Adjustable mA current circuit breaker - Page 1
Forum section: electronics / repair

author: rhb
author_rank: Super Contributor
timestamp: May 21, 2020, 08:59:26 pm
content: For testing stuff that might blow a fuse I'd like to have a DPST relay controlled by a circuit which can be adjusted so that it opens the relay if the current exceeds a preset level to use in place of the fuse.This is to allow testing without blowing fuses which can sometimes be difficult to get. And Murphy says if I blow it I won't have a replacement.Does anyone know of a source for a ready made unit? I'd want 50-100 mA steps at the minimum and would prefer continuous. Max of 10A would be fine. I did a bunch of searches but didn't find anything suitable. Just lots of motor over current breakers and such which are much too high current for this application.What I'm looking for is basically a comparator with an adjustable threshold and a set/reset flipflop capable of controlling a relay with contacts rated at 10 A 220 V. I assume I'm just not describing what I want the way the Chinese ebay sellers would.So I thought I'd ask before building something.Thanks,Reg

author: coromonadalix
author_rank: Super Contributor
timestamp: May 21, 2020, 09:04:19 pm
content: The simplest way would be an sensing resistor with an op amp and some trimpots adjustment, triggering an ne555 who act as an relay controllerIll try to find the circuit in my archives ...

author: rhb
author_rank: Super Contributor
timestamp: May 21, 2020, 09:36:14 pm
content: Designing one is not an issue. I was just hoping not to have a build tool for the task. Seems like a pretty sensible item to have in a busy repair shop. So I thought someone might sell them.Not quite sure how a 555 would work for this, but I did design an alarm system with entry and exit delays and a timeout and auto reset using just 555s. So I expect it can be done. Only problem was it was sensitive to lightening causing a false alarm.Reg

author: MarkF
author_rank: Super Contributor
timestamp: May 21, 2020, 10:10:51 pm
content: Big Clive's teardown of a "Dusk Sensor Switch", where it uses the555 Timer's internal voltage thresholds to turn a relay ON and OFF,may give you some ideas for a starting point.https://youtu.be/cQDIb09cLug?t=585

author: rhb
author_rank: Super Contributor
timestamp: May 21, 2020, 11:01:41 pm
content: A comparator, SR flipflop and relay is less clever. My first thought was a sensing transformer and bridge rectifier, but a Hall effect device might be better and I *think* I might have some.However, I know I have toroids, magnet wire, etc to do the the other. I probably have 555s, but where is a bit more iffy.On reflection, it seems a much better tool than an isolated autotransformer. Check the fuse rating, set the breaker slightly lower and go. Though *not* necessarily fool proof.A shorted cap blew a 200 V 1.5 A bridge rectifier in my Tek 577 curver tracer within 5 minutes of my powering it up for the first time. I don't think it blew the fuse, though I haven't checked. I had turned it off before I smelled the magic smoke. After a couple of minutes fiddling I realized the pots needed cleaning and turned it off. Then I smelled the smoke.I took that as a hint to improve my new TEA procedures. And to test all the caps in the 577 before applying power again. Not everything in it is as easy to replace as the rectifier and cap. I'm sure there is some pure unobtainium in there.Have Fun!Reg

author: aqibi2000
author_rank: Regular Contributor
timestamp: May 21, 2020, 11:42:40 pm
content: What’s wrong with using a current sensing resistor and measuring the differential voltage?

author: rhb
author_rank: Super Contributor
timestamp: May 22, 2020, 12:51:17 am
content: Across a mains line? That was my first thought. It's certainly possible, but is an invitation to serious trouble. Much better to use a transformer between the mains line and the current sense resistor. Or use a Hall effect sensor.If I could figure out a 110 V only electromechanical system I'd do it. But an inexpensive set/reset flipflop at 110 V is not obvious. I'm sure it's possible, but the cost gets rather high for so many relays. Though I think I might have a bunch somewhere I scavenged from the trash 40 years ago..But I'd do it if someone has a circuit. I'm lazy. A comparator and flipflop is dead obvious. Make an SR flipflop from relays is not.Have Fun!Reg

author: bdunham7
author_rank: Super Contributor
timestamp: May 22, 2020, 01:27:22 am
content: I didn't realize you wanted to do this at mains voltage until I read your last. Is there an acceptable minimum setting? You really don't need to limit the current to 100mA do you? I would suggest starting with a small current transformer, load resistor, germanium diode and small capacitor for the sense voltage. The problem is that if 10A = 10V, then 100mA = not enough to get past the diode. I suppose you could use an amplifier that doesn't mind being overloaded at the input. Then you need a potentiometer as an adjustable voltage divider and a comparator and logic circuit that ONLY LATCHES OFF! And a reset button and separate method of latching on. You don't want a stray spike bringing your mains back to life.Oh, and while it's a neat idea, I've never seen such a product for mains testing. An edison socket and a selection of light bulbs seems to be the established method.

author: james_s
author_rank: Super Contributor
timestamp: May 22, 2020, 05:48:47 am
content: I remember seeing a magazine project for an electronic fuse many years ago, I don't think I've ever seen one offered as a commercial product though it does seem like something that would exist.

author: giovannirat
author_rank: Regular Contributor
timestamp: May 22, 2020, 06:02:35 am
content: What about a current monitor relais, like Siemens 3UG4622-1AW30 ?More detailshttps://mall.industry.siemens.com/mall/en/WW/Catalog/Product/3UG4622-1AW30

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2020, 08:11:53 am
content: The design may not be so easy, as with AC current there may be short peaks in the current that one want o accept.So one kind of has to decide on a filtering function between very fast to rather slow. This would possibly be 2 separate limits, like a fast one and slow one.For sensing the current transformer may work, but could also be tricky, as a broken part may draw a DC component that could saturate the CT. So one would at least have to keep saturation in mind. Otherwise the CT is nice that it does not cause much heat and offers isolation.The usual way for testing is to have some incandescent lamps in series.If looking for something ready made, there may be isolation transformers with an adjustable limit.There are also adjustable motor protection "fuses". These are usually adjustable over some range, but may not be available much below 2 A. They are similar to circuit breakers, with a slow thermal and fast magnetic turn off.

author: rhb
author_rank: Super Contributor
timestamp: May 22, 2020, 12:00:01 pm
content: The point is to not blow hard to find AC line fuses in old test gear when powering it up for the first time or making repairs. So it needs to be set to whatever current the fuse in the instrument is rated for. It's *just* for testing.Circuit wise it's just a clamp on ammeter circuit configured to release a relay until reset. Tripping when it shouldn't is not an issue. Not tripping when it should would be a problem.I was simply hoping to avoid adding another design project to my already much too long To Do list and buy something for the task. Everything I found was for large loads with setting increments of 5-10 A which would not provide any benefit.For the immediate task I settled on a power strip with a circuit breaker and used a cheater cord to power the PS I have been working on. The construction was so tight I was worried about having caused a short circuit putting it back together.Reg

author: kripton2035
author_rank: Super Contributor
timestamp: May 22, 2020, 12:05:05 pm
content: there was such a device in a nuts and volts issue september 2007 , named "refuseable box"http://nutsvolts.texterity.com/nutsvolts/200909/?folio=42&pg=42#pg42https://www.nutsvolts.com/magazine/article/the_refuseable_box

author: kripton2035
author_rank: Super Contributor
timestamp: May 22, 2020, 12:09:20 pm
content: there is also this one, without µcontroler :https://www.stmradio.com/SettableBreaker.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2020, 12:10:20 pm
content: I made a box some years ago that does this, but only for DC, up to 30V 8A. You'd need to put one inside a FWB, on a transformer secondary (say 18VAC, rated for some amperes so it has a low series impedance in the "on" state), with its primary in series with the load (its inductance can perhaps be resonated out to improve off-state leakage, but it will never be as good as a relay contact).Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 22, 2020, 12:45:22 pm
content: Hi,I would like to see either of those boxes if they are subject to the fault current of several hundred amps typically found in a home.They may be okay for low voltage, low energy circuits.I use an Elgar 251 AC source if I want I am testing SMPS etc..Jay_Diddy_B

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2020, 01:57:21 pm
content: Quote from: Jay_Diddy_B on May 22, 2020, 12:45:22 pmYep, the relay boxes, probably not going to be pretty; combination with a conventional "dimbulb tester" is just about mandatory.My example at least has an active current limiter, and transient protection so it's safe during turn-off as well; but others, YMMV.I'd love to make one for mains, but I don't do enough mains testing to bother with it personally, and the cost of handling all that SOA is well into the realm of Snap-On Tools(R) pricing; when a "dimbulb tester" will do for almost everything, who would bother with such a thing?Tim

author: rhb
author_rank: Super Contributor
timestamp: May 22, 2020, 03:28:03 pm
content: Quote from: kripton2035 on May 22, 2020, 12:09:20 pmThat's pretty much exactly what I have in mind, except I want to switch an outlet that the unit is plugged into and place a high amperage fuse in the fuse holder. Probably a blown fuse with copper wire soldered in place of the blown element. Everything should fit easily into a deep 4" surface mount electrical box.Inserting a fuse with wire leads doesn't work with panel mount fuse holders which use the cap as one of the fuse terminals.A sudden dead short across the mains might weld the relay contacts if they were too small, but there is the mains breaker backing it up if that happens. A relay with contacts rated for 20 A 220 V should be immune to damage unless repeatedly subjected to dead shorts.Have Fun!Reg

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 22, 2020, 04:10:23 pm
content: Hi,Have a look at this video from bigclivedotcom:At around 5 minutes you will see this:The red structure is an arc arrestor. This is designed to quench the arc when breaking the fault current.You may be able modify a device like this by feeding the trip low current coil with an external circuit.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 22, 2020, 08:54:22 pm
content: You may want to a look at circuit breaker with an accessory called a 'shunt trip coil'.This normally mounts next to the breaker on a rail.I might choose a 6A breaker, this would be the upper limit. Then add electronics that energizes the shunt trip at lower currents.Look at S201 series in this catalog:https://ca-en.alliedelec.com/m/d/e2366fc56acaf11c329a344195a2f28c.pdfand add S2C-A1 shunt trip.Regards,Jay_Diddy_B