 10:47:53 pm
content: Theinputcapacitors? Because you reverse polarized the output transistor - theoutputcapacitors were charged up, and the input was now at zero. This is the same reason why you're supposed to put protection diodes around, for instance, the LM317. You really ought to put a protection diode across any step-down regulator, as well as diodes antiparallel to all the rails in any system with positive and negative supplies. This prevents polarity inversion - if two are shorted together, one regulator will "win" and the negative rail could become positive or the positive rail negative.Reverse polarity is the best way to kill even the most robust chips.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 10, 2013, 11:01:07 pm
content: Quote from: Flávio V on November 09, 2013, 10:45:59 pmbut... why??