LDO regulator blew up - Page 1
Forum section: electronics / projects

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 23, 2014, 06:02:21 pm
content: I'm hoping someone might be able to explain why I had a LDO 3-terminal regulator blow up. I am new to electronics. Every time I think I'm beginning to get a handle on things, something like this happens.The short of it (no pun intended), I hooked up a simple, low current circuit (switched LED, 20mA). It worked as expected, voltages were inline with my design. With or without load (switch was on the load side), the regulator would soon heat up. I googled the heat problem and learned that LDO regulators are very susceptible to oscillations and that the capacitors are crucial, paying particular attention to the correct ESR, information I did not consider in the original design. Further, the load side capacitor apparently plays an important role whereas the regulators I've previously used (LM317, LM350) were far more forgiving in their absence. So, I simply soldered to my board a 10uF electrolytic cap, negative to ground and the other lead to the center pin, the output. When I plugged the device into the wall, it immediately sparked and blew out a chunk of the TO-220 pkg.Particulars: I'm using a TL738,http://www.ti.com/product/tl783for the regulator.Input = 120VAC, Rectified with a DF02.Cinput= Here I ran into a problem. I only had 2 caps of sufficient voltage (aside from an assortment of huge motor run and start caps). One was 220uF. The other, the one I used, didn't look like a cap I was familiar with. It was simply marked with "Sprague .1, +/-10%, 200DC," in an axial 2-lead package. I used my VOM to confirm it would hold a charge (resistance, then volts). It concerned me that there is no discernible polarity markings, but it seemed to take a charge regardless of how I connected it to the ohm meter.R1= 100 ohmsR2= 1k ohmsCload= 10uF. Adding this to the circuit caused the regulator to blow. Keep in mind prior to adding this to the circuit, the LED worked properly, with the exception of the excess heat. Voutmeasured 13V. Using my ammeter, I got unstable readings below 30mA on the input to the rectifier. I would have used my scope, but as a novice, I'm afraid to use it on line voltage. It's a Tektronix 2230. My caution is probably unnecessary and due to ignorance.The load is simple. One white LED with a 505 ohm series resistance. Vf= 3.2V. Switch in series.So, first question is "Why?"Second question is what is the proper procedure, in lay-terms, to evaluate the circuit and select the proper caps? The datasheet (or perhaps it was a tech bulletin on LDO regulators put out by TI?) stated that you need to bench test the circuit using a near-zero ESR cap and evaluate oscillations while varying a series resistor to achieve stability. It doesn't sound like a very elegant design method.Thanks.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 23, 2014, 06:39:53 pm
content: Quote from: chipwitch on January 23, 2014, 06:02:21 pmThe only sensible recommendation to give is to recommend you stay away from such voltages until you know much more.

author: mariush
author_rank: Super Contributor
timestamp: January 23, 2014, 06:46:09 pm
content: Your 125v AC at the wall is a RMS voltage. When converted to DC using a bridge rectifier, you have a peak DC voltage of 125 x 1.414 - 2 x voltage drop of rectifier. So you have a peak of about 174 volts DC. The linear regulator accepts a maximum of 125 vDC... 60 times a second, you're reaching up to around 175v DC on the regulator.The input capacitors are there to make the input dc voltage smoother, removing the "valleys" in the rectified dc voltage. If you indeed had 125v AC rectified to DC, that may explain why it blew up, because you have a peak voltage of up to 175 volts.Assuming you actually mean 125 v DC maximum, the capacitance required to get a minimum voltage can be approximated with the formula capacitance = current / ( 2 x Mains frequency x Vripple) where V ripple is how much you're willing to let the voltage drop from the peak dc generated by the rectifier.For let's say 50mA and considering a peak of 125v DC ( or about 85 AC after it's rectified with a bridge rectifier, because that's the maximum the regulator can handle) and since you mentioned the 13v value/// then keeping in mind the regulator has a voltage drop of at least 5v, then the regulator should have at least 20v or so at the input, so your V ripple would be about 125v - 20v = 105v then C = 0.05 / 2x60x105 = 3.96 e-6 or about 4 uF ( is enough to have at least around 20v all the time, but peaks up to 125v). More capacitance just raises the minimum voltage, or allows more current to be pulled without voltage dropping below 20v.Now, just because the regulator says it can do 1a, doesn't mean it can do it as such large voltage differential... note that the values in the datasheet are for pulses of 1ms or 30ms... in your case let's say with 125v DC input and 13v output and 30mA of current, you would have when the voltage reaches the peak DC voltage : (125-13) x 0.03 = 3.36 watts dissipated. A heatsink is required for pretty much anything above 1 watt. If done as said in the paragraph above, the voltage will fluctuate between 20v minimum given by the capacitor and the peak dc voltage, so the regulator will vary all the time between best case scenario of minimum input voltage (20-13)x 0.03 = 0.21 watts dissipated and the worst case of 3.36 watts... this just for 30mA of current.And seriously, if you run this directly from the mains.. it's very dangerous, stop what you're doing right now. At the very least, use an isolation transformer when doing tests .edits for 60hz mains freq. in us etc...

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 23, 2014, 07:43:31 pm
content: Quote from: Bored@Work on January 23, 2014, 06:39:53 pmI'm not a child. While there is much I don't know, as I've already confessed, I question the "sensibility" of a "recommendation" that doesn't consider the lifetime of knowledge deliberately omitted from the OP, shall we say, in the name of brevity? With all due respect, I do not understand the need to reply to forum questions, while deftly avoiding answering them.Would I be correct in assuming it has something to do with your signature line:I love the sound of a "you are not helpful" whining in the morning.I suppose one shouldn't expect much help from one who derives pleasure from hearing "you are not helpful." Personally, I find it sad endeavor.

author: c4757p
author_rank: Super Contributor
timestamp: January 23, 2014, 07:47:58 pm
content: It's not a bad point, if a bit blunt. High energy (I'd say anything over 20Wpossible*, though that might be a bit conservative) is something you should graduate into, not start with. Try your hand at a lower-power regulator and work out the thermal issues there first.As in, "what happens if I short it out", not "what happens if it's working correctly"

author: Bored@Work
author_rank: Super Contributor
timestamp: January 23, 2014, 07:57:54 pm
content: Quote from: chipwitch on January 23, 2014, 07:43:31 pmThe voltage doesn't care.

author: miceuz
author_rank: Frequent Contributor
timestamp: January 23, 2014, 08:15:40 pm
content: Quote from: chipwitch on January 23, 2014, 06:02:21 pmI congratulate your brave soul and the self learning process you are going thru. No tongue in cheek - this kind of people is exactly what we need, but please, don't touch mains as long as you can allow that. There are other ways to light up a LED from mains, but my suggestion would be - just don't go there yet. There is tons of stuff to to understand and tinker around in electronics.Most of electronics works on fairly low voltages, you need a transformer or a switching mode power supply (which includes transformer) to knock 125V down.Mariush has explained why your LDO gets hot, in addition I'd say that probably electrolytic caused a high current inrush spike to go thru the LDO and that caused it to explode.By converting 125V to 13V, LDO takes the task of dissipating the excessive energy on itself. Power=voltage*current, lots of power - explosion, evaporating metals, damaged eyes, fire, etc.Good luck and don't let this adventure to let you down

author: mrflibble
author_rank: Super Contributor
timestamp: January 23, 2014, 08:27:12 pm
content: If experimenting directly with relatively high voltages this way is your learning style, I suggest:- isolation transformer, so you're not directly on mains- fuse it- wear glasses / goggles for the explodey bitsI wouldn't really expect explodey bits, but eyes are tricky to replace.And reading themax ratingspart of the LDO datasheet probably won't hurt.

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 23, 2014, 08:39:47 pm
content: Hi, Mariush. Thanks for the response.Quote from: mariush on January 23, 2014, 06:46:09 pmYou're quite right. It's been a while. I had forgotten about it being RMS.QuoteI'll have to go back and check with the VOM. IIRC, I checked the output from the rectifier and it was 115v. Maybe it was a 7 instead of a one. Just to be clear, the .1 cap was across the output side of the rectifier from the start. The cap on the load side of the regulator is the one that when added, blew up the regulator.QuoteCorrect me if I'm wrong, but isn't 5v high for a LDO? My understanding is that it would be in the mV area? The 13V was measured. I only needed about 4V for a single LED. Kept the output voltage high in anticipation of adding a series LED. Plus, my thinking was to keep the input-output differential as low as possible. But, I get your point. The cap formula is what I lacked. Can you give me a google term to search so I can read more about the theory behind the formula? Does the formula have a name?QuoteThat's what I needed. I wasn't sure what the power was based on. You're saying I need to use the Vdelta. I knew it wouldn't be all that efficient, but I didn't know it would be THAT bad. That was precisely what I was hoping to learn by wiring the circuit. So, what is the most efficientnon-transformermethod to step down the line voltage to to say 10v?QuoteI appreciate your concern. I'm a retired journeyman electrician who knows better than to stick her hands in the back of a television set. I've comfortably changed out electrical services live. Believe me, if I have any doubt, I don't take risks beyond that I'm willing to lose... discreet electronic components, for example.Thanks again for the informative lesson. It was very helpful.Quote

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 23, 2014, 08:54:27 pm
content: Quote from: miceuz on January 23, 2014, 08:15:40 pmNot so brave. Mains don't bother me. It's all about being insulated. Once, I was helping an engineer friend of mine (mechanical). He had just bought a condo on the beach and was doing a remodel. I was helping him with the electric. The condo was on the 8th floor and I'd just installed a couple circuits to the kitchen. After all was finished, I had to make the connection to the panel. He was looking over my shoulder and said he'd go downstairs to shut off the main. Given the inconvenience of having to trek down the elevator, wait, go back to turn it on etc, I told him not to worry about it. He wouldn't relent. Finally, I got him to just sit still and "don't touch me." As I reached into the panel... before touching a thing... he screamed like a little girl! lol. I was so amused by his outburst, I pointed at the main bus bar and explained to him what that was. Then I proceeded to lay my hand on it. Screams again! It still gives me a chuckle.Working in the field gives you an acute consciousness of remaining insulated and avoiding completing the circuit. Plus, the "one hand rule" adds a measure of safety. However, I must say when working with electronics, it adds a complexity to the circuit that can often be overlooked. So many junctions it's easy to forget what kind of signal you're getting and where. Mains electric is simple. You've got hot... ground, and you are the only component! My lack of knowledge of electronics actual makes me LESS inclined to work it hot. I'm careful, though I wouldn't recommend it to anyone.

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 23, 2014, 08:58:34 pm
content: Quote from: mrflibble on January 23, 2014, 08:27:12 pmTransformer's good advice. Goggles? Not by choice, but aging eyes have made it essential. Explodey bits??? Got to see that today!Oh, and I did read the datasheet. Understanding it all... well??? That's something I'm working on

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 23, 2014, 09:11:50 pm
content: Quote from: miceuz on January 23, 2014, 08:15:40 pmForgot to respond to this part. Chalk that up to age as well? Yes, Mariush explained the heat well. You're suggesting that the cap on the load side caused the inrush. That would explain why it blew when it did. The regulator was already stressed beyond it's capacity, as Mariush pointed out. The cap just pushed it over the edge. This info is precisely what I was looking for. I have checked and rechecked the circuit several times thinking I'd shorted something out or connected it wrong, but couldn't see anything. Now, at least I know I wired it correctly. It was mydesignthat was flawed!It takes a lot more than a couple sparks and a puff of smoke to deter me. Heck, some people would say those are essential elements for a proper celebration! (new years day, independence day, etc) lol

author: dannyf
author_rank: Super Contributor
timestamp: January 23, 2014, 09:13:50 pm
content: QuoteFor learning, that's not bad.Better yet is to build on others' experience so you don't have to re-learn what others have learned.

author: mariush
author_rank: Super Contributor
timestamp: January 23, 2014, 09:24:17 pm
content: QuoteI'll have to go back and check with the VOM. IIRC, I checked the output from the rectifier and it was 115v. Maybe it was a 7 instead of a one. Just to be clear, the .1 cap was across the output side of the rectifier from the start. The cap on the load side of the regulator is the one that when added, blew up the regulator.The 0.1uF capacitor is for decoupling. The capacitor does charge and provides the regulator with some capacitance, but that's not its major purpose in the circuit.The output of your rectifier is like this (with and without capacitor after it):... Each of those bumps is one / 120 of a second. So you can see that even without capacitors after bridge rectifier, for quite a lot of time the voltage will be above a certain threshold, and the linear regulator will output something as soon as the voltage goes above a threshold.. for this regulator the threshold is maybe around 6-8 volts.Even if you set the output to something, as soon as the input voltage is above a threshold it will start to output Vin - DropOut and as soon as the input voltage is above Vout+Dropout, it will output the set Vout. For example, you set it Vout to 12v, you have a 10v input voltage, the regulator will probably output 10v - 5v dropout voltage = 5v.You probably measured 13v because that's what the regulator was averaging 80-90% of the time it was running or maybe your multimeter wasn't fast enough to show you that the output wasn't stable for a few tens of milliseconds (each second) when the output of the rectifier was going below that minimum input voltage the regulator was running at.QuoteThere are all kinds of linear regulators. The dropout voltage varies with type of linear regulator, the temperature it's running at and the current.The vanilla LM317 or 78xx need 2 - 2.5v to work, other plain regulators like 1117 only need 1.2v, and of course there are small current ldos that only need 10-40mV above the output voltage to work. It depends on what components are used in the chip, if it's a pnp transistor, if it's npn, if it's a darlington used.Naturally, to be able to handle such large voltage differential, your regulator needs more dropout voltage... i didn't pull those 5v out of my ass, the datasheet has a graph on page 6 on the top right, it shows 5v drop @ 100 mA. You can see in the picture that for 700mA, at a temperature of 100c (normal if you heatsink and pull that much current) the dropout voltage can be up to about 18v!QuoteI don't know if the formula has an actual name and I don't remember now exactly how it's derived... I used to know.For basics, these videos may help you out. A similar (approximation) formula is used by him in the Capacitors tutorial and the videos are easy to watch and understand :https://www.youtube.com/playlist?list=PLC8A1AD995AB64055And you may also want to simulate circuits using the circuit simulator here :http://www.falstad.com/circuit/(requires java, if you don't want to run in browser click on link on the page to download a zip with the simulator)The simulator has lots of examples including bridge rectifier with or without capacitor and you can play with the examples adding loads or changing capacitance values or input voltage and so on.QuoteThe most efficient would be to use a transformer. Classic transformers are heavy and big, smps power supplies are smaller because they work at higher frequency so they use smaller transformers but smps power supplies are obviously more complex. BOTH solutions are much safer as the transformer isolates you from mains. Get one of those small adjustable voltage power adapters (likethis one, just the first one that came up on Amazon search, not the best or cheapest) and you're in business. If you want smaller, get one of those phone chargers and hack the feedback of the power supply and make it output 10-12v instead of 5v (but make sure the secondary capacitors can handle it).A cheaper and non-isolated method of lightning a led from mains would be using a capacitive dropper. It's still super lousy efficiency, a few watts wasted, but it's small and cheap and simple.You can google for those two words, or you can watch these videos and remember/learn some things besides learning how to make a circuit :

author: mamalala
author_rank: Unknown
timestamp: January 24, 2014, 04:08:03 am
content: Quote from: Bored@Work on January 23, 2014, 07:57:54 pmNor does the amperage, until the mains fuse blows. If the chip blows short between input and output, that will be a lot of fun. Not to mention the energy stored in the caps...Greetings,Chris

author: mamalala
author_rank: Unknown
timestamp: January 24, 2014, 04:15:09 am
content: Quote from: chipwitch on January 23, 2014, 07:43:31 pmQuote from: chipwitch on January 23, 2014, 08:39:47 pmQuote from: chipwitch on January 23, 2014, 09:11:50 pmSeems it has been quite some lifetime indeed. Forgetting about basics due to age, well, i can only concur with Bored@Work then:Quote from: Bored@Work on January 23, 2014, 06:39:53 pmOnly that i would add "again" at the end of his remark.Greetings,Chris

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 24, 2014, 01:25:54 pm
content: Well, bless your little heart!

author: calzap
author_rank: Frequent Contributor
timestamp: January 25, 2014, 12:32:29 am
content: Quote from: mariush on January 23, 2014, 09:24:17 pmI watched the Ben Krasnow videos, and they're a good explanation of impedance but a far shout from how to be safe with using mains to power an LED. I cringed every time he passed his hand over the live circuit to reach the plug or watt meter. At the least, he should of had the circuit and the outlet side by side. Even better would have been a piece of clear plastic over the circuit. The capacitor appeared to be an ordinary polyester type, but should have been an AC-rated safety capacitor. There should have been a fuse in the circuit and a bleed resistor across the capacitor leads. He did a good job of explaining the need for a series resistor to limit inrush current. And he did have two diodes in parallel, but of opposite polarity; however he didn't make it clear than the arrangement is mandatory for their mutual protection. One of the diodes can be an ordinary non-LED diode.I've enjoyed watching Ben's videos and have learned from them, but the ones on impedance aren't the only ones where he isn't using safest procedures. There's one where he uses hot, fuming nitric acid to eat away the case of an IC to look at the silicon inside. It works, but he did it in the middle of his shop without fume extraction or a splatter shield. He was wearing gloves and may have been wearing a respirator and face shield but didn't say so. I think that if one is going to post a tutorial, one should go overboard on safety aspects because some beginners may not realize the full extent of the risks.Compared to the phontonicinduction videos though, Ben's are a paragon of safety.Mike in California

author: Psi
author_rank: Super Contributor
timestamp: January 25, 2014, 12:48:17 am
content: "Afrotechmods" has really good electronics vids on youtube that explain key topics in a fun and easy to follow way.I highly recommend them.Also, "Mehdi Sadaghdar" has really good vids showing how easy it is to make mistakes in electronics thatcouldkill you.His vids are staged with painful but non-lethal voltages of course.. but its still scary sometimes.

author: Rudane
author_rank: Regular Contributor
timestamp: January 25, 2014, 01:20:23 am
content: If you're going to work with the mains I recommend a VARIAC. This is a variable transformer that will allow you to dial down the voltage to your regulator.

author: c4757p
author_rank: Super Contributor
timestamp: January 25, 2014, 01:29:58 am
content: Quote from: Rudane on January 25, 2014, 01:20:23 amTwo comments about that - first, if it's abigone it could actually increase the current output required to trip input-side overcurrent protection, which is more dangerous than the voltage IMHO. Make sure you fuse theoutput. Voltage bites, current sets fires. And second, most don't isolate, so add an isolating transformer.If you want to play with high-ish voltage and rectified mains, I'd recommend spending some time designing and building a sturdy step-up converter to power your circuits from a low voltage input. You'll learn a lot building the power supply itself, and you can build protection features into it (and even without, it'll be quite separate from the mains)

author: echen1024
author_rank: Super Contributor
timestamp: January 25, 2014, 04:12:12 am
content: Hell, I would simply advise staying away from mains until you can understand the datasheet. From what I can see, changing 175V into 13V results in a 162V voltage drop. If we have a modest load of 200mA, that results in 32.4W of heat trying to be dissipated, hence the explodey bits.

author: amyk
author_rank: Super Contributor
timestamp: January 25, 2014, 10:23:37 am
content: I was (only slightly) relieved when I read that OP was using a high-voltage regulator... for a moment I thought he was putting something like an LM1117 across the mains.

author: chipwitch
author_rank: Frequent Contributor
timestamp: January 25, 2014, 03:05:11 pm
content: Yes, it's a problem when we make so many assumptions. Hell, theactualcircumstances can be unrecognizable from what wepresume. Aside from Mariush, who was kind enough to actually attempt to answer my original question, back on reply #3, it seems the responses have been more interested in pointing out my inabilities and shortcomings than actually assisting my learning process. Warning me about the dangers of "mains" power is irrelevant to the OP. However, the very first response was more than enough, if not condescending, in conveying that "concern."I debated on whether or not I should have posted the OP in this forum or the "Beginner" forum. Perhaps, I should have posted there instead. I will refrain from posting in this forum in the future, as I can't help the feeling I have that I've offended the sensibilities of some of you for having the audacity of not knowing all the basics. Much thanks to Mariush for your assistance. The information you provided was precisely what I needed. Though I've watched many videos and read quite a bit about capacitors and other electronic related topics, theapplicationof some principles have been a bit elusive for me, caps being one of them. How they work isn't all that complex, but knowingwhen and howto use them is what I've struggled with. I watched ALL the videos you suggested, though it just about put me to sleepas MOST of it simply explains what capacitors are and what they do... ground I've already covereddozensof times. Nevertheless, I persevered. And, I'm glad I did! Something "clicked" in one of the videos which led to experimenting, giving me a better understanding of their use. Thanks again. There are thousands of videos and tutorials out there. Not all of them will speak to everyone.In any case, I've successfully redesigned my circuit and it is working precisely as it should. I now have a useful circuit from which I can further my self-directed study. Thanks again for the assistance.