Resettable short circuit protection circuit that really works? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: October 07, 2024, 03:49:31 pm
content: Searching the web there are lots of similar looking theoretical op-amp based "should work" solutions, but no actual "proven to work without oscillating" solutions.Does anyone have a circuit that has been proven to work in real world usage?My particular need is to protect a 9v DC pool chlorinator electrolysis cell that draws 15A at the recommended salt concentration. Periodically you have to add salt to compensate for losses and this is a tricky operation as adding too much salt too quickly without giving it a chance to mix properly can put pockets of concentrated saline through the cell with a consequent jump in current that the rectifier and transformer aren't designed to handle.You need to run the chlorinator during this operation as you are using the current draw as the primary indicator of salinity.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 07, 2024, 03:55:09 pm
content: Does it need to turn off, or just limit to a max?Current Limiting is the game.https://electronics.stackexchange.com/questions/262088/limiting-current-without-dropping-voltage

author: e100
author_rank: Frequent Contributor
timestamp: October 07, 2024, 04:27:56 pm
content: Quote from: Randy222 on October 07, 2024, 03:55:09 pmAt the most basic level it just needs to trip off at some adjustable threshold.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 07, 2024, 04:56:32 pm
content: Maybe Littel Fuse PTC RGEF series? PTC is ideal, you won't need powered circuitry to get the protection.https://www.littelfuse.com/products/polyswitch-resettable-pptcs/radial-leaded/rgef.aspx

author: fourfathom
author_rank: Super Contributor
timestamp: October 07, 2024, 05:40:26 pm
content: I don't have an answer for you, but do have questions and comments:Do you want it to recover from overcurrent without requiring a power-cycle?If so, the PTC devices may not work for you as they will stay in the "tripped" (high resistance) state until the current is dramatically reduced. Also, the range trip time and level is somewhat loose, so probably not best if you want a tight limit. I've used these for short-circuit protection and been very happy with the behavior, but if you want to pass 9A and shut down at 10A you should look elsewhere.A regular current-limit circuit could be your answer, but it obviously has to dissipate the power, or be a fancy switching regulator design.I designed one protection circuit that rapidly shut down the power when the current limit was reached, then retried a few times per second. For this to work, your system needs to tolerate the high-current transients. I'm not suggesting that you build one of these, but you should consider your options. There are bi-metallic thermal fuses that behave in this manner (with a slow cycle rate) -- my vacuum cleaner has one of these in the powered beater-brush head. Would that work?

author: e100
author_rank: Frequent Contributor
timestamp: October 07, 2024, 06:44:09 pm
content: Quote from: fourfathom on October 07, 2024, 05:40:26 pm'Press a button' seems like the simplest solution as the person adding the salt stays until a stable salinity level has been achieved.Quote from: fourfathom on October 07, 2024, 05:40:26 pmPTC and other thermal devices are sensitive to the ambient temperature. For example, a PTC setup for winter conditions will have a 30% lower trigger threshold at the height of summer which increases the chance of a nuisance trip, and as you said, once tripped it'll stay that way for several hours until the timer turns everything off in the evening.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 07, 2024, 08:15:35 pm
content: Quote from: e100 on October 07, 2024, 03:49:31 pmSo there is a current meter the user can see ?Rather than a button, you could make a hiccup circuit, that flips in a series power resistor, and continues to do that at some slow duty cycle, until the trip peak reduces.

author: nali
author_rank: Frequent Contributor
timestamp: October 07, 2024, 08:24:25 pm
content: Sounds like a job for an e-fuse?E.g.https://www.ti.com/lit/ds/symlink/tps25984.pdf?ts=1728273033150&ref_url=https%253A%252F%252Fwww.ti.com%252Fpower-management%252Fpower-switches%252Fefuse-hotswap-controllers%252Fproducts.htmlI've used these in smart meters, which must provide a DC output to a Comms Hub, which must trip off then retry in the case of an overload.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 07, 2024, 08:26:23 pm
content: If you want active and very accurate current protection, you op-amp a sense resistor.Inline the sense resistor (heavy metal loop type that can handle 15A, etc) with the load and if AC do it on the gnd side leg, each side of the sense resisor to the op-amp inputs. You adjust amp gain depending on the output stage logic or silicon control. As example, the output of the amp can drive gate of a pFET, when the output of the amp goes high (use a pull down resistor on the amp output, 1k-10k) , when amp goes high the pFET will turn off. The fet or igbt needs to be able to handle the 15A, so one with low 'on' resistance (RDS) is better.Edit: for clarity, my example needs a 2nd opamp as comparator and a voltage reference on input to comparator. This way you can fine tune the threshold at which comparator turns on or off the current. The 1st amp feeds the other input to the comparator, etc.

author: e100
author_rank: Frequent Contributor
timestamp: October 08, 2024, 07:09:25 am
content: Quote from: nali on October 07, 2024, 08:24:25 pmThings are a bit complicated as the chlorinator power supply is simply a transformer and rectifier with no smoothing.All the e-fuse devices I've seen derive their operating power from the supply being monitored so in this case there will be a dead zone from 0 to about 2.5v on every half cycle where the device isn't able to run.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 08, 2024, 07:24:44 am
content: Wouldn't it be easiest in this case if you get a more lossy transformer that is able to have it's output shorted?

author: e100
author_rank: Frequent Contributor
timestamp: October 08, 2024, 07:50:10 am
content: Quote from: PCB.Wiz on October 07, 2024, 08:15:35 pmGood idea, but that feature needs to be built on top of a reliable circuit that can do that switching without having oscillation problems, hence my search for a proven design.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 08, 2024, 07:58:41 am
content: Quote from: e100 on October 08, 2024, 07:50:10 amIt is not trying to regulate the current, there is no linear region to oscillate.It does a time-delay hiccup, where it switches in a (large size) load resistor.You could even use a relay here, if you want proven and simple. It only cycles when someone is there, mixing it not quite right ?

author: e100
author_rank: Frequent Contributor
timestamp: October 08, 2024, 08:45:19 am
content: Quote from: PCB.Wiz on October 08, 2024, 07:58:41 amWhat information are you displaying to the user?I could imagine an on/off switching system that runs at a 1% duty cycle so you only pass 1 out of every 100 half cycle pulses and capture the peak current and display that to the user once per second.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 08, 2024, 09:06:16 am
content: Quote from: e100 on October 08, 2024, 08:45:19 amI was assuming you had a meter display already ?It could be as simple as a reed switch with a few turns of current wire around it, and a heavy relay and resistor + cap for timingReeds look to be available with about 10% AT tolerance so you adjust current trip via turns and maybe some magnetic steering.Once high enough, the reed flips to charge the cap across the relay, which then flips the relay to ballast resistor, which opens the reed.The relay stays flipped until the voltage drops below the relay holding voltage, then it releases. If current is still too high, it repeats.Charge time is set by cap + resistor and discharge time is set by relay coil resistance and cap.An LED + Res across the relay coil could indicate this working, although the relay clicks and current meter dips may be enough.

author: tom66
author_rank: Super Contributor
timestamp: October 08, 2024, 09:25:53 am
content: Quote from: e100 on October 08, 2024, 07:09:25 amThen you want a device with an external FET. You would run the IC from a local semi-regulated supply (a half-wave rectifier and a small cap would work, maybe add an LDO if you want) whilst passing the unregulated supply through the FET and current sense circuit.https://www.ti.com/product/TPS24771I expect this would work OK, since the device only cares about the current passing through the sense resistor, but I haven't tested it. TI may have dev kits available to experiment with though some surgery may be needed to change what supplies Vdd.You would set the fault timer up to be around the order of a few seconds by changing the fault timer capacitor, which would cause the device to periodically retry until the short circuit is removed.

author: e100
author_rank: Frequent Contributor
timestamp: October 08, 2024, 10:15:03 am
content: Quote from: PCB.Wiz on October 08, 2024, 09:06:16 amThe existing current meter is a basic moving coil meter that displays the average (RMS?) current.I'm not an engineer and this is the first time I've heard of a hiccup circuit. Is your magnetic breaker system with a time delay going to work with the unsmoothed lumpy waveform of the rectified AC chlorinator power supply?

author: Zero999
author_rank: Super Contributor
timestamp: October 08, 2024, 10:30:51 am
content: Use a magnetic circuit breaker. Don't bother trying to roll your own. It will comply with the safety standards and a home made circuit won't.The transformer should also have a primary side breaker.It is also a good idea to add bimetalic switch to the transformer, to cut the power if it overheats. Fix it to the core (tap holes or glue with thermal epoxt resin) and wire it in series with the primary.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 08, 2024, 04:41:39 pm
content: Sense resistor, one op-amp, one comparator, one voltage reference, one Omron G3NA SSR.When current gets to the threshold, this setup will go into oscillation, because when the current gets cut the amp output will drop to zero, SSR will again turn on.It's basically a current clamp, so even if you get a short ckt it will clamp to the set max amps. You can control the oscialltion using a resistor and cap.The G3NA series has app note for DC loads.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 08, 2024, 08:01:25 pm
content: Quote from: e100 on October 08, 2024, 10:15:03 amGood question.If this is raw rectified that will cause problem in a simple circuit.Whilst you can with care use reed switches with pulsed fields, that use intends the pulse to remain above some holding level, so it can work on a simple ON/OFF case, but here you have a widely vasrying current that sweeps up and down with the mixture changes.https://www.te.com/en/products/relays-and-contactors/electromechanical-relays/intersection/operating-dc-relays-ac.htmlThat means you need something a little smarter than can detect with a threshold, then count or filter samples above that threshold.A response time in the ballpark of 100ms~800ms would be about right for something with soft overloads and thermal mass like your transformer and rectifier.Addit : AliExpress have something like this, with a wire sense current transformer. Tho most of these seem to use 10A relays, a bit wimpy, but it could control a larger automotive relay.LG-C25X DC24V AC 0-5A / 0-10A / 0-20A / 0-50A / 0-100mA ①Relay output; ②Delay time setting 0-99.9S. ③LED displayhttps://www.aliexpress.com/item/32809338166.htmlIf you wire these so the relay drops the sense current, they will naturally hiccup.This is cheaper, but does current detect onlyhttps://www.aliexpress.com/item/1005004503711502.htmlThese must use some sort of parasitic powering, hence the minimum current given.Here is a smaller unit, that has a higher min currenthttps://www.aliexpress.com/item/1005006412086239.html

author: Zero999
author_rank: Super Contributor
timestamp: October 08, 2024, 09:45:01 pm
content: Quote from: Randy222 on October 08, 2024, 04:41:39 pmAdd a diode to the comparator circuit to make it latching. It can be reset with a push button switch.Here's a simulation, for educational purposes only. The tripping current is also dependant on the MOSFET's RON, which forms past of the sense resistor. It typically has a positive temperature coefficient meaning this circuit will trip more easily when the MOSFET is hot, which is a good thing.Warning: an E-fuse circuit should have a real fuse or circuit breaker, as there's a risk the transistor can fail short circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2024, 12:27:36 am
content: What's wrong with a dumb current-limiting resistor? Electrolysis is a simple process, nothing fancy required here.Tim

author: e100
author_rank: Frequent Contributor
timestamp: October 09, 2024, 03:08:00 am
content: Quote from: tom66 on October 08, 2024, 09:25:53 amIf I'm reading the datasheet correctly this will not work as the absolute ratings section says that the 4 pins (VDD, SET, SENM, FSTP) on the supply side of the mosfet must be kept within a few millivolts of each other.https://www.ti.com/lit/ds/symlink/tps24770.pdf

author: e100
author_rank: Frequent Contributor
timestamp: October 09, 2024, 03:12:00 am
content: Quote from: T3sl4co1l on October 09, 2024, 12:27:36 amDo you mean switched in circuit as a temporary thing while the salt is being added?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2024, 04:49:43 am
content: Just always on. What's wrong with that? Or if you prefer lower losses -- a series capacitor or inductor in front of the transformer/rectifier.Tim

author: nali
author_rank: Frequent Contributor
timestamp: October 09, 2024, 07:57:47 am
content: Or combine the dumb low value resistor & thermal cutout in series, mount the cutout on the resistor. You'll need to tweak values according to the thermal environment but it'll give you a crude auto-resetting device.Put another higher value resistor in parallel if you need some smaller residual current to keep the cell going during trip.Your requirements are so specific you won't find a "proven" solution - apart of course from comercially made devices for this specific application.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 09, 2024, 02:14:18 pm
content: Quote from: Zero999 on October 08, 2024, 09:45:01 pmBut if there's a "short" the manual reset will need to be pressed a zillion times?Might I offer yet another solution. Use a Constant Current PSU and set it to 15A.As a side note, I use Copilot just to see how it would answer. On this specific ask, Copilot suggested using a wrapped toroid core and then stick hall sensor in middle of toroid to sense mag field, hall then fed to op-amp. So basically same as using a sense resistor but perhaps without wasting as much power. Although toroid + hall is more parts and more expensive then just a sense resistor.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2024, 02:27:02 pm
content: Quote from: Randy222 on October 09, 2024, 02:14:18 pmNo it only needs to be pressed once and it will only reset, after the fault has been cleared. I would even consider AC coupling the reset circuit, so it's impossible for someone to hold the reset button, causing it to overheat by driving the short circuit.QuoteI agree. The best solution is to replace the entire transformer and rectifier with a constant current PSU.Perhaps an LED driver with a constant current output?https://uk.farnell.com/mean-well/pwm-200-12/led-driver-constant-voltage-180w/dp/3764248

author: Randy222
author_rank: Frequent Contributor
timestamp: October 09, 2024, 02:41:05 pm
content: Quote from: Zero999 on October 09, 2024, 02:27:02 pmIf the solution drops the impedance yielding a ckt that would be more than 15A 9vdc, then how do you clear the fault? As example, the Z drops low enough where amps would be 16A (the "short"), there's no easy way to know when Z increases enough to be able to do a manual reset.I like idea of CC PSU. Not too pricey and does all the ask.As a side note, someone mentioned the analog amp meter. Although they inline with the amps, the actual meter part is shunted around the main amps. If current is some sort of metric to be known via a sensor, then using such inline meter is a good idea, because perhaps 6A when it should be near 15A also means something to the OP, etc.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2024, 02:59:10 pm
content: Quote from: Randy222 on October 09, 2024, 02:41:05 pmBy investigating what caused the short circuit, removing it, then resetting it.QuoteNearly all over current protection devices have that issue. There are breakers which automatically reset, after a certain period, but it will oscillate and the OP asked for a latching circuit.

author: Seekonk
author_rank: Super Contributor
timestamp: October 09, 2024, 08:54:01 pm
content: Get a cheap current sensing relay or build one and have that trip a time delay relay which turns the output off for a period of time. That will give the salt a period to mix before it resets and turns on.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 09, 2024, 09:03:20 pm
content: Quote from: Seekonk on October 09, 2024, 08:54:01 pmYou can get that delay in comparator by using cap & resistor as a bleed down in hysteresis feedback. Just isolate the cap from comp output with a diode.I would actually use a small cheap MC for all the logic, and then just use sense resistor and op-amp, feed amp to an MC input, apply some logic in code. The MC can control an SSR, etc.Using the MC allows one to plug in a USB cable to flash new code whenever a change is needed. More flexible.Or, CC PSU.

author: fourfathom
author_rank: Super Contributor
timestamp: October 10, 2024, 12:46:22 am
content: If you want to measure high-side current, using a device like the Zetex ZXCT1107 might be simplify your design. You still need a comparator and reference, and the rest of the shut-down or limit circuitry, but you don't have to fight with an op amp's input common-mode range. I'm using the little 3-pin (SOT-23) part and while it's not a precision device, it's certainly accurate enough for this application.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 10, 2024, 01:15:47 am
content: Quote from: e100 on October 07, 2024, 03:49:31 pmComplicating factors are the nasty, pulsed DC nature of this, and your primary focus is current control, with a view to keeping inside the thermal limits of the transformer and rectifier.Since you asked for a circuit, I took a look around.The newest whizz-bang controllers tend to be 'too clever' with inbuilt counters on Hiccup and internal soft start etc, or charge pumps that have start times.You'll need plenty of voltage margin, as the spikes here will be nasty.Looking primarily for easy package, and high-side current sense, with a UVLO and simple pin control - basically a smart comparator and gate drive.Parts like NCP1034, NCV8852, and TPS40200 pop out.TPS40200 drives a PFET with high side current trip, and capacitor set restart time. Gets very close to 100% duty cycle at lower frequencies.These days you can get sub 5 milliohm PFETsOr, an alternative is a LED controller that purely does current regulate, like NCV30161 or FL7760 similar.Those need some circuit inductance and once the trip level is reached, the FET turns off/on within a current band - very stable Hysteretic Current Control.One plus is most of these use an NFET, a minus is the duty cycle is defined by the current and voltage and inductance, so a bigger diode is needed.AL8863 (4.5V to 60V) is similar but adds NTC fade out, which could help protect the transformer and rectifier.

author: e100
author_rank: Frequent Contributor
timestamp: October 10, 2024, 09:07:29 am
content: Quote from: Zero999 on October 09, 2024, 02:27:02 pmI found a similar constant current XLG series from Meanwell that may be suitable.https://www.meanwellusa.com/upload/pdf/XLG-200/XLG-200-spec.pdfIn the spec it says the constant current region is valid from from 8.4-12v.Not sure I undestand what that means. Does it imply that it shuts down if the calculated voltage for the required current is below 8.4v?

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2024, 10:18:43 am
content: Quote from: e100 on October 10, 2024, 09:07:29 amYes, that's what happens. Normally it goes into hiccup mode. It shuts down when the output voltage drops too low, then after a certain period it restarts and will shut down again if the short persists.I have used Meanwell power supplies for LEDs and they're very reliable.