Isolation transformer Over Current Protection - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: September 11, 2021, 11:42:32 pm
content: I bought myself an isolation transformer 6 months ago. It can handle around 4A @ 220V (Brazil). The thing is big and it is tucked in a corner full of wires that is hard to access.The issue Im having is this: As I now use the transformer to work on my projects and fix broken electronics, it is exposed to a lot of potentially dangerous short circuits and current surges from the experiments. I would like to not fry it.Now Ive been using fuses on primary and secondary and that works wonders, except that after a couple of really complicated (to me) projects, I ended blowing several fuses and the fuse holders tend to fail very easily, they either fail to make good connection after a fuse explodes or somehow break the plastic that makes pressure so the whole holder wont sping when you are you opening or closing to replace the fuse. Also, to keep replacing fuses and fuse holders is a rather annoying task given how hard it is to access the huge transformer.Ive been trying to design an electronic fuse with a hall sensor measuring the current, going through a OPAMP comparator that opens a relay if the current is too high, the issue is that I need it to latch. Other wise when it opens the relay, the current will go down and it will close it back up and the cycle will continue.How do I solve this? With a SCR?Also, is this the right route to solve this problem?Ive found this modules online but I think the will have the latching problem I described above.https://www.aliexpress.com/item/33035675183.html?spm=a2g0o.productlist.0.0.5ef011faVBPLOk&algo_pvid=ebe61326-1245-4f83-aab5-7cd6e4efa16d&algo_exp_id=ebe61326-1245-4f83-aab5-7cd6e4efa16d-17&pdp_ext_f=%7B%22sku_id%22%3A%2267354264907%22%7Dhttps://www.aliexpress.com/item/32821876449.html?spm=a2g0o.cart.0.0.1f5d3c00Uqk0iu&mp=1thanks everyone!

author: james_s
author_rank: Super Contributor
timestamp: September 12, 2021, 12:15:40 am
content: An electronic fuse isn't really the best choice here, at least not as the final protection. It sounds like what you need is a good quality fuse holder and a properly rated fuse. You probably want a "slow blow" type, rated for the maximum current the transformer is rated to supply. An even better option is a circuit breaker, small panel mount breakers are readily available.

author: AngraMelo
author_rank: Regular Contributor
timestamp: September 12, 2021, 12:26:02 am
content: Quote from: james_s on September 12, 2021, 12:15:40 amThe fuse and fuse holders will remain, what I wanted was to add an easier and fast access option like the breaker you mentioned in between. I thought about that but Ive been struggling to find the correct type of breaker.Lets say the max current is 4A. The breakers Ive found would either trip and a multiple of that peek current (type C curve) or they would break at the desired 4A but would hold it there for some seconds up to a minute before they break.Could you please instruct me what type of breaker I should be looking for?thank you so much!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 12, 2021, 12:58:59 am
content: You may find alternate practice is better. For example, a "dim bulb test" or other limited-impedance source. This way, power applied to a short simply can't draw dangerous current.Use a variac and increase it gradually, in case there's a transformer you can't measure through, or a breakdown threshold where current jumps up suddenly.You can avoid powering shorts by always measuring the load first. Open it up and check likely components. Understand typical circuits (e.g. SMPS input and power sections) so you know what to probe at.A true electronic fuse is difficult to make here, as the peak power levels are just so high -- you still need to provide some inrush current for loads, say, more than 10A. (You'll have a short-circuit current somewhat over 40A from that transformer, I would suspect.) This is part of the trouble with making even a crude (relay turn-off) one, you have to delay the response for normal loads, but trip at some point, when it's been too much for too long. But that's what an ordinary fuse does, it's just not repeatable.And doing it with a relay isn't ideal, putting fault current across contacts guarantees maximum arcing. Eventually, it won't open any more; or it has too much resistance when closed, and burns up.Ideally, you'd use series transistors or something, like what's often used with low voltage loads. But you'd need a hundred of them in parallel to do this the same way, it'd just be crazy. There is an alternative, using a switching circuit to improve the "dim bulb" circuit: it's normally held closed, but the instant the current rises too high (as limited through an inductor), it switches off, dumping some energy into a load resistor. The resistor can handle much more power, at little cost; the downside is, you need a fast-acting circuit that's more complicated to design and test.So, sticking with traditional methods is probably the best idea.Regarding breakers, they're going to be quite slow at this current level; and fuses and breakers in general, aren't precise devices, their only purpose is to protect the wiring. So a 4A nominal fuse, might take up to a minute to open at 6-8A -- the wire won't get too hot in that time. Likewise they don't open the instant some hundred amps is drawn, they allow inrush current through. The time grade determines how much is too much; slow-blow or type C curves are used with heavy loads like motors. For general electronics, you probably want the A curve, but yeah, hard to find -- there's simply not enough heat at those currents, to make a breaker move in a short time.Tim

author: bdunham7
author_rank: Super Contributor
timestamp: September 12, 2021, 01:03:27 am
content: You don't need anything fancy, just a thermal breaker and a slow-blow fuse rated slightly higher than the breaker. My isolation transformer setup has a 4A fuse on the secondary and a 3.5A circuit breaker on the primary. Your transformer isn't going to melt in one second, you probably want a quick trip at 10 amps or so.These might work, you'd need to study the curves to know for sure.https://www.mouser.com/datasheet/2/240/Littelfuse_Fuse_313_315_Datasheet_pdf-476752.pdfhttps://www.mouser.com/datasheet/2/358/typ_T9_311-1275876.pdfAdditional things like dim bulbs and electronic limiters are nice, but I prefer to have simple backup protection like this in case things go sideways.

author: james_s
author_rank: Super Contributor
timestamp: September 12, 2021, 03:05:06 am
content: Quote from: AngraMelo on September 12, 2021, 12:26:02 amI expected that's what you wanted, a device to protect the transformer. The transformer will tolerate a substantial overload for a period of time but will eventually overheat and fail, that's what a breaker or slow-blow fuse is meant to protect. It will allow an overload for a while without blowing but a sustained overload will cause it to blow.If you want to prevent damage to malfunctioning devices connected to the transformer then an electronic fuse may be useful.

author: AngraMelo
author_rank: Regular Contributor
timestamp: September 12, 2021, 03:50:54 am
content: Quote from: T3sl4co1l on September 12, 2021, 12:58:59 amYou pretty much described the problems I had with the mentioned "complicated" projects. The output of one of them was measuring open circuit but would blow up the instant mains was applied. The other one would be fine with a light load but would short after a significant current (that still was ways away from the designed working power). Those projects gave me a run for my money. I blew up several fuses and the transformer being where it is, made a chore out of an insignificant action like changing a fuse so the whole finding a solution started.But you are right, I will bring the fuses closer to me for easier access and use the dim bulb more often.Thank you!

author: Monkeh
author_rank: Super Contributor
timestamp: September 12, 2021, 04:06:49 am
content: Quote from: T3sl4co1l on September 12, 2021, 12:58:59 amWhat 'A' curve? A is intentionally not used..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 12, 2021, 05:37:32 am
content: Quote from: Monkeh on September 12, 2021, 04:06:49 amRight, if you can even find it.Which actually, let me see, kinda hard to search on but of the scant few references I see at a glance, it looks like 'A' is "semiconductor fuse" territory. Which, well, there you go, protection of electronic equipment, namely in the hope that the rectifier might survive -- there's no hope of protecting a transistor, let alone ICs, with any kind of fuse, but diodes (and relatives: SCRs and TRIACs) are robust enough to have some hope of survival under fault conditions.(It's not even that bad, in a lot of cases, really. I've had 35A FWBs survive shorting across a 240V 50A residential circuit. That's probably on the order of 2kA fault current, at whatever time curve the panel breaker has.)The main downside to such fuses, in general use, is anything with inrush is likely to blow them. Medium to low power electronics, with conforming inrush protection, are probably okay. Power tools, not so much!They're also much more expensive, so I wouldn't recommend using them in a service/repair context.Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2021, 05:49:37 pm
content: Quote from: AngraMelo on September 11, 2021, 11:42:32 pmThat is one way but a better way is to use a relay with multiple poles, and use one pole to latch the relay on. So when the relay power is interrupted by the trigger circuit, it latches off because the relay removes power from itself.A common circuit breaker is probably a better solution however.

author: analogMensch
author_rank: Newbie
timestamp: September 20, 2021, 10:25:31 pm
content: My isolation transformer is also not located at my bench, cause it's a heavy and big 3kW beast. So it takes up space in a shelf and I wired it up to a single socket and a two way push button at my bench.The primary side runs of a 25 amp breaker through a big relay, which is controlled by the two way push button. It's a simple self latching circuit with a normaly open button for turning on and a normally closed button for turning off.On the secondary side I have a motor protection relay. These things are build for protecting motors, so they can handle the inrush current. But also they trip much faster than normal breakers if you overload it.There are two nice things about these motor protection relay...1.) It can be switched between manual reset and automatic reset. I have mine on automatic reset, cause...2.) It have a normally open and a normally closed contact. I wired the normally closed contact in series with the normally closed contact of the two way push button. So if the motor protection relay on the secondary side trips it shuts down the relay on the primary side. The whole system will stay off after that, and I have to turn it on again by pushing the on button (after the motor protection relay resetted itself).My motor protection relay is adjustable between 10 and 16 amps, I run it on 12 amps (thats about 2760W here at 230V). But you also can find one for 4 amps.If you don't want to crawl down to your transformer every time, you could add a small breaker box at your bench and install the relay there. Instead of the motor protection relay you also can take a motor protection switch. It's technically the same, but with a knob on the front for turning it on by hand.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 21, 2021, 05:51:50 am
content: Quote from: T3sl4co1l on September 12, 2021, 12:58:59 am+100%.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 21, 2021, 08:13:03 am
content: a simple 3 euros 6A 230V current disjunctor will do the trick, secondary side. no socket, no problemson primary side i guess you already got one

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 21, 2021, 02:32:48 pm
content: Another plus for the incandescent light bulbs.When such light bulbs are cold, they have a relatively low resistance, and stay mostly off, even when in series with a load. However when the current increases, the bulb heats up, which inreases it's resistance more, etc.So in effect it acts as a self repairing fuse a bit like to a PPTC. When they're cold, they can conduct more current than their nominal current when the light is on, and they have a pretty obvious "tripped" indication that is hard to miss.Ideally you put a few of such lights (with different wattages) in a box and add some switches, to adjust it to the job you're working on.This video from Photoninduction with a 20kW lightbulb clearly shows the resistance difference between hot and cold.The idea of a variac also used to work, but there is a catch with SMPS circuits. These can draw too much current when input voltage is too low and this can result in fuses of the SMPS itself blowing.Isolation transformers are also very robust. It's a chunk of steel and a bunch of copper wire, and they do not get damaged quickly by moderate overloads. The primary failure mode is the thing gets so hot that the wire insulation gets compromised, and it takes time to overheat. (Although, when the secondary has a hard short, it's pulled into severe saturation, which leads to a massive overload of the primary coil.)If you want to go fancy, you can add a current transformer or such an HALL-effect current sensor and a microcontroller, and then write a bit of software to make an adjustable cut off, both for "instant current" and I-squared-t. and then switch a beefy SCR or solid state relay.Another luxury option is to use a PKZ:https://duckduckgo.com/?q=pkz+motor&t=hx&va=g&iar=images&iax=images&ia=imagesThese give protection against both "instantaneous high currents" and slow overloads with an integrated bi-metal.There are also household variants of these, but the PKZ is usually adjustable, while the household versions have some fixed current limit.

author: analogMensch
author_rank: Newbie
timestamp: September 28, 2021, 09:12:56 pm
content: Quote from: Doctorandus_P on September 21, 2021, 02:32:48 pmOh, yeah, that is what I was talking about. Didn't know it is called PKZ.