AC crowbar circuit - Page 1
Forum section: electronics / projects

author: ogoun
author_rank: Contributor
timestamp: February 13, 2022, 10:59:34 pm
content: Hi,More hazardous environment stuff! (ATEX and IECEx , in case anyone is searching these terms on this board for this type of stuff)I am looking at using an AC crowbar circuit on the power input to my device. Essentially a bipolar version of a DC crowbar, so that anything over 50 volts, either positive or negative (ref to the power return/ground), will crowbar the input, and blow the input fuse.The design is actually 24 volts DC in, and uses a protection circuit that isolates the rest of the device from input voltages outside of 0 to +28 volts (including negative levels).This protection circuit is good/safe with inputs up to about +/-50 volts, so the crowbar would be placed before this circuit, and is there to handle more extreme conditions, where positive or negative excursions exceed this.I considered a zener based solution, but clamping the input could still lead to a corner case where the external power supply goes high, but not high enough for the zener to conduct enough current to blow the (3 amp) input fuse quickly enough. In this case, the zener rapidly overheats, and bad stuff happens.BTW.. The device will use around 2.5 amps worst case when operating normally/within spec, and the external power supply module will be capable of providing around 20 amps.Anyway, to my question. I cannot find any reference schematics for such an AC crowbar circuit. Does anyone here have any suggestions or pointers?Oh, and assuming this crowbar works as expected, and is appropriate, I am also considering using a polyfuse for the input, but have some concerns about heat dissipation in the polyfuse while the crowbar is active, and before power to the device has been (externally) cycled..Cheers,Pete

author: bob91343
author_rank: Super Contributor
timestamp: February 13, 2022, 11:08:55 pm
content: Why reinvent the wheel? Use a pair of crowbars back to back with diode isolation such that if the peak voltage, negative or positive, exceeds the threshold, a thyristor (or two) turns on and a fuse blows.

author: ogoun
author_rank: Contributor
timestamp: February 13, 2022, 11:16:53 pm
content: Quote from: bob91343 on February 13, 2022, 11:08:55 pmThis is actually my fallback position if I can't find something that will do bipolar (triac?). Two dc crowbars just seems a bit inelegant to me, hence the original post.Cheers,Pete

author: bdunham7
author_rank: Super Contributor
timestamp: February 13, 2022, 11:18:02 pm
content: I'm sure there are many cleverer solutions, but the most obvious to me is to simply put a bridge rectifier ahead of a DC crowbar circuit.

author: bob91343
author_rank: Super Contributor
timestamp: February 13, 2022, 11:29:06 pm
content: My traditional response involves a rectifier bridge. You rectify the input and send it to a single crowbar circuit. That way the thyristor gets to short both halves of the cycle.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 14, 2022, 01:34:05 pm
content: Can a simple triac dimmer circuit be adapted to do the job?

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 01:51:57 pm
content: Quote from: ogoun on February 13, 2022, 11:16:53 pmThe difficulty here starts with finding thyristors that are rated for crowbar operation. Crowbarring slams a lot of current very suddenly through the thyristor and if one is not careful the thyristor may only ever operate once, if one is really unlucky the thyristor will give up the magic smoke before the fuse does. There are some thyristors that have been designed and are specifically rated for crowbarring. Finding them is another matter, and usually means reading a lot of datasheets of candidates until you find one with the magic words in.The difficulty for triacs is even higher. They are not typically used for higher powers anyway, and they are not typically used for crowbarring. So finding one that you can convince yourself from the datasheet will tolerate the current pulses involved in crowbarring is going to be even more of an uphill struggle than finding suitably rated thyristors.In the case of either device you're looking for a device that has an I2t rating, and if it has, has an I2t rating significantly greater than that of the fuse it is intended to blow.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 14, 2022, 02:04:29 pm
content: Quote from: Cerebus on February 14, 2022, 01:51:57 pmCurrent limiting resistor in series? It won't have to run for long, so could be underdimensioned.

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 02:17:14 pm
content: Quote from: SilverSolder on February 14, 2022, 02:04:29 pmErm, if you limit the current (and time) to where you don't exceed the I2t capabilities of the thyristor/triac then you also limit the I2t pulse on the fuse and it won't blow. Whichever way you cut it, the I2t of the shorting device has to exceed the I2t rating for the fuse. Given that semiconductors tend to be more fragile than than fuses (as anyone who has been misguided enough to try and protect a semiconductor with an FF or even FFF fuse will tell you) you're on a hiding to nothing trying to limit the current. Also time is of the essence here, you want the crowbar to act before anything downstream of the PSU can be damaged.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 14, 2022, 02:40:25 pm
content: Quote from: Cerebus on February 14, 2022, 02:17:14 pmI see what you mean, you kind of want the fuse to be the current limiting resistor...So there is no hope of getting something like this to work?

author: bob91343
author_rank: Super Contributor
timestamp: February 14, 2022, 06:01:08 pm
content: That looks as though it will only trigger on a positive transient.

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 06:28:06 pm
content: Triacs trigger in all four quadrants.

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 06:32:54 pm
content: Quote from: SilverSolder on February 14, 2022, 02:40:25 pmThe OPs requirement is for a crowbar that triggers for both polarities but in a DC supply. That diagram is more a circuit that is intended to trigger on alternate cycles (e.g. dimmer, motor controller). Think what happens for DC that might go overvoltage either way, but will provide a positive supplyfor a long time. (Hint: What will happen to the voltage on the capacitor.) Anyway, DIACs are almost certainly not precise enough.

author: bdunham7
author_rank: Super Contributor
timestamp: February 14, 2022, 06:33:44 pm
content: Quote from: Cerebus on February 14, 2022, 06:28:06 pmMany, but not all of them do. But the main problem I see here is that this circuit will reset itself each cycle and not retrigger until the next cycle exceeds the threshold. That might not be ideal.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2022, 06:36:48 pm
content: You need beefy thyristors, like Littelfuse S8055W, which can handle a little bit of power, and might survive longer than the fuse needs to blow. So l long as the fuse needs lower than 1750A2s to blow, and the peak current is under 550A, it will survive, and if the fuse takes 10 cycles to clear then the current should be below 300A. Looks like even a regular slow blow fuse will work with these as a mains clamp, though your gate current will have to be over 40ma to make sure it turns on fast enough.Just use an even beefier bridge rectifier to only need one trigger, or duplicate it with 2 identical circuits back to back instead. Thyristors are almost invariably more robust than the equivalent triac, though you only get a single quadrant trigger on them instead of all 4.

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 06:54:46 pm
content: Quote from: bdunham7 on February 14, 2022, 06:33:44 pmReally? Name a part number of a triac that does not trigger in all four quadrants so we can all learn something. As far as I know it's an intrinsic property of a triac. The sensitivity in all four quadrants is not the same (QIV is the least sensitive) but they trigger in all.

author: bdunham7
author_rank: Super Contributor
timestamp: February 14, 2022, 07:09:29 pm
content: Quote from: Cerebus on February 14, 2022, 06:54:46 pmhttps://asset.conrad.com/media10/add/160267/c1/-/en/001113558DS01/datasheet-1113558-nxp-semiconductors-bta312y-600c127-thyristor-scr-triac-to-220ab-12-a-600-v.pdf

author: SilverSolder
author_rank: Super Contributor
timestamp: February 14, 2022, 07:47:11 pm
content: Quote from: Cerebus on February 14, 2022, 06:32:54 pmThe capacitor is just there to suppress noise - it is meant to follow the AC sine wave. With DC, it would charge up to the DC level and stay there - and the DIAC won't fire no matter how long the DC is there.If the DC goes over 32V (or below -32V), or if AC with > 32V peaks is present, the DIAC will fire the TRIAC and hopefully burn the fuse.So, this circuit should protect against both AC and DC overvoltage?If the DIAC precision is not good enough, it could be replaced by back-to-back Zeners?

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 08:00:29 pm
content: Quote from: bdunham7 on February 14, 2022, 07:09:29 pmAh, I see. You're thinking so called "3 quadrant" triacs don't trigger in QIV, They do. All triacs are less sensitive in QIV, so called "3 quadrant" triacs are just doped to make that characteristic more extreme. Hit them with a big enough gate pulse in QIV and they will trigger.

author: bdunham7
author_rank: Super Contributor
timestamp: February 14, 2022, 08:05:32 pm
content: Quote from: SilverSolder on February 14, 2022, 07:47:11 pm24VAC (sine) would have ~34 volt peaks, so you'd definitely need something other than the DIAC. Other that that, yes it will work but if the fuse doesn't blow in 1/4 cycle, it may get ugly.However, if I understood him, the OP is using a 24VDC input and only asked about an AC crowbar to cover AC or reverse polarity mishaps. He could just use a DC crowbar plus a reverse-polarity shunt diode.

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 08:20:14 pm
content: Quote from: SilverSolder on February 14, 2022, 07:47:11 pmYou could get it all to work, but for aforementioned reasons, it's really not worth the trouble figuring out the niceties of doing it with triacs (unless one is minded to do it as an academic exercise) - thyristors are really the way to go.Diacs are essentially the same as zeners, so yes you could, it's just that for most diac uses they don't need to be all that as closely specified so they aren't.On the capacitor side, I was thinking more that (1) this will charge to rail voltage, then, (2) in the case of a negative transient it will take time to discharge and once it has will it pass enough gate trigger current quickly enough to fire the triac in time? You'd have to run the numbers to find out but it looks potentially problematic. If nothing else, it makes the response asymmetric and that might be problematic in itself.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 14, 2022, 08:51:14 pm
content: Quote from: Cerebus on February 14, 2022, 08:20:14 pmI thought a TRIAC can be thought of as two back-to-back thyristors - is that a mental shortcut too far?

author: Cerebus
author_rank: Super Contributor
timestamp: February 14, 2022, 09:10:13 pm
content: Quote from: SilverSolder on February 14, 2022, 08:51:14 pmPossibly, possibly not, as a mental shortcut it's probably harmless, but the practical issue is the one I brought up earlier, of finding devices with aspecifiedI2t rating so they you don't end up "using semiconductors to protect the fuse". The currents in even a small crowbar firing can be huge, and it happens quickly becuase that's the whole point of a crowbar, to get rid of the overvoltage before it can do damage (said damage probably to semiconductors). Youcanfind thyristors with a specified I2t rating, (although more often than not parametric search tools don't have that parameter available so it can be an uphill struggle) and there may be some triacs with an I2t rating on their datasheets but I've never seen one.

author: ogoun
author_rank: Contributor
timestamp: February 15, 2022, 03:21:19 am
content: Thanks for the reminder about I2t.. I will look for something suitable.So at this stage I have a fuse in both legs of the power input, followed by a schottky(for minimising power dissipation due to (2xVf)×If ) bridge (80v rated), followed by a 70 volt tvs(to clobber fast spikes that might kill the bridge), followed by an scr based crowbar, with delayed action (so narrow spikes dont blow the fuse), crowbarring at around 50 volts. Then the surge gobbler cct, and then 24 volts nominal to power the device.I am also considering a 60 watt isolating converter immediately after the surge gobbler, to make compliance testing a bit easier, since if the converter has 1500vrms (equiv to 2250vdc) isolation or greater, the iso barrier is considered by the standard to be "infallible".Regarding thyristors, littelfuse's Sidactor range look pretty good.. one component crowbar devices, so long as the app requirements match what they make (not sure they are close enough for me)Cheers,Pete

author: ogoun
author_rank: Contributor
timestamp: February 15, 2022, 04:32:41 am
content: FYI..The Littelfuse Sidactor part that I was looking at is P0640FNL. They make a range of these.. Looks like they are often paired in series with a MOV, and used on AC mains ccts, maybe similar to a gas arrestor in operation...The surge gobbler I have been referring to is the AD (Linear Tech) part LT4356.. Looks like a nice piece of siliconI am pretty much resigned to using a bridge in my design, even though I would have liked to eliminate the power loss due to the bridge's VfxIf. Dumping heat in a haz enviro design is rigorous..Cheers,Pete

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2022, 04:38:55 am
content: Quote from: ogoun on February 15, 2022, 04:32:41 amYou don't need to put the normal operating current through the bridge, you can just have the bridge feeding the crowbar.

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2022, 04:47:14 am
content: Quote from: SilverSolder on February 14, 2022, 08:51:14 pmYes, it is going too far if you start to think about gate drive. A back-to-back SCR setup requires that the gate trigger polarity alternate with the supply, IOW operate in quadrants 1 and 3. That works with some designs like the diac circuit you posted, but won't work with logic-driven designs that use the same gate drive polarity for either polarity of supply, IOW operate in quadrants 2 and 3 (typically).

author: ogoun
author_rank: Contributor
timestamp: February 15, 2022, 05:29:52 am
content: Quote from: bdunham7 on February 15, 2022, 04:38:55 amNow this is a great example of why I love this forumWhat a good solution to this problem!Cheers,Pete