Jim Thompson wrote:> On Sat, 01 Dec 2012 15:49:26 -0800, Joerg <invalid@invalid.invalid> > wrote: > >> Jim Thompson wrote: >>> On Sat, 01 Dec 2012 13:35:06 -0800, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> Fred Abse wrote: >>>>> On Fri, 30 Nov 2012 14:17:47 -0800, Joerg wrote: >>>>> >>>>>> #3 is the only option I ever consider. Personally I like to have the SCR >>>>>> right at the circuit side of the fuse. If the fuse were at the input and >>>>>> the SCR were at the output you'd have the upper FET and the inductor in >>>>>> the path. This will slow down the fuse tripping and there is a chance that >>>>>> the FET grenades before the fuse trips, something that is generally not >>>>>> desired. >>>>> Seconded. >>>>> >>>> Hey, we have the absolute majority in the house now :-) >>> Sounds like a couple of amateurs when it comes to computing I^2*t >:-} >>> >> Huh? >> >> This works, and fast, done it many times. One has to make sure that the >> SCR triggers with gusto and is big enough. > > I misconscrewed you as the one saying "inductor"... or maybe not >:-} >That was my point, ideally you don't want to have the inductor in the path. -- Regards, Joerg http://www.analogconsultants.com/
Over Voltage Protection Crowbar Circuit
Started by ●November 30, 2012
Reply by ●December 1, 20122012-12-01
Reply by ●December 2, 20122012-12-02
On Sat, 01 Dec 2012 10:35:14 -0800, mike <ham789@netzero.net> wrote:>On 12/1/2012 5:22 AM, legg wrote: >> On Fri, 30 Nov 2012 14:43:08 -0800, mike<ham789@netzero.net> wrote: >><snip>>>> >>> Is this a dedicated supply where you KNOW what it's gonna be hooked to >>> in EVERY case? >>> There are times when it's not a good idea for the output to short. >>> Like when you're charging a battery. >> >> With the battery attached, the overvoltage condition would be >> difficult to detect, until the battery was toast. A current regulator >> failure in this condition would likely blow the fuse without external >> aid. >> >> If it's a back-up battery, it should be connected into the circuit in >> such a manner that source failure (including the shorted condition) >> doesn't interfere with the intended back-up function. >> >> There ARE issues with scr failure, when improperly sized for large >> storage capacity and supply/fuse combinations. Parts intended for >> crowbar applications have I^2t ratings that are considerably larger >> that jellybean devices. MCR67,68 and 69 were examples of parts >> designed for this use. The latter two are still available. >> >> RL > >I spent a significant portion of my career trying to train that kind >of thinking out of engineers. > >Some engineers leap directly to telling you why your example is wrong. >Many times, I've had to go back and fix their designs. > >The ones you want working on your project use the example to consider >what real users might do with the product and come up with a >more robust design. > >I don't remember the exact one, but I once had a commercial bench >power supply that >I wanted to use for charging batteries. Turns out, if you turned >off the power switch before disconnecting the battery, it made >a LOT of smoke.I don't follow your reasoning. Is it the battery back-up, overvoltage protection method, or the power supply design you're questioning? The subject is crowbar ovp methods. You're point was the ovp condition occurring in a battery charging situation. I think was addressed. Power supply compatibility in parallel redundant or backup applications is a subject in itself. Misapplication of products, or their immunity to such conditions can also be an interesting discussion, along with the sad state of the equipment and the amazement or frustration of the misapplicator. Rest assured that you won't see a product that wasn't immune to the application of external voltages occupy or originate in my work area. This is, in fact, a crude test method to confirm the function of OVP circuitry, though alternative methods exist that may perform the same function automatically and with reduced hardware, in both design and in production test. I do not, however, assume this capacity in any equipment that is unfamiliar to me, until proven otherwise. The crowbar method of protection isn't always used, and it's external application in any black box situation should probably be evaluated. Your battery charging or back-up situation isn't exactly a black box, so you should be able to get your mind around it, with a little effort. There's no reason why a commercial bench power supply should smoke on application of external voltage, within it's normal output compliance, save the naivete of it's designer, or the complaisance of it's purchaser. RL
Reply by ●December 2, 20122012-12-02
On Fri, 30 Nov 2012 14:17:47 -0800, Joerg <invalid@invalid.invalid> wrote:>panfilero wrote: >> I'm needing an over-voltage protection circuit to put at the output >> of a switching regulator that takes in 100V input and drops it to >> 24V... I'd like to protect the output from going over 28V. >> >> Here's the 3 options I was considering: For all the options the >> point is to blow my 5A fast acting fuse that's at the input, so I'm >> trying to decide on a way to short my output >> >> 1. A Zener - I'm thinking these get rather bulky for higher current >> handling, and they're not very accurate >> >> 2. A TVS - Probably better than the Zener option as far as current >> handling and bulkiness... I think TVS kills Zener for this >> application >> >> 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate >> trip voltage and I think I could probably find a decently sized SCR >> that could handle the current required to make the fuse pop >> >> Anybody have any thoughts or recommendations for OVP circuits like >> this... I'm likeing the SCR option right now >> >> much thanks! > > >#3 is the only option I ever consider. Personally I like to have the SCR >right at the circuit side of the fuse. If the fuse were at the input and >the SCR were at the output you'd have the upper FET and the inductor in >the path. This will slow down the fuse tripping and there is a chance >that the FET grenades before the fuse trips, something that is generally >not desired.Applying the crowbar to the output is the most effective way of reducing potential damage to the load - which is it's intended function. The end user would usually prefer that your power supply turn itself into rubble, before any harm can come to the load. Best to test the effectiveness of any protection method, as this has priority. A grenading fet would only pass single-fault abnormals in an enclosure (if isolation barriers are not compromised in the process) and is best avoided by design. In many cases (though not in this simple buck example) the presence of an isolation barrier will alter design considerations for ovp protection. RL
Reply by ●December 2, 20122012-12-02
On Sat, 01 Dec 2012 15:47:25 -0800, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>I keep a bench supply at home and up in the cabin, mostly for charging dead car >batteries. Most "modern" car battery chargers refuse to put any current into a >zero-volt battery, but a bench supply will.<http://802.11junk.com/jeffl/pics/drivel/slides/dead-battery-chargers.html> What's wrong with this picture? Hint: All but one of the chargers are dead. There were various possible failure modes, but mostly, they were from inability to distinguish between a depleted battery, and one that might turn into a hot battery acid belching volcano. While the chargers seem to be fairly well protected against charging a dead short, they're not so good at dealing with one shorted cell out of six cells. They just continue to pump power into the battery until the electrolyte boils. I'm not sure exactly what failed, but I did find shorted diodes in one charger, and a burnt hole where the controller chip had previously lived in another. The whole pile went to the recyclers. I switched out all the above chargers and about 5 more with Xantrex/StatPower chargers and have lived happily ever after since. No more hot acid volcanoes.>I think the car supply places sell >the "smart" chargers so that people will think their batteries won't charge, so >the auto supply crooks\\\\\ people get to sell people a charger *and* a new >battery.Good conspiracy theory, but I don't think that's the reason. Litigation avoidance seems a better excuse. If there's any possibility of the charger starting a fire, or in any way damaging a customers equipment or facility, the charger should shut down. Charging a nearly "dead" battery is a tricky borderline case, where the battery could be either severely depleted, or suffer from a shorted cell. The way it's usually handled is to trickle charge such a low terminal voltage battery. If it doesn't recover, then nothing is lost and no damage is done by the low current charge. If it recovers nearly normal voltage, then the charger will switch to a higher current charge. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●December 2, 20122012-12-02
On Sat, 01 Dec 2012 16:49:06 -0800, Joerg <invalid@invalid.invalid> wrote:>Jim Thompson wrote: >> On Sat, 01 Dec 2012 15:49:26 -0800, Joerg <invalid@invalid.invalid> >> wrote: >> >>> Jim Thompson wrote: >>>> On Sat, 01 Dec 2012 13:35:06 -0800, Joerg <invalid@invalid.invalid> >>>> wrote: >>>> >>>>> Fred Abse wrote: >>>>>> On Fri, 30 Nov 2012 14:17:47 -0800, Joerg wrote: >>>>>> >>>>>>> #3 is the only option I ever consider. Personally I like to have the SCR >>>>>>> right at the circuit side of the fuse. If the fuse were at the input and >>>>>>> the SCR were at the output you'd have the upper FET and the inductor in >>>>>>> the path. This will slow down the fuse tripping and there is a chance that >>>>>>> the FET grenades before the fuse trips, something that is generally not >>>>>>> desired. >>>>>> Seconded. >>>>>> >>>>> Hey, we have the absolute majority in the house now :-) >>>> Sounds like a couple of amateurs when it comes to computing I^2*t >:-} >>>> >>> Huh? >>> >>> This works, and fast, done it many times. One has to make sure that the >>> SCR triggers with gusto and is big enough. >> >> I misconscrewed you as the one saying "inductor"... or maybe not >:-} >> > >That was my point, ideally you don't want to have the inductor in the path.Presence of the inductor will not reduce the effectiveness of ovp crowbar protection that is located on the output terminals. Neither will a grenading fet. RL
Reply by ●December 2, 20122012-12-02
On Sat, 01 Dec 2012 21:46:56 -0800, Jeff Liebermann <jeffl@cruzio.com> wrote:>On Sat, 01 Dec 2012 15:47:25 -0800, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>I keep a bench supply at home and up in the cabin, mostly for charging dead car >>batteries. Most "modern" car battery chargers refuse to put any current into a >>zero-volt battery, but a bench supply will. > ><http://802.11junk.com/jeffl/pics/drivel/slides/dead-battery-chargers.html> >What's wrong with this picture? Hint: All but one of the chargers >are dead. > >There were various possible failure modes, but mostly, they were from >inability to distinguish between a depleted battery, and one that >might turn into a hot battery acid belching volcano. While the >chargers seem to be fairly well protected against charging a dead >short, they're not so good at dealing with one shorted cell out of six >cells. They just continue to pump power into the battery until the >electrolyte boils. I'm not sure exactly what failed, but I did find >shorted diodes in one charger, and a burnt hole where the controller >chip had previously lived in another. The whole pile went to the >recyclers. > >I switched out all the above chargers and about 5 more with >Xantrex/StatPower chargers and have lived happily ever after since. No >more hot acid volcanoes. > >>I think the car supply places sell >>the "smart" chargers so that people will think their batteries won't charge, so >>the auto supply crooks\\\\\ people get to sell people a charger *and* a new >>battery. > >Good conspiracy theory, but I don't think that's the reason. >Litigation avoidance seems a better excuse. If there's any >possibility of the charger starting a fire, or in any way damaging a >customers equipment or facility, the charger should shut down.A charger is supposed to put current into a battery.> >Charging a nearly "dead" battery is a tricky borderline case, where >the battery could be either severely depleted, or suffer from a >shorted cell. The way it's usually handled is to trickle charge such >a low terminal voltage battery. If it doesn't recover, then nothing >is lost and no damage is done by the low current charge. If it >recovers nearly normal voltage, then the charger will switch to a >higher current charge.I had a zero volt battery, some light left on for a weekend or something. I went to Kragen Auto Parts and bought a charger, and it wouldn't even try. Zero output measured current. I took it back and they said the battery was destroyed and that I needed a new one. So I hacked a charger: an old DSL power transformer, a diode, and a belt sander as a current-limiting resistor. After a couple of hours it started up, and the battery was fine. Now I keep a bench power supply around. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●December 2, 20122012-12-02
On Sat, 01 Dec 2012 22:14:05 -0800, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>On Sat, 01 Dec 2012 21:46:56 -0800, Jeff Liebermann <jeffl@cruzio.com> wrote:>><http://802.11junk.com/jeffl/pics/drivel/slides/dead-battery-chargers.html>(...)>>Good conspiracy theory, but I don't think that's the reason. >>Litigation avoidance seems a better excuse. If there's any >>possibility of the charger starting a fire, or in any way damaging a >>customers equipment or facility, the charger should shut down. > >A charger is supposed to put current into a battery.Sure, but how is it suppose to tell the difference between a depleted but chargeable battery and a battery with a shorted cell? Without some manner of ESR measuring algorithm, they look the same to a commodity charger. The problem is that putting current into a shorted cell is the same as a room heater.>I had a zero volt battery, some light left on for a weekend or something. I went >to Kragen Auto Parts and bought a charger, and it wouldn't even try. Zero output >measured current. I took it back and they said the battery was destroyed and >that I needed a new one.Then the battery charger controller did it's job. It protected itself from a potential hazard. If it were a more sophisticated charger, it might have started with a trickle charge, which should have brought the terminal voltage up to the point where the charger would be willing to charge it. Incidentally, Kragen is where most of those dead chargers were purchased. Returning it was the right thing to do.>So I hacked a charger: an old DSL power transformer, a diode, and a belt sander >as a current-limiting resistor. After a couple of hours it started up, and the >battery was fine.I had a similar problem on a mountain top radio site, where the available resources tend to be rather limited. So, I dragged the battery out to my truck, and put it in parallel with truck battery using jumper cables. I did NOT run the engine. Judging by the sparks, there was quite a bit of current flowing. After about 2 minutes, the terminal voltage was high enough that the Schumacher(?) marine charger would agree to charge it normally. A trickle charge would have been better, but I didn't want to wait. I like the belt sander as a resistor trick. However, it won't work with a variable speed controller or solid state switch in the belt sander.>Now I keep a bench power supply around.Don't forget a series protection diode. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●December 2, 20122012-12-02
"Jim Thompson" wrote in message=20 news:pciib85mttaob0g1ur0frlafu2d98baam8@4ax.com...> On Fri, 30 Nov 2012 13:54:33 -0800 (PST), "langwadt@fonz.dk" > <langwadt@fonz.dk> wrote:>> why not something like this: >> =http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf=>> no need for fuses then> Yep. I always like my own circuits >:-}> The fuse blower design was done before MOSFET's were common.I see some problems with that circuit. The IRLM6401 is only rated at 12V = and=20 the gate only +/-8V. Of course this is apparently for a low voltage = battery,=20 probably 6V, considering the 3.3 ohm load. M1 seems to be connected so = that=20 its internal diode normally conducts, although the 0V biased gate would = turn=20 it on. I think there should be a resistor on the gate. The shunt = regulator=20 DZ1 is set to 5.2V, so I see this may be for a 5V logic supply. I see = that a=20 voltage over about 5.8V will start to shunt the Vgs of M2 and turn it = off=20 when it becomes less than about 0.6 to 0.95 V gate threshold. But it = still=20 does not seem to provide any voltage regulation to the load, and with = 0.05=20 ohms or each MOSET at 1.5A loses 150 mV. If the voltage is only = marginally=20 high, or if the load increased, there could be about 2A and 1 or 2 volts = on=20 the MOSFET which would exceed its power dissipation of 1.3W. I don't see what is so great about this circuit. It does not have = voltage=20 regulation or current limiting and it does not have any hysteresis to = keep=20 it from entering a destructive linear range or possibly oscillate. The = SCR=20 crowbar with a fuse (or maybe a polyswitch device) is more robust and = far=20 simpler. Maybe you or someone can explain the merits of your circuit. Paul=20
Reply by ●December 2, 20122012-12-02
"legg" wrote in message = news:elmlb8h64pf5q2dgsjr1tbhaahhelhqcm2@4ax.com...> There's no reason why a commercial bench power supply should > smoke on application of external voltage, within it's normal > output compliance, save the naivete of it's designer, or the > complaisance of it's purchaser.There is not that much difference between charging a battery with a = bench=20 supply, or charging a large capacitor in a circuit. I have a large = capacitor=20 that I put across my bench supply so that the circuit I'm using it on = can=20 draw current surges much higher than the current limit. I haven't had a=20 problem if I leave the capacitor connected and turn off the supply. Paul=20
Reply by ●December 2, 20122012-12-02
On 12/1/2012 9:14 PM, legg wrote:> On Sat, 01 Dec 2012 10:35:14 -0800, mike<ham789@netzero.net> wrote: > >> On 12/1/2012 5:22 AM, legg wrote: >>> On Fri, 30 Nov 2012 14:43:08 -0800, mike<ham789@netzero.net> wrote: >>> > <snip> >>>> >>>> Is this a dedicated supply where you KNOW what it's gonna be hooked to >>>> in EVERY case? >>>> There are times when it's not a good idea for the output to short. >>>> Like when you're charging a battery. >>> >>> With the battery attached, the overvoltage condition would be >>> difficult to detect, until the battery was toast. A current regulator >>> failure in this condition would likely blow the fuse without external >>> aid. >>> >>> If it's a back-up battery, it should be connected into the circuit in >>> such a manner that source failure (including the shorted condition) >>> doesn't interfere with the intended back-up function. >>> >>> There ARE issues with scr failure, when improperly sized for large >>> storage capacity and supply/fuse combinations. Parts intended for >>> crowbar applications have I^2t ratings that are considerably larger >>> that jellybean devices. MCR67,68 and 69 were examples of parts >>> designed for this use. The latter two are still available. >>> >>> RL >> >> I spent a significant portion of my career trying to train that kind >> of thinking out of engineers. >> >> Some engineers leap directly to telling you why your example is wrong. >> Many times, I've had to go back and fix their designs. >> >> The ones you want working on your project use the example to consider >> what real users might do with the product and come up with a >> more robust design. >> >> I don't remember the exact one, but I once had a commercial bench >> power supply that >> I wanted to use for charging batteries. Turns out, if you turned >> off the power switch before disconnecting the battery, it made >> a LOT of smoke. > > I don't follow your reasoning.You're sounding a bit snippy, but let's see if we can wrap our heads around it.> > Is it the battery back-up,I never mentioned backup. overvoltage protection method, or the power> supply design you're questioning?Overvoltage protection IS part of the power supply design. The subject is crowbar ovp methods.> You're point was the ovp condition occurring in a battery charging > situation. I think was addressed.My point was that people use power supplies in ways that don't show up on a spec sheet. Having a stiff voltage supply hooked across the output is a proxy for one type of potential application. It's important for designers to consider what conditions might occur and do whatever is reasonable to prevent failure of the supply or destruction of the DUT. Transient conditions inside the power supply are part of the analysis. In particular, it's easy to design a circuit that crowbars the output when it reaches 28V. What's not so obvious is when the output is held at 24V by a battery or a big bank of caps, or back EMF from a motor when the input power glitches. Anything that triggers the SCR while the internal power supplies are dropping out can cause problems.> > Power supply compatibility in parallel redundant or backup > applications is a subject in itself. Misapplication of products, or > their immunity to such conditions can also be an interesting > discussion, along with the sad state of the equipment and the > amazement or frustration of the misapplicator.Well, misapplicators don't realize that they're misapplicators. It's the designer's job to protect them from themselves. If you do nothing but meet the numbers on the spec sheet, you're not doing enough.> > Rest assured that you won't see a product that wasn't immune to the > application of external voltages occupy or originate in my work area. > This is, in fact, a crude test method to confirm the function of OVP > circuitry, though alternative methods exist that may perform the same > function automatically and with reduced hardware, in both design and > in production test.Not clear exactly what you mean. If you are also testing shutdown transients due to power line fluctuations with external voltage applied, I'd agree.> > I do not, however, assume this capacity in any equipment that is > unfamiliar to me, until proven otherwise.It's not about you. It's about designing robust products. Users are very creative with unintended applications. My experience from design reviews is that designers tend to have tunnel vision. A skilled designer with ability to see the big picture is rare. I used to build test equipment for a living. In many a design review, I witnessed a circuit that probably met spec, but would have been a failure in the hands of a typical user. Hanging a SCR across the output without serious consideration of all the ways it could get triggered would qualify in that respect.> > The crowbar method of protection isn't always used, and it's external > application in any black box situation should probably be evaluated. > > Your battery charging or back-up situation isn't exactly a black box, > so you should be able to get your mind around it, with a little > effort.Again, it's not about me or my application. It's a caution that an SCR crowbar can be very effective or a ticking time bomb if you're not paying attention. Designers sometimes don't pay attention. We all like to think that we're the world's greatest engineer. But the sad fact is that half of us are below average. The cutoff point for incompetent is well above average.> > There's no reason why a commercial bench power supply should smoke on > application of external voltage, within it's normal output compliance, > save the naivete of it's designer, or the complaisance of it's > purchaser.As sentences with no content go, that's a good one. I'd have to agree. Power supplies are invulnerable unless they aren't.> > RL
