Soft starting larger transformers (<1 kVA) and power supplies - Page 1
Forum section: electronics / projects

author: Benta
author_rank: Super Contributor
timestamp: April 06, 2022, 08:00:03 pm
content: A semi-recent thread about this problem got me thinking:https://www.eevblog.com/forum/chat/1000-va-230-vac-50hz-how-much-inrush-current/This due to the fact that I'm installing a 1 kVA 230/230 V isolation transformer for my workbench. And it keeps popping my fuses.I never got around to solving it when I had the same problem ~5 years ago, but this time I put on my thinking hat.I designed the attached circuit. It's simply a Triac phase-control to limit the start current and trigger the transformer at the right time, with a relay cutting in after a predefined time for full power.(Yeah, yeah, I know, "throw an Arduino at the problem". Not my style, sorry).It's not built yet (waiting for parts), but simulates perfectly.Feel free to use it for your own projects.Cheers.

author: free_electron
author_rank: Super Contributor
timestamp: April 06, 2022, 08:21:42 pm
content: a simple NTC of 40 ohm or so will do it. bridge that with a relay after 5 seconds. no need for triac kerjiggery...

author: Benta
author_rank: Super Contributor
timestamp: April 06, 2022, 08:43:40 pm
content: Now we just need to source a +200 W NTCDo you really think I didn't consider that? Or did you miss the little "k" before "VA"?If you don't like my circuit, fine. Suggest your own instead. Just ridiculing everything else is not really constructive.

author: KaneTW
author_rank: Frequent Contributor
timestamp: April 06, 2022, 10:10:14 pm
content: Two solutions:1. Replace your circuit breaker with a C or D type.2. Use a peak switching SSR, e.g. RM1C40D25 for a 25A model.No need to DIY anything.

author: Benta
author_rank: Super Contributor
timestamp: April 06, 2022, 10:19:32 pm
content: KaneTW, I don't think you understand the issue. The relay you linked to is quoted at 90...100 Euro. Are you from this planet?Try to inform yourself and read the thread I linked to.

author: jbb
author_rank: Super Contributor
timestamp: April 06, 2022, 11:26:01 pm
content: Nice concept!In the past I just used a wirewound resistor instead of a phase controlled TRIAC.Absent blowing fuses, this approach might help those experiencing nuisance tripping of Residual Current Devices (RCDs) or Ground Current (?) Fault Interruptors (GFCIs).A couple of suggestions:- how about a discharge resistor across C2 so that it ‘empties’ quicker and isn’t prone to switching in early when the incoming supply switches off and back on quickly?- I haven’t done a serious TRIAC design before, but I recall that SCRs can be sensitive to sharp dV/dt transients and may need an RC snubber between the anode and cathode.

author: Benta
author_rank: Super Contributor
timestamp: April 07, 2022, 05:00:45 pm
content: Quote from: jbb on April 06, 2022, 11:26:01 pm1: no real need, the Zener and BE junction of Q1 plus R1, R2 and RD will discharge both C1 and C2. However, adding a reverse diode across R2 would speed up things.2: the selected Triac is a so-called "snubberless" type and is not really sensitive to this.Thanks for the feedback, I'll think about incorporating the diode for a "version 2".

author: tkamiya
author_rank: Super Contributor
timestamp: April 07, 2022, 05:13:25 pm
content: Is your load always connected to this transformer? If not, how about using a series incandescent light bulb to cause a voltage drop? Then second later, short that out and feed full voltage to the transformer? I don't have an access to it right now, but Linear Amplifier circuit for ham radio often contain soft start circuit. Perhaps you can do something similar?

author: KaneTW
author_rank: Frequent Contributor
timestamp: April 07, 2022, 06:03:21 pm
content: Quote from: Benta on April 06, 2022, 10:19:32 pmYou didn't mention it was a cost sensitive application. These are just the standard industry practices to avoid it. A circuit breaker is like 5-10 euro and will solve the problem (assuming your short circuit current in the wiring allows it). The relay prevents it entirely, and does basically the same as your circuit, just as a finished and certified product.

author: Benta
author_rank: Super Contributor
timestamp: April 07, 2022, 06:20:51 pm
content: I see I need to explain the background of this circuit a bit better.The problem is larger transformers driven into core saturation when switched in at a "bad" time during the 50/60 Hz cycle. This will very often slam the mains fuses.This happens when the transformer is switched on at the mains voltage zero-crossing point.The circuit presented will switch on the transformer at peak mains voltage (rather, shortly after), which is ideal for avoiding transformer core saturation and resulting current spike.An additional advantage is, that the charge current for large secondary storage capacitors is also limited. This was not the original intention, but is a nice side-effect.I attach the voltage simulation over the "LOAD" terminals [OUT (L)] and [OUT (0)] before the relay closes. This is independent of load size/resistance (up to 1 kVA).

author: Benta
author_rank: Super Contributor
timestamp: April 07, 2022, 09:18:26 pm
content: Quote from: KaneTW on April 07, 2022, 06:03:21 pmEverything in Electronics Engineering is cost sensitive. That's why industry pays good salaries for good engineers. And this is an EE site.Your approach is more like an electrician, where the boss pays the materials. Totally OK, but off-point here.

author: profdc9
author_rank: Frequent Contributor
timestamp: April 07, 2022, 11:54:53 pm
content: Here's a soft start I have used with 800 - 1100 VA toroids and charged capacitorshttps://github.com/profdc9/PowerAmpAudio/tree/master/SoftStartThere are resistors switched in briefly before shorting them with a $2 relay.

author: KaneTW
author_rank: Frequent Contributor
timestamp: April 08, 2022, 12:07:49 am
content: Quote from: Benta on April 07, 2022, 09:18:26 pmMy approach is minimizing total cost, NRE + unit*volume. For this case I don't see a situation where the DIY solution would be cheaper than a premade solution.It's a different situation if this an exercise or such of course.

author: bdunham7
author_rank: Super Contributor
timestamp: April 08, 2022, 12:47:13 am
content: Quote from: Benta on April 07, 2022, 06:20:51 pmFirst, thank for posting a workable project. I think it is one of the best contributions one can make to EEVBlog, even a simple one.However, I've seen this claim made about the perils of switching on at the zero-crossing point and I'm really not clear on exactly why this is claimed. Your mains are 50Hz, so 20ms per cycle or 10ms per half-cycle. If you switch on at the point 5ms after the positive-going zero voltage transition (peak voltage), and the core is already magnetized in that direction (the worst-case fuse-blowing scenario) you are less likely to saturate the core that cycle as both the average and RMS voltage applied over a half cycle will be half of a full-cycle. And in addition, if you do happen to get to core saturation right at the end, the voltage will be cut off very soon instead of going on for another 1/4 cycle whilst saturated. That makes sense to me.However, I don't see what zero-voltage crossing has to do with this. Would the result be different if you used a power MOSFET and turned the current on for only thefirst1/4 cycle instead of the second? I can't see why.And in regards to charging filter capacitors, I would think that peak voltage turn-on would result in the highest possible instantaneous current into the filter caps. Granted, the power tapers off quickly, but I think it would be very, very high for a 1-3ms. Whether that blows a fuse is another issue, but at a high enough current I think a typical panel breaker will initiate the trip in under 1ms via the magnetic trip mechanism. I have several large audio receivers from the 70's and I've worked on a few as well--the ones with the large power supplies and toroidal transformers all use a soft-start circuit that uses a relay, timer and large power resistor. The startup time is quite long--like 5 seconds--and I've always assumed that was due to charging the very large filter caps. One example I'm restoring now has 6800uF x 4 charged to ~80V, so perhaps 90J. I would think if you gave that the output from your device, the average input current during the several cycles of charging time would be more than half of what it would be with no soft-start at all (which would be a lot of current) and the peak currents would be no lower.

author: magic
author_rank: Super Contributor
timestamp: April 08, 2022, 07:33:45 am
content: The transformer starts unmagnetized. The worst-case core saturation scenario is application of a full half-cycle to it, which would normally reverse the magnetizing current from negative peak to positive peak or vice versa, but now it attempts to bring it from zero to 2x peak, which may end up being even more. To put an unmagnetized transformer into its steady-state operating point right away, it has to be turned on at peak voltage.If a transformer is pulsed at 50% duty cycle then the choice of quarter-cycle used doesn't seem to matter, although triacs make it easier one way, while output capacitive loads would prefer the other.

author: bdunham7
author_rank: Super Contributor
timestamp: April 08, 2022, 02:09:44 pm
content: Quote from: magic on April 08, 2022, 07:33:45 amI thought the worst-case would be if the the transformer had previously been turned off right at the end of say a positive cycle there would be residual magnetism (remanence) in the core and if, by random chance, the transformer was turned back on right at the beginning of another positive cycle, the combination the remanence and the magnetization from the half-cycle of current would push the core into saturation. Thus the phenomenon observed by the OP where the fuse only blows occasionally and never on the first try (remanence fades with time?).QuoteJust to be clear, this is just so that the first period has half of the magnetization of the following full cycles? And you simply have to do a full turn-on right in the middle of a half-cycle and then keep the power on full after that point? That makes sense if you are starting from zero magnetization, so you're ignoring remanence if you do it that way in practice?

author: magic
author_rank: Super Contributor
timestamp: April 08, 2022, 03:45:31 pm
content: Well, yes, I'm ignoring remanence. TBH, I don't know if it's a major problem in real world transformers.At any rate, you are unlikely to know the exact state of permanent magnetization of the core (if any), so starting at zero-crossing is most likely to give trouble, and peak is least likely, on average.

author: IanB
author_rank: Super Contributor
timestamp: April 08, 2022, 04:18:45 pm
content: Quote from: bdunham7 on April 08, 2022, 12:47:13 amFor example, I saw an example recently where someone had moved to the UK from the USA, and had bought some step down transformers of various sizes to help operate US appliances. They worked nicely, except that the largest transformer (1 or 2 kVA) could not be plugged in without tripping the breaker, therefore making it impractical to use. One can debate the underlying reason, but nuisance tripping from inrush current on larger transformers is clearly a problem.

author: Robotec
author_rank: Contributor
timestamp: April 08, 2022, 04:50:16 pm
content: This happened to me and I solved it with 2 2x ohm PTC in series is the most cheap way and i´ve seen it as a way of doing a inrush current limiter and works pretty well.Edit: my transformer is a isolation one of 3KVA, true that i always used it for ~300W applications (but i got the transformer for 50 bucks)

author: joeqsmith
author_rank: Super Contributor
timestamp: April 08, 2022, 05:32:49 pm
content: Depending what's upstream, you may have to not only soft start but also soft stop. Showing a small transformer saturating on a 220 bus. Decent feed driving it, pulling about 350A. Fun stuff...

author: harerod
author_rank: Frequent Contributor
timestamp: April 08, 2022, 06:59:35 pm
content: Quote from: free_electron on April 06, 2022, 08:21:42 pmThis is a pretty standard approach to that issue. Works like a charm, until an intrepid product manager comes along and tries to solve application problems by a series of quick power cycles... (no time for NTC cool-down)

author: Terry Bites
author_rank: Super Contributor
timestamp: April 09, 2022, 09:38:23 am
content: I've used this useful but tedious calculation from Anathermhttps://www.ametherm.com/blog/inrush-current/transformer-inrush-current-40va-transformer.So I got a bit bored and made up a spreadsheet I can’t vouch for Ametherms factors, but they seem sensible. The calculated result reduces the inrush by a factor of 5. I think you have to be careful not to over specify the energy rating or the thermistor will run too cold. Note that these things have very long time constants so a restart could be an issue if the thermistor is already hot. It depends what charge is still on the caps at restart. Simple one component solution but far from ideal.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 09, 2022, 10:24:15 am
content: The assumption that inrush current would be 30x the normal transformer current is a very broad and probably faulty assumption.quite some time ago I was designing a switch-on circuit for a 230V to 12V 600VA transformer and I tried to measure inrush current with a 1 ohm shunt resistor in the primary winding, and that shunt resistor had peaks of over 300V during an "inrush event". So the only thing I'm sure about is that the inrush current is limited by pure ohmic resistance of both the transformer winding and the utility wiring. Normally the primary winding of an (unloaded) transformer has a primary inductance of several Henries. However, once the core goes into saturation, that drops to a very low value, and the real Ohmic resistance is all that's left.Inrush current is also very much dependent on the brand of the transformer.There are a few factors which influence the inrush current.Normally the capacitors on the secondary side form a quite hard short circuit as long as they are not charged. I had the luxury of an auxillary power supply that kept those capacitors charged.Cheap transformers are used very close to their saturation point, and the inrush peak only happens after the core is saturated. I could see this very clearly on my scope. Depending on the phase shift at the shifting moment, there is zero inrush current, upto "extremely big".My final solution was to use a solid state relay without zero crossing detection (I think it was sharp S202S02 or S01 variant) and once figure out the phase angle at which there is no inrush current. I think it was about 60 degrees after a zero crossing. After that, I still sometimes had inrush current peaks, and those baffled me, untill I realized they occured about half of the time. The cause was the remanent magnetism in the transformer, and the solution was to always switch the transformer off in the same half phase of the power supply cycle. I had already built a zero crossing detector for the switch -in events, so it was just a few extra lines of code in the uC.Using either an NTC or a resistor + Relay are simpler options.It was quite a good learning experience for me though, and I had some special requirements for this power supply.

author: harerod
author_rank: Frequent Contributor
timestamp: April 09, 2022, 11:48:48 am
content: Benta, I just had a closer look at your design and the components. Thanks for sharing.How would you expect this design to fare at CE accreditation? I am thinking about the EN 61000-4-x series, industrial.https://en.wikipedia.org/wiki/List_of_common_EMC_test_standardsNever mind the control circuit, which would be pretty easy to protect.How would the TRIAC cope with Burst-4 and Surge-5?Is it necessary to protect that device?Any simple concepts to protect that component? Snubber, maybe?Edit: Yes, I see that the relais would shunt the TRIAC in steady state operation and thus protect the device under standard test conditions. However, I see a difference in just passing tests and designing for the test's intention.

author: Bud
author_rank: Super Contributor
timestamp: April 09, 2022, 01:36:15 pm
content: @OP I would add a bleeder resistor across the timing capacitor for repeatability of delay between short power off-on intervals.

author: nctnico
author_rank: Super Contributor
timestamp: April 09, 2022, 02:03:49 pm
content: Quote from: Benta on April 06, 2022, 08:43:40 pmUsing an NTC is the standard solution.Farnell even has a dedicated section for inrush limiting NTCs:https://nl.farnell.com/c/circuit-protection/thermistors/inrush-current-limiting-icl-ntc-thermistorsFollowing free_electron's suggestion to use a 40 Ohm one:https://nl.farnell.com/ametherm/sl22-40005/ntc-thermistor/dp/1703898

author: free_electron
author_rank: Super Contributor
timestamp: April 09, 2022, 03:12:44 pm
content: Quote from: Benta on April 06, 2022, 08:43:40 pmwhy ? this has been done for ages this way. even the computer power supplies do this !. you don't need a kilowatt ntc. that's the whole point.It is only to dampen the inrush current. Even if that transformer behaves as a dead short a 40 ohm ntc at 220 volt is a maximum of 5.5 ampere. easy peasy. the NTC's easily handle 30 ampere.https://www.digikey.com/en/product-highlight/e/epcos/p27-series-ntc-inrush-current-limiters?utm_adgroup=xGeneral&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic%20Search_EN_Product&utm_term=&utm_content=xGeneral&gclid=CjwKCAjw3cSSBhBGEiwAVII0Z0WGZvb49hPQjjfHICyj6N9YaShSxeOYK7_AET0BLNhrmd6_rRNIIhoCZIQQAvD_BwESo your fuse will not pop, the ntc will heat up , lower its resistance and provide a smooth start.The relay kicks in a after a few seconds bridging the NTC and switching on the load. the NTC is out of the loop.So next time before blowing your fuse think about what i wrote. Yes i noted the small K. You don't need kilowatt NTC's. you have 5 ampere tops.

author: Benta
author_rank: Super Contributor
timestamp: April 12, 2022, 07:25:21 pm
content: Quote from: Bud on April 09, 2022, 01:36:15 pmYes, @jbb had the same comment, so I went back and looked at this point.I analyzed again and have to agree it's an issue. Unfortunately it's not solved with a simple bleed resistor: an active "C2-reset" is necessary.I attach the "Rev1" version of the circuit which implements this. Additionally the simulations.Sim1.png:Red trace: C1 voltage.Blue trace: C2 voltage.Green trace: BRT22 LED current.Sim2.png:Purple trace: applied mains voltage. The interruption in the middle is for provoking problemsnone happen.The Triac/Diac circuit can be replaced with a resistor or self-heating power NTC or whatever according to your needs. I'm agnostic there. But the delay circuit works well and is built from jellybean parts, except for BRT22 and relay.

author: nctnico
author_rank: Super Contributor
timestamp: April 12, 2022, 08:37:46 pm
content: Quote from: Benta on April 12, 2022, 07:25:21 pmWhich I why I typically use a dedicated chip (voltage supervisor) for these kind of designs instead of a discrete solution. Less parts and better behaviour.

author: Benta
author_rank: Super Contributor
timestamp: April 12, 2022, 08:48:25 pm
content: Quote from: nctnico on April 12, 2022, 08:37:46 pmLike an MC33164? Nice idea, I like it.Why don'tyoupublish anything, being so smart?

author: floobydust
author_rank: Super Contributor
timestamp: April 12, 2022, 09:00:44 pm
content: I measured primary inductance about 60mH on a 1,000VA EI core 115VAC primary I have lying around.

author: Benta
author_rank: Super Contributor
timestamp: April 12, 2022, 09:34:13 pm
content: Quote from: floobydust on April 12, 2022, 09:00:44 pmI find that implausible. It would imply a magnetizing current of around 5 A.

author: nctnico
author_rank: Super Contributor
timestamp: April 12, 2022, 09:50:38 pm
content: Quote from: Benta on April 12, 2022, 08:48:25 pmAs I wrote: I'd use an NTC for a transformer application. Nice and simple. I suspect anyone can wire up a component with 2 leads.However, if you really want a circuit with a predictable startup sequency, then a voltage supervisor chip is a good option. Something with an adjustable delay can be handy if the secondary circuit needs time to discharge itself. The LM8365 for example.

author: Benta
author_rank: Super Contributor
timestamp: April 12, 2022, 11:18:58 pm
content: @nctnico:Since you're the super-expert here on this subject, why don't you just present your own super-circuit?Just commenting and critizing others in a 360-degree-circle isn't very helpful.I'm certain that with all your experience, you can post functioning circuits from your library within minutes. That would be helpful to everyone.

author: nctnico
author_rank: Super Contributor
timestamp: April 12, 2022, 11:48:14 pm
content: Feel free to PM me for a quotation for my design services. Other than that I have to refer you to the datasheet of the chip I mentioned so you have a good starting point. IOW: I can tell you which ingredients to use to bake a good bread but I'm not going to hand out bread for free.

author: free_electron
author_rank: Super Contributor
timestamp: April 13, 2022, 12:24:54 am
content: Quote from: nctnico on April 12, 2022, 11:48:14 pmhe want you to put it in his mouth , move his jaw and clean up the day after as well...

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 13, 2022, 12:30:45 am
content: If the efficiency loss of the NTC is not acceptable, an easy fix is to bypass it with a relay connected to the main capacitors. It will close once there's a decent amount of time to allow them to precharge.