Ceramic capacitor is of  too low voltage?...som,etimes blows up at switch on. - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: July 05, 2018, 05:04:03 am
content: We have an LR8 linear regulator downstream of our mains diode bridge (240vac). The capacitor at the bridge output (at the input to the LR8) is just a 100nf, 1812, x7r, 630vdc capacitor. It sometimes blows up at switch on. (one in 50 times)It surely cannot be blowing up because of over voltage above 630v, because the sensitive LR8 would surely have blown up first.What we suspect is that our board stuffer has accidentally placed a 250V capacitor there, and most often these are surviving in circuit, but every now and again the product is switched on at mains peak, and this 250v part blows up. What do you think?Do you think this is likely?...ie the board stuffer has accidentally placed a 250v capacitor here, instead of the required 630vdc capacitor?Surely it’s the only explanation?Strangely, the capacitor , when blown up, has a hole in it and there’s tarnish on the PCB near it, but when desoldered the capacitor actually still measures 100nf.LR8https://www.microchip.com/wwwproducts/en/LR8

author: Giaime
author_rank: Regular Contributor
timestamp: July 05, 2018, 06:46:09 am
content: Maybe too strong of an inrush current?

author: rs20
author_rank: Super Contributor
timestamp: July 05, 2018, 06:51:12 am
content: Quote from: Giaime on July 05, 2018, 06:46:09 am^ This. Inductance in the wiring leading up to the capacitor can create an LC circuit; an LC circuit can (ideally) see a hard switch-on event leading to the voltage peaking to twice the steady state. Interestingly, and perhaps unintuitively, simply adding a capacitor with ESR (i.e., a bog standard electrolytic capacitor) in parallel to the ceramic cap can alleviate this a lot; the ESR damps the LC tank.

author: Delta
author_rank: Super Contributor
timestamp: July 05, 2018, 07:27:13 am
content: Don't forget that the mains voltage in the UK regularly reaches 292vAC RMS for periods of several minutes.

author: Giaime
author_rank: Regular Contributor
timestamp: July 05, 2018, 08:38:25 am
content: Quote from: blueskull on July 05, 2018, 06:53:49 amYep we don't know. Maybe that's the input of a boost PFC stage? In that case, I would still expect a plastic X2 capacitor for EMI suppression

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2018, 09:01:53 am
content: Quote from: Delta on July 05, 2018, 07:27:13 amIf the mains supply regularly does that, then there's a problem with your electricity supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2018, 09:14:26 am
content: Quote from: blueskull on July 05, 2018, 06:53:49 amDid you check which OP we're talking about here?Tim

author: Terry01
author_rank: Frequent Contributor
timestamp: July 05, 2018, 09:21:18 am
content: Quote from: rs20 on July 05, 2018, 06:51:12 amI get that you guys will see this as easy stuff but to a newby like me using the ESR in a cap like that is genius! You learn something new every day right?

author: wraper
author_rank: Unknown
timestamp: July 05, 2018, 09:49:06 am
content: Quote from: Hero999 on July 05, 2018, 09:01:53 amThat was a joke about some insane thread in the past.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 05, 2018, 09:53:31 am
content: Have you, oh, I don't know... Measured the voltage on the cap?

author: Gyro
author_rank: Super Contributor
timestamp: July 05, 2018, 09:56:40 am
content: Quote from: wraper on July 05, 2018, 09:49:06 amYes, there should really have been a smiley on that!(https://www.eevblog.com/forum/beginners/mains-in-uk-can-go-up-to-292vac-for-periods-of-several-minutes/for context reference, butpleasedon't resurrect it!)

author: exe
author_rank: Unknown
timestamp: July 05, 2018, 10:08:52 am
content: Inductive kick of some sort?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 05, 2018, 10:18:19 am
content: The fact it goes at switch on suggests to me the issue could be current surge. Regardless of voltage rating, there will always be some peak current value that will make it unhappy.What limits the peak current into this cap? Is there any series resistance before it?

author: Gyro
author_rank: Super Contributor
timestamp: July 05, 2018, 10:21:01 am
content: Of course you don't actually know that it didn't fail short at switch-off, only to manifest itself at the next switch-on.

author: BrianHG
author_rank: Super Contributor
timestamp: July 05, 2018, 10:55:13 am
content: Your all crazy, he just needs a 6300vdc cap. There, problem solved...Next thing to blow up would be the LR8 linear regulator...As for inductive kick, not much may be needed at all for such a small cap value while just the AC snap on with capacitance in the diode bridge as well, you can hit a massive combo double peak source voltage, + a super fast V/us slew rate into that poor 1812 cap it you switch on the power at just the right time in the AC phase.If you are going to stick with this cap, the absolute cheapest thing I would do is add an appropriate sized series resistor either just before the rectifier or just after. However without knowing the rest of the schematic, do not follow me here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2018, 10:39:07 pm
content: Keep in mind that the value is significantly less than rated. This exaggerates the peak voltage during transient startup or surge conditions. X7R are bad already, and worse at high voltages.Note that C0G have more value (at voltage) for the same package volume or cost.Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 05, 2018, 11:04:24 pm
content: My guess is that the inrush current into the capacitor is so high that Ohmic resitance heats a small part of the capacitor too much and it vaporizes some bonding wire or similar.Did a few experiments a long time ago with a capacitive dropper to light a LED (with anti- parrallel diode) directly from 230Vac. Quite some leds exploded on turn on untill I learned to put in an extra series resistor to limit the inrush current.A wall power socket can easily deliver several hundreds of Amps without blowing a fuse for a short time such as a switching event.The LR8 has a current limit between 10mA and 30mA:http://ww1.microchip.com/downloads/en/DeviceDoc/20005399B.pdfYou can probably use a series resistor between a few hundred Ohm to a few kOhm between 230Vac and your capacitor to limit the inrush current. Make use of this to turn it into a usefull RC filter.If you have doubts about accidentally wrongly placed caps, then take one of those caps and subject it to a high DC voltage untill it breaks. Compare that voltage to a known good cap.Beware that you capacitors may explode or release some nasty magic smoke during this experiment, but it is one of the experiments where destroying these components is actually usefull.

author: chris_leyson
author_rank: Super Contributor
timestamp: July 05, 2018, 11:32:02 pm
content: No room for a film X2 cap ?

author: Terry01
author_rank: Frequent Contributor
timestamp: July 05, 2018, 11:35:50 pm
content: Quote from: Doctorandus_P on July 05, 2018, 11:04:24 pmI thought a wall socket was only good for 32 Amps? I don't know for sure myself, i'm still learning. I read it on the forum somewhere.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2018, 12:38:18 pm
content: Quote from: Terry01 on July 05, 2018, 11:35:50 pmSome sockets? I guess??You can easily draw over 2000 amperes from a typical outlet... it's just a question of, for how long. At that rate, the breaker will trip within a cycle (< 10ms). Which, mind you, is an electrical eternity -- say a transistor was dropping that voltage (even while drawing far less current -- transistors can't draw fault current, but a fault condition like SMPS shoot-through, or driving a shorted load, will still quickly be fatal), it will die in about 20us. So the breaker opens after about 500 transistor deaths, by which time the one transistor has thoroughly turned into an expanding plasma ball.Over longer time scales, you want to keep the RMS current below the fuse/breaker rating (whatever that happens to be -- and for UK circuits, I believe that includes a fused plug and cord, versus the US system that assumes any outlet may be asked to deliver full rated current). RMS is an averaging process, so the peak current can still be much more than rated -- again, it's just a question of, for how long.In the present case, blowing a ceramic cap will have an equivalent RLC circuit, which will peak at around 1.5A in 3.5us, then 680V at 7us (worst case, switching at the instant of maximum line voltage, and assuming mains inductance ~500uH and capacitance 0.01uF constant). In practice, the ceramic's capacitance will drop off rapidly as voltage rises, so the peak current will be lower, and the peak voltage will be far higher, probably over 1kV. At that point, the LR8 is guaranteed to be unhappy; it may well happen that it can bear modest avalanche currents (<1A in this case), but it may also be gradually damaged in the process. Of course if the capacitor fails internally, you don't get a chance to see if that happened...Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 06, 2018, 01:21:30 pm
content: In this case it is not the (stable state) of an RLC circuit which has to be considered, but the inrush current.Inductance of your main cables is hard to guess / measure and varies upon location.Go blow a few leds like I did a long time ago to get some respect what 230Vac will do with unprotected electronics.BigClive (youtube) has a very nice switch box for working relatively savely with Mains voltage & electronics.

author: Terry01
author_rank: Frequent Contributor
timestamp: July 06, 2018, 02:36:10 pm
content: Quote from: T3sl4co1l on July 06, 2018, 12:38:18 pmI've read this 4 times now and still don't fully understand it but I will...even if I have to read it another 4 or 5 times! It's very very interesting reading.Thanks for the info..

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2018, 03:46:19 am
content: Quote from: T3sl4co1l on July 06, 2018, 12:38:18 pm2,000A seems a bit high for a real world situation. Looking up wire resistance I see 12AWG is 1.588 Ohms per 1,000'. If you figure a 50' run between the panel and receptacle then that's 100' round trip so 0.1588 Ohms. If we assume 170V peak and ignore the source impedance of the distribution feeding the panel, resistance of the breaker, receptacle, power cord, etc then I get 1,070A as the absolute peak you could get. Now that's still a whole lot of current, but it's not 2,000A. Apologies for a pedantic moment.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 08, 2018, 07:25:21 am
content: No worries about pedantism, 2kA was merely a rough ballpark figure. YMMV, depending on many things, of course.Tim

author: exe
author_rank: Unknown
timestamp: July 08, 2018, 11:27:43 am
content: Quote from: james_s on July 08, 2018, 03:46:19 amThere are some counties with 230VAC RMS

author: SeanB
author_rank: Super Contributor
timestamp: July 08, 2018, 03:48:55 pm
content: Well, getting the electric system at the old building checked, and the supply impedance at the furtherest plug outlet is 0.4R. Take that with the voltage being pretty constant at 238VAC ( 200kVA transformer supply feeding this panel via 20m of 16mm copper cable and a 200A distribution fuse and a 100A main breaker) and you get a prospective fault current of over 500A, and this is a best case current, the closer sockets are higher, and you need to have a prospective fault current over 400A to blow fuses and trip breakers in the instantaneous ( single cycle of 20ms) range.Now, if you are not feeding a lot of 2.5mm cable, but instead are using 16mm cable as typical in a street lighting circuit, where you have only 10m of 1.5mm cable at most providing supply impedance ( the supply will be as close to 0.1R as you can think, just from the cable size, irrespective of the length of the cable used and that it will be fused with a 100A fuse at the point of supply) your switch on surges can be pretty high, and without inrush limiting resistors ( could be a PTC only, but better to have a combination of PTC and resistor and a damped power line inductor to take the sharper spike edges off) you can get really high current inrush.

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2018, 04:27:00 pm
content: Quote from: exe on July 08, 2018, 11:27:43 amSure, but Tim is in the US so I was assuming he was referring to a standard 15 or 20A 120V socket. I don't know what wire sizes are typically used in other parts of the world so I did the calculation based on what I'm familiar with and didn't have to go look up. Not that it really matters, I was mostly being facetious.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 08, 2018, 09:12:07 pm
content: Quote from: james_s on July 08, 2018, 04:27:00 pmI actually originally did the same calculation when I cooked off a FWB, on a 240V 50A circuit. Which had about the same length but obviously much lower wire gauge than your estimate. (If you're curious, only one diode failed in the FWB. The other one ((s) at least) started at the same initial temperature, yet survived the fault surge!)FWIW, inductance makes up about 5-10% of most circuits, IIRC. It's not all that much at 60Hz, but enough to make note of, and it's significant for elevated frequencies, like for surge protection.Tim