Simple amplifier soft start for inrush protection? - Page 1
Forum section: electronics / beginners

author: Per Hansson
author_rank: Unknown
timestamp: October 17, 2021, 08:56:58 pm
content: I have been fixing up anancient LEM amplifierfor a friend of a friend.It has been a learning experience: it has two 550VA toroidal transformers and originally one of the 4x 15000µF 80v bulk capacitors was shorted above 50v.That is to say it tested fine on my ESR meters but when you hooked it up it blew the fuse.Through that I also learned the hard way how much energy such a transformer requires with no load!So I felt quite confident that with new capacitors it will work fine but it does not. (Probably because their ESR is much lower than the originals).I boughtthese on Mouserrated for 100v instead since the unit runs nominally at around 80v.Now with these to get the amplifier to start is a matter of luck:With the toroidals on their own you could at least get lucky and time it with the AC mains cycle at its peak and then it worked.But with the new capacitors it pretty much trips the 10A house circuit breaker 9.5/10 times...I first researched to use inrush limiting NTC thermistors but soon found that they are not available to handle such large capacitance values.So a soft start seems mandatory now, and I thought I had figured out the most elegant solution:Just a 100VDC relay with its coil connected to the bulk capacitors.They would charge through a high wattage resistor and as they reach 70v the relay should pull in (at 70% of its rated voltage).But then I read that this is bad because the pull-in will be "soft" and create more arcing and wear on the contacts than normal.So I made the circuit a bit more complex by using a MOSFET with built in zener protection diode:I have tested this on a breadboard with a 24VDC relay and have made the following conclusion:With the resistor values shown in the attached schematic I reach 70V before it pulls in, so that is very good.However when it does pull in it does so at only 12VDC, i.e. half of the rated coil voltage, so the pull-in is very soft.My question is is there any way around this, i.e. to give the relay a hard turn-on with 24VDC?(I should clarify that the coil does see 24V+ when I reach 80v).Or would I then have to admit defeat and go for one of the more complex designs like these:Project39orSST-01that use a dedicated power rail?I have tried to understand if for example a SCR would have a less linear region than a MOSFET does.Or if some relay driver could help but I'm not sure really when ramping up the voltage like this?

author: Benta
author_rank: Super Contributor
timestamp: October 17, 2021, 09:42:45 pm
content: Your analysis is good.Bringing two 550 VA transformers on line is a challenge in itself, and then charging the caps afterwards makes it even worse.Your approach is good, and I hope the 100-ohm resistors are powerful enough.What you're looking for is a "snap-on" response for your relay.An idea would be to insert a Zener diode in series with R5. As a first try: 56 V Zener, R5 and R6 both 10 kohm. Additionally, I'd reduce R1 and R2 to 33 ohms just to get the voltage to increase faster.

author: Per Hansson
author_rank: Unknown
timestamp: October 18, 2021, 08:37:11 pm
content: This morning when I woke up I had the idea "schmitt trigger!" in my head.So since I anyway needed some power resistors and other misc stuff I added aTCA345Athat I found on the same website I order small orders from.To be continued when the parts arriveQuote from: Benta on October 17, 2021, 09:42:45 pmThanks, yes as I alluded to this has been a learning experience, these two threads where really helpful:can a transformer alone blow fuses?andAC Active Soft Starter for Inductive Load.Quote from: Benta on October 17, 2021, 09:42:45 pmIt is a 50W power resistor I happened to have, when the design is finalized I might go for something lower both in wattage and resistance, but then again that will require some testing to see what works best.Quote from: Benta on October 17, 2021, 09:42:45 pmI'm not at home now so I can't bredboard this but I did the next best thing and ran it through a simulator but the end result seems to be quite the same as I have now?I.e. the MOSFET will see a linear increase in voltage, so at around 63v or so it will not be fully on anyway?

author: Benta
author_rank: Super Contributor
timestamp: October 18, 2021, 09:12:12 pm
content: You're right, I had a brain fart in my first response. I meant DIAC, not Zener.Use 68 kohm for R5 plus an ST DB3 DIAC in series. Stay with 10 kohm for R6. That should trigger the MOSFET and after triggering keep the gate voltage at 10...12 V.Unfortunately DIACs are rare today.

author: Per Hansson
author_rank: Unknown
timestamp: October 24, 2021, 01:51:48 pm
content: Quote from: Benta on October 18, 2021, 09:12:12 pm(TLDR: Skip to my last paragraph)I had a brain fart moment too, I tried using the TCA345A I mentioned in my previous post to drive the MOSFET gate.But it has a NPN transistor output, doh!So I connected it to drive the relay directly, as that should work I thought, and well, it did once!Those that already had a coffee before me might realize the mistake:The open ciruit voltage is up to 70v before the relay closes.But the TCA345A has a built in 13.6v Zener to protect its output, bye bye zenerQuote from: Benta on October 18, 2021, 09:12:12 pmI tried the DIAC with the recommended circuit.But it has the same problem with the 24v relay switching on at only 12v as in my original schematic.The reason is that the voltage formed after the DIAC is not sharp either:I.e. it switches on at 32v but then if we increase the voltage to 35v we have only 3v at the output.So same problem...So now I drew the attached schematic, compared to the TCA345A here I have a complexity advantage:The LM393 comparator I'm using has the advantage that the non-inverting input is not tied to VCC.So as I ramp up the voltage it becomes much easier to calculate the divider ratio for the input resistors.Since both the TCA345A & LM393 will start working with a supply of only 2v, so we must make sure to not trigger early!As the voltage rises beyond the D1 Zener voltage the divider ratio gets smaller.And with the selected components it triggers at around 70v.The MOSFET sees instant 18v from the Zener and the relay triggers with a hard snap-on.Then the selected hysteresis resistor turns it off at around 67v.The comparator even offers 16mA sink so in theory I could directly drive a sensitive relay with it if freewheeling protection diodes where added.However I'm still not so convinced that the initial intention / subject of this thread has been maintained anymoreAt least I had some fun! Have I made any significant mistakes in my current design? This is my first ever op-amp circuit...

author: Terry Bites
author_rank: Super Contributor
timestamp: October 24, 2021, 04:56:34 pm
content: The real solution is to just not use torroids of course.A separate supply for the control circuit seems like a good idea. Dont get involed in solid state switching and Zeners that will give you more compexity and issues down the road when the go bad- and they will. I love the smell of scorching dust in the evening, better than a single malt.See Mr Elliots idea(s)https://sound-au.com/articles/soft-start.htma fab website for audio- not for audiofools, just striaight up years of experience.Mmmm NTCs. According towww.ametherm.com/blog/inrush-current/transformer-inrush-current-40va-transformerYou would have 70A of inrush per transformer! Requirng a pair of 1.2KJ thermistors. That's what, a 1000 kr. Madness. So a 44089 18V 1.5VA or similar is more like 50kr. The 2N7000 is a bit of a fossil but dirt cheap and still readily available. It looks like your relay draws about 20mA? So a 200mA MOSFET will do. I dont see a freewheel diode acoss your coil. Its a must. Delay type fuses too. You might also fit a snubbers across your NO and COM contacts- 100R in series with 100n will do. Get some big fat bleeder resistors on those massve caps. 80V from a stored 50J will shorten your life to a brief look of surprise.

author: Benta
author_rank: Super Contributor
timestamp: October 24, 2021, 07:24:36 pm
content: A problem I see with yout latest circuit is, that your resistance values are far too high. This makes parasitics and bias currents play roles in the functionality.As an example: an LM393 powered from +18 V is fine. But using 1M resistors to bias an input from there is utopic.The same thing probably happended with the DIAC. The leakage current prevented it from working as a DIAC.Don't be squeamish, you need to run several mA through this kind of circuit, not uA. And if a couple of resistors protest, use larger ones.

author: Benta
author_rank: Super Contributor
timestamp: October 24, 2021, 07:28:04 pm
content: Quote from: Terry Bites on October 24, 2021, 04:56:34 pmWrong.The real solution is to use quality transformers that are not so starved on copper and iron that they saturate at the slightest overload. Whether toroids or EI makes no difference.But cheapness is cheapness, and Alibaba products rule.

author: james_s
author_rank: Super Contributor
timestamp: October 24, 2021, 07:45:00 pm
content: I have used a very similar approach to what you proposed except it was to bring up a 4kVA distribution transformer wired backwards to get 4160V to power a series lighting regulator. I used a large power resistor in series with the primary and then a contactor with a 240V coil wired in parallel with the primary with the contacts bypassing the resistor when closed. I have not had any problems with contact wear, the contactor pulls in quickly, there is some inherent hysteresis due to the way magnetic fields behave. Try it and see what I mean, connect a relay coil to a variable bench supply and try to make the relay pull in slowly, it won't happen. Due to the inverse square law, the closer the relay armature pulls to the coil, the stronger the pull, you get inherent positive feedback.

author: pqass
author_rank: Super Contributor
timestamp: October 24, 2021, 08:10:17 pm
content: Quote from: Per Hansson on October 17, 2021, 08:56:58 pmBreak R4 into two separate series-connected resistors; R4a and R4b.R4a is sized so that the coil receives 24V when the secondary is fully powered-up (assume R4b=0ohm for this calculation).R4b is sized to say, half the coil resistance. Then put a 470uF cap in parallel with R4b.The 470uF cap in parallel with R4b will give you snap-on action but after it is charged R4b will limit the holding current to save power. See Figure 4 athttps://sound-au.com/project39.htmIn case I'm not understanding the last part, "...see 24V+ when I reach 80V", you can over compensate. That is, at 70V, choose R4a such that the coil sees 24V, then add some more resistance to R4b so that it doesn't cook the coil at fully-on and save some power too.Also, WRT the relay switching, why not just use two 24V zeners+resistor(s)? See attached screenshot.Notice the LED turns-on only when close to the top of the hump (includes a slight mechanical delay). The capacitor appears as a short (1.2V) effectively removing one of the resistors giving a decisive snap-on action.Play with it here:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEAWB0DsZnMwZgEwBsAHAJySkgKRVU0ICmAtGGAFC7Ij7IBu44DFlwJ8xMKTFh4+ZuijdY8ZSsjEuPcGwBe4dIXBh8eg4Q3c+OkyDPWwB4z15sw6YtzUWa+TwRoGIGGMYSDEEMARqXBIcSARiXChYJGkVJNCPdIj0uK0AJ2seGlcA-QVUyDYAc2s-WtEFSv4WMuR3JjaUd2LwGmJgxugENgATEBaDTpYjcYJuEBGGADMAQwBXABsAF1HxzqnWYyY540XVzZ2x6eMjdpKu+bP17d3r8DE9907T5eedmret3Gh3A2EabAA7sCZscbmVYVA2AV8AgDKQHKjZgMKpV7I5MejuJiio9fhcmBsGGNAoNIOwoSiDAj7gjKlCJMY6vc6pUAA7A0pM+xeAQ0NkgQlcso8yEShzIYplEmVMaSnxykU-c4vKGE+76sqVaJ3GFzN5MWxi2U+dwkxkiyoFA2CwwDWKyljymiE5Ue4X3PWGtgAYw1Aa9tPgEDAxGgIkI9nQ+iICFwMZy7Fw6EVTLm+oC4IA9oYbFwaArSO57GLoDSbhBEgA1ADCzbYxeFlpQIi4CqSNIgAAUAPIAdQAogAlJgAZQAgo3xwAdADOk-HM4AkjOACrDyft8CJLsV+RIEJGc-zMCJRIAGXHABFD-h5CeKPJq+kr455Ilm8Om53quzYAKqTuuAByO5sEAA

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: October 24, 2021, 10:10:26 pm
content: 1/ relay and resistor.2/ relay and thermistor3/ Triac controlled soft start1 or 2 are simplest. 3 needs a microcontroller to change phase angle for triac.

author: MarkL
author_rank: Unknown
timestamp: October 25, 2021, 04:42:49 pm
content: Quote from: james_s on October 24, 2021, 07:45:00 pmThis is exactly what Heathkit does (did) in their higher power amplifiers. See snippet below from their AA-1640 2 ch, 200W/ch amplifier. I've had one for 35 years, and the surge protection relay, and everything else, is still working fine.Another option is a settable time delay relay. Typical delay ranges are 0.1 to 30 seconds, and are adjustable via a built-in knob.

author: james_s
author_rank: Super Contributor
timestamp: October 25, 2021, 07:26:32 pm
content: Interesting that Heathkit wired the relay coil before the resistor though, I wired it after the resistor, the thought being that once the surge has passed and the transformer core is magnetized the primary voltage rises and the relay pulls in. The way Heathkit has done it is going to create a delay that is apparently enough.

author: Per Hansson
author_rank: Unknown
timestamp: October 25, 2021, 08:14:43 pm
content: Quote from: Benta on October 24, 2021, 07:24:36 pmI did use the resistance values you suggested for the DIAC circuit, do you suggest I go much lower?As for the comparator circuit I posted I did breadboard it and it works fine on the breadbord.Well with the obvious side effects of adding the 10M of a multimeter in circuit can make it act up of courseWhat would you suggest, around 100K instead?Quote from: Benta on October 24, 2021, 07:28:04 pmAgreed, and this is not a cheap product, it is made in Italy, transformers included.It is also a rack mount unit and only 2U high (88mm) so good luck fitting a big EI core transformer in thereI don't have a decent picture of it but I shamelessly stole one I found on an auction site and attached it to this post.It also has a very manly solution to the problem it seemsQuote from: james_s on October 24, 2021, 07:45:00 pmThis is a good suggestion, my tests have been with a 24VDC relay and I can for sure make it pull in slowly, that is very obvious when increasing the voltage to it using a bench PSU...But your idea of putting a 230VAC contactor directly on the mains does sound feasible, I'm just not sure if the caps will have time to charge enough so maybe I will blow the fuses (or power resistor) anyway?Quote from: pqass on October 24, 2021, 08:10:17 pmThank you for explaining that circuit for me, I had actually read it before creating this topic but not fully realized the implications of it.Quote from: pqass on October 24, 2021, 08:10:17 pmRemember that a 24v Zener lets through any voltage lower than 24v, so this creates the problem of a slow pull in (around 12VDC using a 24VDC coil in my tests)But maybe together with your other ideas above that may indeed be feasible.

author: Benta
author_rank: Super Contributor
timestamp: October 25, 2021, 08:36:52 pm
content: Wow, nice rack! I like it.

author: Benta
author_rank: Super Contributor
timestamp: October 25, 2021, 08:42:15 pm
content: Quote from: Per Hansson on October 25, 2021, 08:14:43 pmMore like 10k for the LM393 biasing.Look, you're trying to power a couple of 550 VA transformers. This kind of "green" power saving is too Thunberg, somehow.

author: Per Hansson
author_rank: Unknown
timestamp: October 25, 2021, 08:55:06 pm
content: Quote from: Benta on October 25, 2021, 08:42:15 pmWell, I'm not trying to power them with the comparatorAnd just to clarify again that picture was something I found online, the LEM amplifier sits one up from the bottom.Do note the 3-phase power cable on the top!

author: pqass
author_rank: Super Contributor
timestamp: October 25, 2021, 09:16:00 pm
content: Quote from: Per Hansson on October 25, 2021, 08:14:43 pmNo.Look closer at the attached screenshot or at the falstad.com simulation.The zeners are stacked and in-series with the coil. They effectively block 48V from Vcc before they letanycurrent flow.They behave like a switch. The LM431 is effectively used as such in 99% of all switching power supplies out there.See first IV plot here:https://en.wikipedia.org/wiki/Zener_diodeJust like a forward-biased diode has a voltage drop, zeners can have a lot more drop when they are connected in reverse.

author: Per Hansson
author_rank: Unknown
timestamp: October 26, 2021, 05:24:37 pm
content: In reply #2 I show the reason for the slow turn on of the relay in my OP.The circuit suggested by Benta has the same drawback as in my OP:The voltage on my MOSFET increases linearly and as such the coil sees a gradual increase in voltage.Then in Reply #4 I switched to a DIAC but it behaves much the same way.And I think I understand why now, it really is just a diode that works in both directions.So the behavior is the same and the MOSFET is still not out of the linear area as the voltage ramps up.Then in the same message I posted the comparator based design, this works fine and fires the relay hard.I'm trying to understand the circuit by pqass because I really like it's simplicity.But I just run into thinking that it has the same issues as the first two above.That is that the 24VDC relay will pull in at around 12VDC or around 11mA of its rated 22mA as in my first circuits.Unfortunately I'm not at home so I can't breadboard it right now for a test...I'm also thinking that maybe I'm overthinking it

author: MarkL
author_rank: Unknown
timestamp: October 26, 2021, 06:35:48 pm
content: Quote from: james_s on October 25, 2021, 07:26:32 pmAh, right. They are clearly relying on the relay delay.Reading a bit in the "Circuit Description" section, they say the relay delay is about 15ms. So that's enough for about two 60Hz half-cycles of pre-charging for the main capacitors.

author: Per Hansson
author_rank: Unknown
timestamp: October 27, 2021, 03:50:26 am
content: Quote from: MarkL on October 26, 2021, 06:35:48 pmUnless my math is off to achieve a full charge in 15ms would require a 0.25Ω resistor, its peak power would be around 25kW!60000µF at 80v is 192J of energyOr put another way the capacitors have an ESR of 12.5mΩ that should result in a initial circuit load of 512kW.So while that 25kW number sure looks silly at first for sure it is a big improvement over no resistor!Another idea I had this morning would be to use two relays: this way snap action of the main relay can be assured.The resistance value R5 in this example is critical and might not be correct: it should be sized so it pulls in when the main relay sees 24VDC.

author: pqass
author_rank: Super Contributor
timestamp: October 27, 2021, 08:04:03 am
content: Quote from: Per Hansson on October 26, 2021, 05:24:37 pmSo I decided to play around some more with both my proposed approach and yours, then ended-up with a third.See attached screenshot for a side-by-side comparison.Since the last simulation, I've now correctly defined the relay (1.09K@22mA=24V) and lowered my zeners to a total of 36V (12V+24V) to block the low and slow Vcc initial ramp-up. I've also added a "power-save" resistor to both circuits (the one with the parallel cap) to give an initial punch of over-current before settling back to a lower coil holding current. Wherever you see a 1R resistor, I've effectively removed it from the circuit.As per the simulation, my  circuit would ramp-up the current faster (first to reach 22mA in the COIL CURRENT graph, 0.75 vs 1 division). But I'm not sure this would make much of a real difference. However, using the power-save resistor in your  circuit did improve it a bit.Then I remembered how crow-bar circuits work in power-supplies. The third circuit uses a triac (or scr) that is triggered when 72V is reached. You can use 3, 24V zeners or a Vbe multiplier circuit (the BJT+2 base resistors arrangement) to make the equivalent of a 72V diode drop. As you can see, this results in an instant sub-millisecond coil current ramp-up!Watch out for the power rating of: the zeners (500mW), BJT (900mW), and coil resistor (>1W). Also BJT Vce >80V.Let it run for 20ms and then stop it. See:http://www.falstad.com/circuit/circuitjs.html?ctz=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-Zx1LrINKyMckWB6ZexOZNqUA4AY3Dkwr8YjVce-WU7HkXGI0CmliSbLqu1xgyUrFQAZiZtmsX1B0BtvTtozDFW63Te1yIEQ-S4wvSGHtm8Px4j+SP8oZ7CPttbN-tYj2PtPK8uyE+J4OIofb9fDraVQ41-B1E23ACDzXM13C2QsKwWY1VSzJleW2A5J1g5CoN5TR5yWStPVAp880IdZCHtJCmGtdIKw1OltV1AR2xKMY9HI4sqIDNDwGUOMeHtVNOJozgDmNdcsPYmVRJWOpbSCNdZI2S0ZM-TZsNtTZpKUuTzHEnIggOLFRB0jS4I0aNNWhY1jMTZs9NTetTJSExtNskSO3gLs93Ycp2CkdxyjyDBdyGVj1LmS49NiKhunlMToJcmyYK0hNZxiTQUtbGDQOSlgAAdG0rBMYLw40RBw3SZwylgGwYS8sPqowfkeJRyCPaB10QXB7mmcocFwDtWAZRqKVhAVWXZZ1jUa3NQNzA4CpbcUXITVM8MWzB-VdeTPVK7aPSdJawxS+SiyOttnQ207m02ksmKm-bhMKtznSSSKMietcPv0zgCk6Youh89Q1DULgsDAZAewKFpAI6Epuh4voWLHOARmA8DEB2tTMZE-L0nAwTypMvawLMpyXoW-HyRcomXpJ2nsqS+Y8dJysGZMPC8gKaUcZANljv6QolHuHiqEhUZQToSDCBMqiBa7K82M-Q6ZZyc80zdJWJM+Eq83GDGnWYDcnQvLTRplcL5rjVXc26C1LpAZtYmbNa825F2K2EJtcYKj3fo9II9ubXmvZMg43u4RTUyhky-qKbyuAcLBJ0sNREEgVZEdh5Q9x6GHBZR4YtHd5MNHS3Hxjtp6q4A7g8mk0OXprhyYUbkPpWjiy6RdNughr3npJr3MP2Qweo4rEeKYPSekMn+bRflUid2XE1l8+TuiQxBi9X2QW2E1oe-nL+7pINqgMYM-D+JM6+112PNljbw3pWIl8hItm20JZ-mIveT9A7dn-ZylxVa5WND-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

author: pqass
author_rank: Super Contributor
timestamp: October 27, 2021, 05:26:00 pm
content: CORRECTION:Forget my first proposed zener+resistor solution. Go with the last (triac) circuit.My last simulation was unrealistic. Its time-frame was on the order of a quarter 50Hz wave-length. The one below is 1000x slower!I've since added in the 15MF cap and 100R slow-start resistor and that changes everything. It takes at least 4 seconds (2 divisions) to full charge the rail to 80V! In my first circuit, even if the zeners block passage until 36V appears, there is still a very gradual slope to 80V that doesn't have the ummpf to pull-in the relay. And the power-save resistor/cap is too short to do anything useful.With your circuit, the slow rise also lets the MOSFET linger in the region between Vgs(threshold) and when it's fully-on. Still 2 seconds spent there (1 division).In the last circuit, the Vbe multiplier snaps the triac fully-on when the rail reaches 72V however long that takes. With the 2k2 coil resistor @ 72V gives enough current to pull-in the relay quickly.Click the Reset button and stop 12 seconds into it:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0mRBmBB2ayAcA2ZBOBZsEBWYokIyc8yogUwFowwAoBAFhACY2A3ccNGDbFO6MDlFg4kTg2RQusacpWR0HbuGYAvEPUyb6ySB32HkRTly69WHMJz4Qwg4ZbESx02UYWclqqrq4FYsumamFlYReuhw1g48zGAYekQSaciYaRn0bDjZ2RAwVjAyROlEWJhwolii4JA4lFIqULCcoU2UZWBkZZBkLABOegammGyUMUYmCiwA5nqcyNHuy6t6bJMKkMx89DhZelnosWxnRphnLdTgfbvtRMwAJnpH2VeXOA7nZ1YvWgAMwAhgBXAA2ABdXn8Ttd3r96Op-iBAaDITC3ockddvscvtZ0eDobCcdE8XDCQDgSSYUtyWN0N9cZseswAO6xeLIi4nTjTFFQZijehENiRAVpCVwlrKZIGaWS6bi0xEY40jHQ+gQ2hvYq7WAsLlimW8y7jWWcsZqLYFMaGfLZPYm4ymJ0O932l0nOZigmW-3Osluu2fUNBom0zHW2YTKaesYJvYIBGM83vXJC9km9JrAmqzLB0WBybTQPq50ga57ADGvvj0wjlcNKjY8hS0CmyHEPwFCBWWHacBYKHLhiFMzit3ZQL0SE+hfoKzWMoHIAQw84Iq2cSVu+mFx61enewZR4Pl7yOxzV6XC-3e1FF9N7r3yPicpUsZf06264-dlRRXBsNgpZMtFzM1JyIJFs1YYwwKpEDAP4dkGUDD0Zm2Dh2QIaICCbKVGBqJNKE1OkdT1NDWxGPRCP5aYGIzL84FjbhywTEjONvZdNgzbirRNTgiEXUjl1EtIGh9CTFwaPi1mk2MRDScSUNE9lsRUsVxO0ssoy1GETT0riUP0ut50sUD6BM48BmkdstzYEh0E4TBLAcZy3IeAY6IU6yUL-ciBRklCI200MZIjP8p0-WMIwjQNIuYAAHayYoSuYDRk4RohiwK4vrVCBJiykeiNMgxC7FI4gQeAwDEbZh3gZgGVK5luXLNlhTSuNrMShNstSk5gkjIxRo9IbXQm+0+ryb14tGls5pbPZeowRsRrVY4pq2sjrPM5JUmXLj9JOlVSKKdpSg6ZyEC8AoGru9VGmaRo2jKTpXvK+B+iUIYfwAjTLLWUi1v-cDDyB0jdryACztsnqIes3L9th1G5oiuZwbhvKeQx0MhoFIozWB2aPR+4olFc96QChYYwVoHcSNUpjPgpkB2zYhkCDDej2b3dkx35qleb6obRgAyakQ5hBmljWC8r5fyWNjMWhU6Ux4LSnZUN1-Tdv1wazUOnXNt14HDbVGHSbBo7-kk-QExEsTKGyG6bpkZyaiaTgB2QbYVkcqQ3taZQty6N7ej+uAAa5XW-01zrhVGJOneCrXPy4ELrSETa08O-VbZaE3nbRaNSS5PPWez63bzToUH1Vrk6s2pvDsl1NGI3LuWPetj8J78MpXbsviUxKj9UeXoEIzuEG-iXapftaH0K4DqdPIjecN2VrwDNSaD5vYVXWIydA2195wz9WbLd3k1ZojMXkof12Rc34UoXfy14c0W4jH8MyVYTRjBCFcpgZ00AIHGFTFkfAECKi4FpuPUkooxaBmftjDczQ4BQL8ugwwYtgaUDlj6VeG4KS2jIZDChoFqHWUwbhNWb9yFPkRErS4MVszvWYYKZW6s+Q+g4jXYRYpfg+gxugWafZYgLQfnBaRcFBHWhdrEd8XDBasARE3IUUw+E3BPsnZE6ieTwS5Ko9OXBHbMnrlxRWnMgbiOtBjexkiFoAHt7jVlwpzJoZxCLQBwPkEwGB4BqEmJA6QBpQiIBAAANQAMIJOYJ4-I3iFBTBwGcC4sBXISjlpwEOogA7NXgPMEAAAtAAogAOSqQAJQAMopOzukygmT-guHbMTZAqYGoFA+KU6JrTNwAAUADyAB1BpAAdAAzo0gAgnEqp8z6lVMaQASUaQAFXGfUlpmhj6ZPkPZKmkB8DoFEsyHA4oMDIB7EEvwNgNwgASeMzZAAZeZCSACq9T1k1J2S0sAm5jkgKoL0ESzzQWvM+VUgAIj8-5gLgWeKYG03xWTwA1ECcE9scQpBYCamc-gMTXmJOSZ4-AmKOlbgkOqCUXsLmQADuckcZKNyxImdM+p8ylkrLWRs7ZeyDmeIaOC7FUhoDcCyBIBqAofiTFOQEZw1gGibgAOKLJ2VUw5ZAdjyEyTgSF0A1VU3wNCgAsuMxpAAxKpOz5k8oaS0pAtKIWkrKBcSYolLASHcrgOW9hrDus3O8r5yKAW1LRRuA1PiTmmvgNCiAVhiBwsRVG1FLSfgeqlbi7AfQIEQJQHLBqQz1UgFzZuSlOarCSrOILM1CR4kACNaDzKtb8z5OzNmjM+Zs11nj0DyElfIGO0KrBWBHa8iN3y5l-OjUClpQ4x1JocGQKdIAhybnhUihdKKY2sDON6fgYd4j1NoHMgAlnMqEIIAB2tYmZAA

author: mawyatt
author_rank: Super Contributor
timestamp: October 27, 2021, 06:50:03 pm
content: Don't know if this has been mentioned, but a simple method would be to place a light bulb in series with the mains, then bypass the bulb with a proper sized relay that is driven by the supply output voltage. The inrush current won't be any higher than the bulb by itself, and the large amount of energy to charge the supply caps will get spread over the bulb since the bulb resistance increases as it heats up.Best,

author: JohanH
author_rank: Frequent Contributor
timestamp: October 27, 2021, 08:23:30 pm
content: I've used this circuit (from Elektor 7-8/97) for many years for a subwoofer amplifier with a 330 VA toroid transformer. For some reason the R3 resistor has burned at some point. This resistor limits inrush current at switch-on, whereafter C1 limits current (C1 should preferably be X2 rated or similar). I changed it to a 2W resistor now to be on the safe side. Otherwise the circuit has worked fine. The circuit is meant for 230V and the relay coil is 24V (1200 ohm).

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2021, 07:09:58 am
content: Quote from: mawyatt on October 27, 2021, 06:50:03 pmwhat a great simple solution,hats off.

author: Per Hansson
author_rank: Unknown
timestamp: October 30, 2021, 08:22:53 am
content: Quote from: pqass on October 27, 2021, 05:26:00 pmThanks for your updated schematic, I will try to breadboard it this weekend and compare them.I did try to simulate the comparator circuit I posted in reply #4 on falstad but I failed to get it working.(I could get it working with an op-amp but that has the wrong characteristics).I still think the comparator is the nicest solution because it is easy to set the hysteresis also.(Since we are using the capacitors to hold the relay it is important that it does not pull out if the voltage sags)...Quote from: mawyatt on October 27, 2021, 06:50:03 pmI actually used this exact method when testing the amplifier:I brought it up first on a 60w then 300w incandescent light bulb and it worked fine.Then when I bypassed the lamp I instantly blew the 10A fuses on the transformers secondary!The reason was as I stated in my OP that one of the capacitors was shorted above ca 50v.Though as crammed as the unit is fitting a light bulb inside it would not really be feasible.Unless a really low wattage one would work, I may experiment because it is a fun way to do it!And it has the added bonus of the thing not burning up if the relay fails to activate!Quote from: jukk on October 27, 2021, 08:23:30 pmThanks, this circuit is similar to SST-01 that I posted in my OP, the reason I prefer to measure the voltage on the output capacitors instead is to get rid of the time-delay logic.And instead rely on the actual charge buildup of the capacitors to act as a timer, it may be overkill, but it is my goal with this circuit

author: Per Hansson
author_rank: Unknown
timestamp: October 30, 2021, 10:21:42 pm
content: Numbers time!So I did first a control shot, it came out at 2.5ms, it is just the relay driven directly with 24VDC.The scope is hooked up with channel A on the normally open contact and channel B the normally closed contact.There is no actual load on the relay: I'm just sending a small signal voltage to have something to measure...First is a test with the schematic in my OP (original post) in this thread, I'm calling it resistive divider + MOSFET.The time to switch was recorded as 16ms but this really is not the worst it can do:I can make it take up to 50ms if I increase the voltage really slowly!Next up is pqass VBE multiplier from reply #39, I had high hopes for this one and it takes 3.6ms.The reason for this can be understood by watching:w2aew video #198: Basics of a Vbe Multiplier: what it is, how it works & where it is usedFinally is the comparator circuit in my post #4It takes 1.6ms and this is really weird because this is quicker than my control test with direct 24VDC!I think the reason is that I was not actually using the same power supply for the control, so we can probably ignore that.But all else being said this is a very impressive switchover time! The reason is that there is no linear region: the MOSFET gets a sharp signal.

author: Per Hansson
author_rank: Unknown
timestamp: January 31, 2022, 07:26:23 pm
content: It's been a few months but finally I had this thing finished and thought I'd post the final schematic.There are some minor changes since the schematic in post #4:The inrush resistors should always be in circuit like the circuit MarkL showed in post #11I decreased the resistance of the bias resistors for the voltage dividers as Benta recommended in post #6I also had to adjust the divider ratio slightly because the inrush resistors I used would not reach the target voltage.So now I switch the relay at 60v instead of 70v like in that schematic.I also decreased the dropper resistor for the relay to 2.2K, I then measure 25v across the relay coil.I have also for clarity drawn the snubber diode for the coil, and included the fuses that are part of the original unit.It does seem to work well: the unit has a "protect" board and it remains on till after my relay energizes.So there is no pop in the speakers, and the inrush resistors only increase in temperature by about 5 to 10°C.I attached oscilloscope shots, red shows the voltage on the big capacitors, and blue is the relay.Thanks for all the advice given! This was a great learning experience in so many ways