Add "soft start" to Isolation transformer - Page 1
Forum section: electronics / projects

author: staze
author_rank: Frequent Contributor
timestamp: June 03, 2016, 11:32:02 pm
content: All,I just picked up a 120V 15A isolation transformer for $1. Yes, $1. It's huge, and weighs a ton. Opening it up, there's no soft start, and I'm worried the power on inrush current will trip a breaker. It looks like I should be able to remove the "jumpers" on the terminal block (upper right of the overhead picture), and replace those with NTC Thermistors... but I'm unsure how to calculate the value needed. Or should I just throw in a couple high wattage resistors? Or am I crazy, and you really only have to worry about inrush with toroidal transformers?Any info would be great.

author: uncle_bob
author_rank: Unknown
timestamp: June 04, 2016, 12:02:14 am
content: HiWith no load on the secondary, a proper isolation transformer presents no load on the primary. There's no need for a "soft start" unless you do something silly. Even then it's no worse than plugging in whatever load you already have attached to it.Bob

author: staze
author_rank: Frequent Contributor
timestamp: June 04, 2016, 12:05:31 am
content: Then are toroidal transformer based isolation transformers just that much worse? Because I know they generally have soft start (through relays or NTC thermistors) (the one I have certainly does). I thought there was a inrush current due to the magnetic "mass" of the core (sorry, can't think of the word I'm looking for there)...Thanks!

author: uncle_bob
author_rank: Unknown
timestamp: June 04, 2016, 12:12:04 am
content: Quote from: staze on June 04, 2016, 12:05:31 amHiI suspect what you are looking at is a fuse. Transformers, regardless of the design do not have an inrush issue. Capacitors have inrush issues.... Some loads with capacitors in them have inrush issues.Bob

author: Ian.M
author_rank: Super Contributor
timestamp: June 04, 2016, 01:33:49 am
content: So why do the experts contradict you?http://sound.westhost.com/articles/inrush.htm#s4http://www.ametherm.com/inrush-current/transformer-inrush-current.htmlThe inrush current of a E&I core transformer is typically much less than a similarly rated toroidal transformer, but the O.P. has one big-ass transformer so it may in fact be a problem. One can get all fancy and try and measure the inrush current to determine if a soft start circuit will be needed, but why not simply see if it ruptures a T6.3A fuse if switched on and off repeatedly with no load? If it does, its going to need a soft start circuit - either a NTC thermistor and a time delay relay to bypass it, or a series power resistor and a time delay relay, with another relay slaved to it so the output is disconnected until the resistor has been shorted out.

author: uncle_bob
author_rank: Unknown
timestamp: June 04, 2016, 01:41:45 am
content: Quote from: Ian.M on June 04, 2016, 01:33:49 amHiThe first one seems to be *very* confused about motors vs transformers. Hint: they are not the same thing at all. The second one is talking about a transformer that has not been properly wound and they *really* would like you to buy their inrush gizmo.Transformers have been used for over 100 years now. Go climb up a pole and take a look (carefully). Seen any inrush limiters? Nope. Tear open several thousand pieces of gear with transformer based supplies. See any inrush limiters? Nope. Take a look at the one you just bought. See an inrush limiter? Nope. Do they all work just fine? Yup. Have they been doing it for about a century or more ... yup.Bob

author: Ian.M
author_rank: Super Contributor
timestamp: June 04, 2016, 01:47:43 am
content: *BIG* transformers on limited current capability supplies can have an inrush problem. Big toroidal transformers are known for it. Try fusing one with an ordinary F fuse, using the next value up from its rated current some day. It wont last very long and you may even see the fuse wire glow red and sag momentarily at switch-on.

author: IanB
author_rank: Super Contributor
timestamp: June 04, 2016, 01:59:17 am
content: An explanation would be helpful.The primary of a transformer with an open secondary looks exactly like a big inductor, meaning it will tend to resist any sudden changes in current. If you connect AC to it it will naturally present a high impedance and try to prevent a large current from flowing.If there is to be an inrush current problem, something else must be at play. What might that be?(In contrast, a transformer with a shorted secondary has a primary that also looks like a short circuit. For instance, smoothing capacitors on the  secondary side look exactly like a short circuit. So if you apply power to the primary of such a transformer you had better expect something interesting to happen.)

author: Ian.M
author_rank: Super Contributor
timestamp: June 04, 2016, 02:15:45 am
content: Residual magnetisation of the core is one issue. If the supply is switched on at the beginning of a half cycle that will give the same flux direction, unless the transformer has a significant excess core area, its likely to saturate.Our workshop Variac often tripped the breaker on the isolating transformer even if it was switched on with no load. It certainly didn't have any capacitors. The only reason you don't see problems more often is that most fuses and circuit breakers have a surge capability significantly in excess of their continuous rating.

author: Zeranin
author_rank: Regular Contributor
timestamp: June 04, 2016, 03:12:32 am
content: Quote from: Ian.M on June 04, 2016, 02:15:45 amYou are exactly correct, core remanence is one culprit, and the resultant inrush current can in some cases be very large indeed. Remanence is the magnetic property of steel, whereby it stays in a magnetized condition after removal of a magnetizing coil current.How much of an inrush you get on any particular switch on is purely a matter of chance. In the worst case, the transformer is switched off at the peak of the magnetizing current, with the result that the core remains in a substantially magnetized condition. That is a matter of chance. Then, in the worst case, the supply is switched on at the beginning of a half cycle, of phase such that the magnetizing current magnetizes the core still further, in the direction in which it is already magnetized. If you are unlucky enough for both these events to be worst case, then there is a fair chance the core will saturate, after which the inrush current is limited only by the winding resistance. After a few cycles, the remanent magnetization disappears, and all is well.Toroidally wound xformers are generally much worse that EI laminated types, because they use a strip-wound core that has zero air gap in the magnetic path, whereas conventional EI lamination types do have some magnetic air gap because the laminations are crudely butted together. The air gap helps to resist remanent magnetization in the first place, and also reduces saturation when inrush currents do occur, limiting the magnitude of the inrush.I have a 2kVa Variac, toroidally wound as Variacs are, and every 4th or thereabouts time I turn it on, it trips it's breaker, as per the above, and as Ian has also experienced.You may or may not have a problem with your 'huge' conventional lamination xformer. Sounds like it might be conservatively designed and thus less likely to saturate, so with luck you will be OK.

author: edpalmer42
author_rank: Super Contributor
timestamp: June 04, 2016, 05:27:51 am
content: I have a 250V, 1.6KVA variable autotransformer (i.e. Variac). Many years ago, I wondered how much current it drew at no load. I carefully hooked it up with my bench DMM in series to measure the current. When I turned on the power,LIGHTcame out of my DMM!Note that there were no gaps for the light to come out, but it did anyway! I quickly unplugged everything and pulled the top off the DMM to let the smoke out. But there was no smoke. No leads that were missing a component. No damage at all.Eventually I saw that the 2 amp glass fuse had a perfect mirror finish on the inside of the glass. That's where the light came from. The initial current surge vaporized the fuse wire and deposited it in a perfect, mirror smooth layer on the inside of the fuse.So yes, a transformer with no load can generate a scary current surge. However, Staze, since you're in the States, your circuit breaker is probably a thermal breaker, rather than a magnetic one. If so, I'd be very surprised if the surge was large enough or long enough to blow the breaker.If you decide, or testing shows, that you do need some form of soft start, you won't be able to use an NTC because the NTC required to suppress the starting surge would be totally wrong if you load the transformer with a 15A load. You'd have to use power resistors with a time-delay relay to short them out. Even then, you could find that they would interact badly with some loads.For now, I'd recommend that you just run it as is and see what happens.Ed

author: staze
author_rank: Frequent Contributor
timestamp: June 04, 2016, 05:53:23 am
content: Ah, interesting. Actually, this whole thread has been.The confusion, I suppose, is that my 5.28A toroid isolation transformer does have two NTC thermistors for "soft start". That's it. No relays, etc.So given the huge additional current capacity, those NTCs would have to be pretty large I guess. So hence the power resistors and a relay?Interestingly, the unit isn't fused. Probably for this reason. :pI'll give it a go.Thanks for all the input. I don't really NEED a 15A one... But who could turn down a dollar. :p

author: johansen
author_rank: Super Contributor
timestamp: June 04, 2016, 06:10:46 am
content: Quote from: Ian.M on June 04, 2016, 02:15:45 ammy experience is this is only a problem with relatively new breakers. i think they are now designed to be sensitive to the magnetic fields from the 100 amp inrush current on the first cycle.

author: edpalmer42
author_rank: Super Contributor
timestamp: June 04, 2016, 07:23:37 am
content: Quote from: staze on June 04, 2016, 05:53:23 amI'm actually surprised that a general-purpose isolation transformer would use NTCs at all.QuoteBut if the NTCs were big enough to handle a 15A (~1875W) load, a 10W load, for example, might be too small to heat them up, or it would take so long that it would affect the load. If the transformer was built-in to a piece of equipment, the NTCs could be sized appropriately, but not for a general-purpose, variable-load isolation transformer.QuoteI wondered about that. Even a crappy power bar has a 15A breaker. Is the switch just a switch or does it have a breaker built in? By the way, for 15A, the line cord had better be 14 AWG.Ed

author: TerraHertz
author_rank: Super Contributor
timestamp: June 04, 2016, 08:21:23 am
content: I'm surprised at all the posters insisting there's no inrush current problem with big transformers. Don't they have any big transformers themselves?Even my measly 240V 2.2A to 240V isolation transformer sounds like it's going to blow up if I happen to switch it on in the 'wrong' instant of the 50Hz cycle. Takes several complete mains cycles for the "BRRRMMMmmmmm" to fade away.I had thought it was due to the magnetic field taking a while to build up in correct phase to the mains voltage. Bit simplistic I know. The explanation involving remanance magnetization is much more satisfying.

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 04, 2016, 08:59:22 am
content: What happens when you switch on a transformer? Or an iron cored inductor? At that moment the iron is not magnetised. Now when you switch at the zero crossing (zero voltage) you will get a DC component on the magnetisation current of the iron. This DC slowly fades out finally due to the transformer losses. The net result is at switch on: Twice the peak induction as without DC. To cope with this you will need twice the amount of iron as is needed for normal steady operation. It is sometimes done for medical transformers but for normal transformers it will be a waste of material and money. In that case the core will go into saturation for the peak induction for a while. When that happens, no self induction any more, only the primary DC resistance to limit the current. With small transformers that is no problem, primary copper resistance will limit inrush current, but with larger ones the fuse will trip. It is the way inductors work.

author: TheAmmoniacal
author_rank: Unknown
timestamp: June 04, 2016, 10:16:47 am
content: Quote from: TerraHertz on June 04, 2016, 08:21:23 amI'm asking myself the exact same question, I've had high inrush current problems on all my large transformers, they would trip the circuit breaker for every cold start. The ferrite core isn't instantaneously magnetized!I just throw an EPCOS NTC in series to fix the issue. Still not confident on which values to pick though, but they've all worked so far.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 04, 2016, 11:04:43 am
content: A transformer will haveat least3x Inom inrush current. This will depend on the impedance of the source.Inrushing transformers from an inverter or generator will cause low inrush, from mains this will cause high inrush. Very high if you turn it on at 90 or 270 degree.Using a solid state relay can reduce it significantly.I remember on this forum someone strapping simple water heaters in series to reduce the impedance of the source, then bypassing them when core magnetization it complete.This generally takes only 10 periods.

author: VK5RC
author_rank: Unknown
timestamp: June 04, 2016, 11:18:41 am
content: I can vouch for the in-rush current issue, built a 28V 30A linear PSU (for some RF gear - so wanted to avoid SMPS). The in rush current did vary from time to time at switch on - in keeping with a phase type issue.The only time I have worn safety glasses in addition to my corrective glasses was when I was testing that puppy, scared the daylights out of me when I had a tantalum in backwards! O0 (had straight hair before I started)

author: Zeranin
author_rank: Regular Contributor
timestamp: June 04, 2016, 11:54:02 am
content: Quote from: Pjotr on June 04, 2016, 08:59:22 amAgree with all you say, but core remanence is another important player. Here is a paper analysing xformer inrush current in detail.http://www.ee.ktu.lt/journal/2011/03/04__ISSN_1392-1215_Calculation%20and%20Analysis%20of%20Transformer%20Inrush%20Current%20Based%20on%20Parameters%20of%20Transformer%20and%20Operating%20Conditions.pdfTake note of Figure7, showing that the initial remanent flux density has a very large effect on the inrush current. Even if switched on at zero remanent flux, there will be a large inrush current exactly as you describe, but if you get unlucky and combine this with a peak remanent flux in the wrong direction, then the brown matter really hits the fan.

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 04, 2016, 12:04:04 pm
content: The large inrush current is due to the iron core getting into saturation. If the core goes into saturation and how much depends on what phase the transformer is switched on. Switching on at peak voltage doesn't cause the inrush problem. Remanance magnetism add or substract to this but for normal silicon grain oriented transformer steel it is very small. In the end the inrush effect fades away due to the transformer losses. For that time you need to limit the current with an extra resistor if the primary resistance can't do.

author: Zeranin
author_rank: Regular Contributor
timestamp: June 04, 2016, 12:28:02 pm
content: Quote from: Pjotr on June 04, 2016, 12:04:04 pmSo presumably you believe that the paper that I linked to is wrong, and the effect due to remanence is 'very small' ? Who are we to believe? Did you actually read the paper? Can you provide evidence for your claim that the effect of remanent magnetism is 'very small'. How small is 'very small' ? What is your reference on this matter? I'm not saying you are wrong, but you will need to provide some evidence.

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 04, 2016, 12:36:11 pm
content: I didn't say the effect is small. I did say that remanence magnetism for transformer steel is usually small. Note that the area of the BH loop represents iron losses. You want to keep that as small as possible.But finally it doesn't matter much. The iron will go into saturation now and then at switch on. For that situation, anyway you need to take provisions to limit the current in some way.

author: timofonic
author_rank: Frequent Contributor
timestamp: June 05, 2016, 01:49:00 am
content: I just to say I'm amazed by that big cat. Please put more photos of that great feline?

author: staze
author_rank: Frequent Contributor
timestamp: June 05, 2016, 02:18:29 am
content: So starting the unit up resulted in a whole lot of nothing. As in, disappointment that it didn't trip a breaker, or make a bunch of noise, etc.To answer the question, what I thought was just a switch is actually a breaker, a 3120-F321-P7T1-R01D, or on Digikey (http://www.digikey.com/product-detail/en/e-t-a/3120-F321-P7T1-W02D-10A/302-1219-ND/509821) a $40 part!So, I won't worry about it tripping the breaker. Now I just have to find a place for this monstrosity. =D Might use it to hold up a shelf below my bench. =P

author: ekyle
author_rank: Contributor
timestamp: June 05, 2016, 07:00:16 am
content: You could try using a solid state relay to power it up. Most of them turn on during a zero voltage crossing so you wouldn't have the problem of it being energized during the peak of a cycle. Also, turning off using a solid state relay won't cause the other problem of burning up switch contacts when it's turned off from the field collapsing. They can be had on Ebay for about $5.

author: SeanB
author_rank: Super Contributor
timestamp: June 05, 2016, 11:01:18 am
content: I added soft start to my variac, simply because of that inrush tripping a 20A breaker ( thermal one, old Fuchs that was likely 30 years old, and which had no magnetic trip at all, as that was not required at the time of manufacture, being handled by wire size selection only, so the wiring was 4mm for a 16A nominal load, as opposed to now being 2.5mm for a 25A load) so added a resistor and a shorting relay for them, along with a thermal fuse between the 2 10W 47R resistors ( what was on hand to use) to limit down, and then a 6.3A fuse on the primary and another on the secondary.

author: IanJ
author_rank: Unknown
timestamp: June 05, 2016, 01:08:58 pm
content: Hi all,I picked up an isolation transformer from the medical world which I have adapted for use in my workshop.It's a 1000VA isolation transformer that was supplied as part of a urodynamic system, sold in 2011 namely a Duet Logic G2 Urodynamic system. It's mounted in the base of the cart and supplies isolated mains to the various components of the system and an addition 24V to connect to the patient unit module. The secondary output is switchable 120/240VAC.I ripped out the 24VAC psu and re-wired the transformer as originally the earth in/out were connected, as was one side of the secondary AC from the transformer (neutral). Now it's fully isolated and has the option via two banana's I've fitted to allow interconnection of the earth in & out. I've also fused one side of the secondary.Yes, there is visible inrush current (the lights in my workshop flicker).Some pics, a mix of before/after.Ian.

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 05, 2016, 01:48:17 pm
content: Those medical ones are often of the "low inrush" variety. That means they have a 50% (or even more) bigger core than needed. So saturation of the core at switch on is much less severe or does not occur.

author: bitslice
author_rank: Frequent Contributor
timestamp: June 05, 2016, 04:16:01 pm
content: Quote from: staze on June 03, 2016, 11:32:02 pmDon't people just use a 100W incandescent in line with these things?

author: bitslice
author_rank: Frequent Contributor
timestamp: June 05, 2016, 05:44:41 pm
content: Quote from: AcHmed99 on June 05, 2016, 04:32:25 pmfor this kind of thing I guess?

author: staze
author_rank: Frequent Contributor
timestamp: June 05, 2016, 06:02:38 pm
content: But lightbulbs have low resistance on turn on, and then it goes up. So this would be the opposite, would it not?

author: bitslice
author_rank: Frequent Contributor
timestamp: June 05, 2016, 09:17:49 pm
content: Oh yes lamps are just for shorts, but the OP seems concerned about breaker tripping. Just throwing it in there.

author: staze
author_rank: Frequent Contributor
timestamp: June 05, 2016, 09:46:15 pm
content: I am the OP. :p I'm just not sure a lamp would help in any way. The inrush would still happen. :/ I'd want something that has a high initial resistance, and then a low ongoing one.

author: ian.rees
author_rank: Contributor
timestamp: June 05, 2016, 09:55:18 pm
content: If the lamp is in series with the transformer primary, then the max inrush current is limited by the lamp - hopefully turning on a 100W lamp won't trip the breaker. The lamp can be replaced by a short a few cycles later to get more power through the transformer.

author: ian.rees
author_rank: Contributor
timestamp: June 05, 2016, 10:29:32 pm
content: Clearly there's more than one way to do it, and sure lamps aren't ideal (though they are a fine solution in this context). I was answering the OP's question about how the lamp could help.

author: ian.rees
author_rank: Contributor
timestamp: June 06, 2016, 12:20:39 am
content: No worries.I think people use lamps for mains current limiting like this because lamps are simple, and everyone used to have a few around the house/shop.If I had to deal with a similar transformer inrush issue today, it would be just about as easy to find an NTC as an incandescent lamp! Of course the lighting at my home workbench is terrible, and it's chilly, so that lamp does have some follow on benefits.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 06, 2016, 05:43:19 am
content: The lamp also features as visual short circuit indicator when put in series with a dut.

author: johansen
author_rank: Super Contributor
timestamp: June 06, 2016, 03:03:43 pm
content: Quote from: staze on June 05, 2016, 06:02:38 pmsince when do 100 watt lightbulbs trip breakers?

author: staze
author_rank: Frequent Contributor
timestamp: June 06, 2016, 03:07:15 pm
content: Good point... Though I'm sure I could gang enough of them together.Anywho, as someone said, it's hard to find 100W bulbs anymore. And the unit seemed perfectly happy powering on in my tests. Maybe due to what someone said about the medical units having oversized cores...

author: Ian.M
author_rank: Super Contributor
timestamp: June 06, 2016, 03:24:30 pm
content: Load it with the maximum resistive load you are ever likely to use on it. Switch it on and off a number of times, depending on the distance to and difficulty of resetting the circuit breaker feeding it - 20 times is probably plenty if its at the bench, but if its in a cupboard the other end of the building, you'd want to test it 100 times with no trips.If it doesn't trip, all is good, provided your bench PC and other test equipment that doesn't like power being cut unexpectedly is on a different circuit. If it trips, you'll need to consider implementing one of the soft start circuits that have been discussed. If it needs it, I'd go for a timer relay shorting out a power resistor (with a manually resettable thermal trip strapped to it) + slave relay to hold off connecting the secondary side load so that the resistor only has to handle the transformer's inrush and magnetising current.

author: uncle_bob
author_rank: Unknown
timestamp: June 07, 2016, 01:52:13 am
content: Quote from: Ian.M on June 06, 2016, 03:24:30 pmHiOr .... ummm ... plug it in... leave it plugged in and don't mess with all this stuff. That of course *assumes* you have a problem, which you will not with a laminated transformer (like the one in the picture) that has been properly designed.Bob

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 07, 2016, 06:01:44 am
content: Just to be clear on the soft start thing ... what level of peak inrush current should one expect from a toroidal transformer rated at, say, 120V primary and 750VA? If it's 10X the 6.25A then we're talking 62.5A give or take IF there's no series limiting resistance. Let's say we use a 10 ohm resistor in series with the primary then the max current would be about 12A but decline as the core sorts itself out.So, if the outputs were disabled during this time and then, once the core was OK the input resistor is bypassed and the outputs enabled. The input resistor would not need to be rated at 1440W as the length of time the resistor would see that level of power is small and over the 10 cycles, assuming the current drops to near zero, the resistor would likely see an average of about 800W for 1/6 second or about 133 joules. Many camera flash units dump more than that energy during a flash. I would think even a 5W resistor would be enough.So, to layout the components needed for a softstart circuit you'd want a time delay relay at the input that picks in, say, half a second, and a 5W 10 ohm resistor in the hot leg of the input to the transformer that will be bypassed when the input relay picks. The output could be controlled by a time delay relay and pick a half second after the input one picks OR the output could be turned on manually by pressing a button that picks it -- perhaps using a circuit in the input relay to inhibit turning on the output until after the input relay has picked. Sort of a modified EMO circuit...Brian

author: uncle_bob
author_rank: Unknown
timestamp: September 07, 2016, 11:45:10 am
content: Quote from: raptor1956 on September 07, 2016, 06:01:44 amHiPlan for at least a million amps for a few hours based on some of the tales ....It's not rocket science: measure the primary inductance and model it. Unless you have a really weird transformer, it will all sort out in less than two cycles. If you can not measure it take a look at a paper on transformer design. That will give you a pretty good idea.(secondary open, the primary inductance is what " takes the current to zero " ).Bob

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 07, 2016, 07:18:35 pm
content: Quote from: uncle_bob on September 07, 2016, 11:45:10 amIt is pretty amazing that this issue has such divergent views. My first thought on this was to use a zero crossing solid state relay but TE explicitly stated that that would be just about the worst thing you could do. Toroid's are a different beast with no air gap and the videos I've seen say they really do have a turn on issue so I'm not going to pretend they don't.So, if you're going to go to the trouble of building a soft start into it you might as well go the full nine and have an output that can be switched on and off separate from the transformer. Parts list then gets to: toroidal transformer, time delay relay at input, 5W 10 ohm resistor, relay for output, main on switch, circuit breaker or fuse, output on switch and output off switch, some indicator lamps.Look like a fun project...Brian

author: Circlotron
author_rank: Super Contributor
timestamp: September 07, 2016, 10:15:27 pm
content: The way I understand this transformer inrush current thing is like this: when the transformer is running steady state, one complete half cycle of mains voltage above the line pulls the core magnetism from negative max to positive max. No saturation. If though the transformer is switched on at the voltage zero crossing the core magnetism starts from zero, not negative maximum, and so somewhere during the following half cycle of mains voltage the core magnetism hits saturation and the current is only limited by the DC resistance of the windings. Plus other remanence factors too, but I think that's the guts of it.

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 08, 2016, 01:16:43 am
content: Yes, with larger transformers and most certainly toroids, the magnetic effects can't be avoided. For most things we as hobbyists work on magnetic effects are generally insignificant beyond the need to calculate and control for inductance etc. But with bigger motors and transformers a whole new realm of factors come into play and few of us have the experience to know ahead of time what the pitfalls might be.I will probably be building an isolation transformer kit in the coming months and I'll probably use a toroidal transformer as it's core. I could make do switching on and crossing my fingers that the CB doesn't trip and reset it when it does but rather than deal with that I plan to incorporate a soft start into and while I'm at it make it a more complete and professional job as described in my priors. I should be able to build a complete isolation transformer with 750VA output for about $250-$300 USD which is about what you'd pay for something from Amazon that is less capable and less usable. And again, it looks like a fun little project...Brian

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 08, 2016, 03:43:41 am
content: I have a 1000 Watt isolation transformer that would trip the breaker every few times you turned it on. I added an Anatherm NTC in line with the primary a year ago, and it hasn't tripped since.Simple solution. Not much downside, since this transformer had a slight step up anyway (120V to 124V). I never saw any reduction in output voltage anyway.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 08, 2016, 04:42:29 am
content: Quote from: raptor1956 on September 07, 2016, 07:18:35 pmNah, it's just everyday stuff on internet forums (or with humans, anywhere). For some people, it's so hard to admit being wrong in such a simple, beginner-level, well-known subject, that they easily end up in a defensive loop (even after such a long time!), which, of course, only makes them look funny. It often happens that the original "being wrong" was not embarrassing at all -- it happens to all of us, all the time; but the later "explanations" are hilarious.Such as in this case talking knowingly about inductance for modeling the inrush current; in reality, inductance value is meaningless because as described by others, the core saturates, which by definition reduces (or nearly removes) the inductance, causing the issue to start with. Resistance defines the maximum upper limit.If the inrush current is not specified by the manufacturer, then you have to measure to be sure. But it should be on the same order of magnitude as the nominal maximum current.

author: edpalmer42
author_rank: Super Contributor
timestamp: September 08, 2016, 05:29:03 am
content: I have a worklight on my bench that uses a 300W projector bulb. To protect the bulb from the turn on surge, I used a technique from the ARRL.If you look at this documenthttps://archive.org/download/arrl-1981-radio-amateur-handbook/arrl-1981-radio-amateur-handbook_text.pdf, on page 153 of the pdf (page 5-12 of the document) figure 20 shows a surge-suppression method that can be scaled to any situation. It's described in the section called 'Surge Protection'. Basically, put a power resistor in series with the load. Size the resistor to fit the requirement. Put a relay coil across the load and use a normally-open contact on the relay to short out the resistor. When the surge hits, the load tries to draw so much current that the voltage drop across the resistor reduces the voltage across the relay and the load to the point where the relay won't operate. As the surge passes, the voltage across the load increases until the relay pulls in and shorts out the resistor.For my 300W bulb, I ended up using a 20 ohm, 25W resistor. It needs to be high wattage because it has to carry the initial current surge. The actual dissipation in the fraction of a second that the surge is there is minimal. Remember that the relay contacts also have to be rated for the current surge and/or the system's full load current.This is better than an NTC because if you turn the power off and back on, the NTC doesn't have time to cool off so you get a large surge. The relay drops out immediately and is ready for another cycle.Ed

author: Ian.M
author_rank: Super Contributor
timestamp: September 08, 2016, 06:36:44 am
content: You are right about the need for a high power resistor with an adequate surge rating, but the problem with the voltage sensing relay approach is that a toroidal transformer surge tends to be highly asymmetric, due to core saturation, therefore the relay coil will see effectively half wave rectified DC from the beginning of the surge. A time delay relay is a much safer choice.

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2016, 08:53:48 am
content: Quote from: Ian.M on September 08, 2016, 06:36:44 amI agree. Ideally you want a timer relay which can run off the mains but if you can't find one you can make your own from a 555 timer, run off a bridge rectifier + smoothing capacitor, connected to a lower voltage winding wound on top of the toroidal transformer.

author: edpalmer42
author_rank: Super Contributor
timestamp: September 08, 2016, 04:27:03 pm
content: First, the OP doesn't have a toroidal transformer, he has a regular transformer. IanJ, who has the toroidal transformer, doesn't feel he needs a soft-start. So the characteristics of a toroidal transformer, while interesting, aren't part of the original conversation..Second, a timer is an acceptable solution for a device that presents a stable load to the AC network, but the OP has a general-purpose isolation transformer that will most likely have different loads - which may have starting surges of their own. The timer has no knowledge of the load or the surge so the delay may be too short or too long. If too short, there will still be a surge when the relay operates. This might require a larger relay to prevent burning the contacts. If the delay is too long, the series limiting resistor will have to be much larger to handle the surge current and power dissipation. In my example of a 300W bulb protected by a 20 ohm 25W resistor, the resistor is instantaneously carrying a load of something like 10 KW due to the bulb's low initial resistance. This is the second resistor I put in the circuit. The first one couldn't take the surge. Even though the total power dissipation is low, the surge current, which is far beyond the resistor's rating, caused the resistance wire to open.So for variable load situations, a time-delay relay is not a good solution. If transformer characteristics upset the relay's operation, then some form of voltage-sensing circuit could be used to provide more reliable operation of the relay. Note that for anything other than hobbyist uses, a voltage-sensing circuit is required. Pull-in voltage for relays is not well-defined.Ed

author: uncle_bob
author_rank: Unknown
timestamp: September 08, 2016, 04:42:18 pm
content: HiThe gotcha with all of the ARRL surge suppression stuff is that it is *not* targeted at the "isolation transformer problem". All of it is designed for another purpose: To take out the surge caused by charging the capacitors on a big DC supply. That is indeed a very real issue and you can calculate what needs to be done to take care of it. Capacitors slurp up a big dose of current when they are given a voltage step. That's the way they work. You *can* put a choke in series with the caps to suppress the current spike. That is an expensive thing to do. The choke does *not* immediately move a lot of current when presented with a voltage step, thus no problem.Bob

author: edpalmer42
author_rank: Super Contributor
timestamp: September 08, 2016, 06:45:54 pm
content: Hi Bob,Yes, the ARRL idea was originally used to suppress the surge of capacitors charging and that could be handled by a rather large choke. But does it matter? A surge is a surge. The transformer surge will likely be shorter than the capacitor surge, but the relay can handle both. It quite happily handles my incandescent lamp surge.I like the idea that the resistor is in the circuit for as long as necessary, whatever that is, but no longer. Yes, there may be some cut-and-try work required to get things working with your particular transformer and relay. I've noticed that some relays don't spec pull-in voltage and different transformers would certainly have different surge characteristics.Ed

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 08, 2016, 08:14:05 pm
content: If the core saturation is well understood to last, at most, 6 cycles, then a time delay relay can be set some short period longer than that -- perhaps 0.3 seconds or there abouts. I'm not sure I see the need for a higher power resistor given the small total ENERGY it needs to handle -- the bigger issue I'd think is current handling ability. But, if you used a 20? resistor with a mains voltage of 120VAC the actual max current should be less than 8.5A no matter what the core is doing or, in the case of a PS with huge caps, what the caps want to do. So, someone need to explain just how and why a larger power resistor is needed given the relatively low energy it need to handle.Brian

author: julian1
author_rank: Frequent Contributor
timestamp: September 08, 2016, 08:28:29 pm
content: QuoteWikipedia doesn't give the reference but states,QuoteI've seen similar multiples quoted elsewhere.

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 08, 2016, 09:14:17 pm
content: Quote from: julian1 on September 08, 2016, 08:28:29 pmGood info...So 60X the 6.25A rating would be ... 375A -- that would be worst case but no CB in the 15A-20A range would handle that without tripping. But, with a 20? current limiting resistor on the input of a 120VAC mains supply the peak current would be less than 8.5A no matter what the core was up to or the phase angle of the voltage.Brian

author: edpalmer42
author_rank: Super Contributor
timestamp: September 08, 2016, 09:36:47 pm
content: Quote from: raptor1956 on September 08, 2016, 08:14:05 pmThat was my thinking when I used the first resistor to protect my projector bulb. I think it was still 20 ohms, but maybe 10 watts. It worked for awhile, but then it opened like a fuse. So I did the calculation. 120V, 20 ohms, 8.5 amps. So? Wait. 20 ohms, 10 watts means that full load current is ... 0.7 amps. Oh. Maybe it didn't like a current surge over 10 times its rating. Oops! Let's try a 25W resistor. That's only about 8 times its rating.Well, it works. Whatever.In case anyone is wondering, you'd need a 1500 watt resistor to get 8.5 amps through a 20 ohm value. I could do it. I've got a couple of electric stove elements in my junk box that would be perfect, electrically. But I don't think I will.Ed

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 08, 2016, 09:41:52 pm
content: Quote from: edpalmer42 on September 08, 2016, 09:36:47 pmAw, why not? Then you would have a coffee warmer to go with it!

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 08, 2016, 09:57:27 pm
content: Quote from: edpalmer42 on September 08, 2016, 09:36:47 pmI've been playing around with some 30ga wirewrap wire which is rated at 338mOhm/m and a rated ampacity of 0.86A. It can handle 10A for 10sec, 15A for 1 sec, and 86A for 1msec.But, it's hard to know how the power resistor is constructed so there may in fact be something in the construction of that type of resistor that has a brick wall limit of current that's in the single digits...Brian

author: edpalmer42
author_rank: Super Contributor
timestamp: September 08, 2016, 10:14:09 pm
content: Quote from: raptor1956 on September 08, 2016, 09:57:27 pmOr maybe there was a manufacturing flaw in that resistor. Don't know. It was one of those gold colored aluminum/ceramic types so there was no way to get into it. I didn't even think to try, I just realized that I'd put too much current through it and moved on.Ed

author: Ian.M
author_rank: Super Contributor
timestamp: September 08, 2016, 11:51:59 pm
content: Quote from: edpalmer42 on September 08, 2016, 10:14:09 pmMeaningful brick wall current limits are rarely found in resistance elements. However brick wall energy limits are common.Consider a wirewound or foil resistance element of some known resistance alloy. The ultra short non-repetitive pulse power limit is directly related the thermal mass of the resistance element itself, and for a known alloy can be estimated from the energy that would be required to raise the resistance element to its melting point.For longer pulses the thermal mass of the former or substrate becomes significant and once you get out to the whole second timescale, its the thermal mass of the whole resistor.However stresses in the element may cause it to neck near its melting point causing hot-spotting on subsequent pulses, it may gradually oxidise, and thermal degradation of its insulation may produce byproducts that corrode it, so the repetitive limit will be considerably less.*SOME* resistor manufacturers have good datasheets with pulse power ratings, typically 5 to 10 times the continuous rating for whole second timescale pulses, and a few even have application notes: e.g.http://www.ttelectronicsresistors.com/pdf/application_notes/Pulse-Overload_AN.pdfA mains rated heating element will certainly withstand the pulse power of a mains soft-start application, but it would be advisable to add a thermal fuse or cutout rated for an inductive load in case the shorting relay fails.For bench use, you can also use NTC thermistors designed for anti-surge applications, but for reliability reasons, even though they may be rated for continuous operation, one should use a shorting relay, which also reduces the risk they will still be hot, and thus ineffective if the supply is briefly interrupted. One *ASS*U*ME*s the user will be smart enough not to repeatedly frobulate the upstream switch, as that can cause worst-case core saturation with the thermistor(s) already hot enough to be near minimum resistance.Controlling the surge while staying within the resistor's pulse power limits is $EXPENSIVE$ and either requires a very high wattage resistor or a higher value resistor (to limit the peak power) than would initially be expected. The purpose of a bench isolating transformer does not and should not include soft-starting its load, as the load should have its own soft start circuit if required. As such, the use of a second relay to delay load switch-on till after the anti-surge resistor has been shorted and the transformer current has stabilised, is likely to be cost-effective at higher VA ratings, as a much smaller resistor can be used if it only has to handle the unloaded transformer surge.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 09, 2016, 12:07:48 am
content: Quote from: Ian.M on September 08, 2016, 11:51:59 pmOf course for a piece of lab gear, to be used by people who in theory know (some) what they are doing, you can just put another switch after the isolation transformer. Turn on the transformer, which has an NTC or relay, or both to keep it form tripping the breaker. Then hook up your load and turn on the load switch by hand, or ramp it up with a Variac, etc.If you are going to make a product then it needs to be idiot proof. If you are making something for use in your own lab then it only has to be you-proof.

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 09, 2016, 02:25:54 am
content: Here's a first pass on what I'd do to build a soft start isolation transformer. As you can see (hopefully) until input time delay relay K1 turns on at, say, 0.5 seconds the toroidal transformer is current limited by R1 -- a 20 ohm 25W resistor. Additionally, until K1 picks the output relay, K2, is inhibited from turning on because it needs K1-2 to be closed in order to turn K2 on and K1-2 is open until K1 times out. Once K1 times out K1-2 closes and the output relay can be turned on. To turn K2 on K1-2 must be closed as above and momentary switch "Output ON" is pressed. Once that happens and relay K2 picks the output as activated and relay K2 is latched with K2-3.So, with this arrangement the output can not be on until K1 picks AND the output is manually turned on with "Output ON".As I mentioned in a prior the output latching arrangement is very like an EMO circuit.Brian

author: Ian.M
author_rank: Super Contributor
timestamp: September 09, 2016, 03:25:17 am
content: IMHO that's *Waaaaaay* over-complex. Also heavy duty relays/contactors with auxiliary contacts don't come cheap. As long as your time delay relay has sufficient isolation between its poles not to compromise the transformer's isolation rating, simply use a DPST time delay relay, one pole to short the resistor, the other to connect the load to the secondary. If the relay's pole-to-pole isolation is doubtful use separate SPST relays, either both time delay or driven by a master delay relay or circuit.I also don't like the idea of pushbutton output switching much - simply use switched outlets or if you want something that's actually safe to work on the load without total manual disconnection, use an isolation switch designed for lockout-tagout.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 09, 2016, 04:14:16 am
content: Looks pretty cool if you are making a product to sell. However, Installing an NTC takes about five minutes and one part (plus some heat shrink perhaps). It may not work well for the once a year flickering power outage, if that turn on happens to line up with the wrong phase of the AC cycle, but otherwise it just works, and lets you move on to other projects. Plug a power strip into the output and you have your DUT power switch, and can leave the transformer on for your whole work session.Do get me wrong; I love over engineering stuff too.

author: SteveyG
author_rank: Unknown
timestamp: September 09, 2016, 07:54:58 am
content: OTT soft starter - although the soft start bit only consists of resistors for current limiting:

author: julian1
author_rank: Frequent Contributor
timestamp: September 09, 2016, 08:43:26 am
content: I've experimented a bit with NTC's and toroids. There's a couple of problems with using them by themselves.- if power is cycled or if there's a short interruption to power, the NTC stays hot, and therefore offers no unrush protection. In fact it would take a minute or more for mine to cool.- With no load on the secondary, there's not enough voltage drop, to even keep the thing warm. That means it will thermally-cycle between times the transformer is loaded and when it's not.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 09, 2016, 03:26:32 pm
content: Quote from: julian1 on September 09, 2016, 08:43:26 amYes, it takes a while for the NTC to cool and reset. That is why you pit the DUT on a separate switch or on a power strip. You just leave the transformer on the whole time you are at your bench. This is like old tube gear. You need to wait a little while before turning it back on.Personally I run a Variac after the isolation transformer. Then that goes into a switch box with a volt/amp meter. It's certainly not foolproof, but it was easy and cheap (free sample NTC).If I were going to make something more complex I think I might make it as a separate outlet box, with its own plug. That way I could use it to limit inrush on anything I plugged into the box, and I could change the order around or remove it per application. In that case the added complexity would be useful, as it would be more of a generic inrush limiting tool. Now that I might consider building!

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 09, 2016, 08:37:39 pm
content: Quote from: Ian.M on September 09, 2016, 03:25:17 amYes, higher current time delay relays are fairly pricey and that would drive the cost up. The push-button output is kind of driven by the need to make sure there is no output load until the input resistance is bypassed and that means relays. The thermistor approach doesn't make sense to me given the potential cycling issue while there still hot.The cheapest solution would be a couple of DPDT switches with the necessary rating and then manually throwing them in the right sequence at the right time -- throw the first one after a second to bypass the series resistor then throw the second one when you want to turn the output on. Doing that to avoid the need for more expensive relays could save $150 or so. That approach should be fine for the guy that knows what there doing but it has risks that limit it ability to be sold or even given to someone.Perhaps a cheaper approach, if you still want to use relays, would be to add a third relay. Use a cheap, low current ON delay relay to pick a higher current relay to bypass the resistor. So, instead of buying a high current ON delay relay at, say, $150, buy a $20 ON delay relay with lesser current rating but only use that to pick the input relay and enable the output relay. Both the input and output relays are simple dumb relays that only need to have sufficient current handling and no smarts and would therefore be much cheaper. With this approach the relay cost in total could be kept below about $60 and perhaps quite a bit below it.Brian

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 09, 2016, 09:41:56 pm
content: Quote from: raptor1956 on September 09, 2016, 08:37:39 pmHow about a four position rotary switch. OFF -> START (resistor in line) -> ON (no resistor) -> LOAD (turn on load).That is actually a simple, elegant solution that is fairly foolproof unless you whip the switch around crazy fast. Just get one with enough clicking force in each stop that you have to move it slowly. Would just have to be sure the resistors could handle being left switched in permanently.QuoteHoly crap! Those are expensive relays.

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 09, 2016, 10:57:57 pm
content: Here's an interesting technique:http://www.bonavolta.ch/hobby/files/MotorolaAN1542.pdf