Soft Start for a Power Supply - Page 1
Forum section: electronics / beginners

author: bsikar
author_rank: Contributor
timestamp: October 12, 2024, 05:32:17 pm
content: I am in the process of designing a power supply. The exact specifications shouldn't matter, but I am currently trying to implement a soft start.I was reading this article:https://sound-au.com/articles/soft-start.htmI attached a relevant schematic from the site, but I this makes the circuit not floating anymore since they attach the output to the neutral of the input. Is there any way to solve this problem? My transformer does not have an extra tap so I cannot use that as a reference.I was thinking that I could do a transformer less soft start that then feeds into another transformer, so the output is floating, but I am not sure if this is ideal.If possible, please send a good soft start circuit or even a completed power supply with a dedicated soft start circuit could be valuable. I have seen some soft start circuits that use the LM317 part of a powersupply to do the soft start, but this still has the bridge rectifier and some filtering capacitors experience large in rush current.

author: inse
author_rank: Super Contributor
timestamp: October 12, 2024, 06:31:48 pm
content: But you wouldn’t have to ground the secondary side.Maybe this is because the circuit originates from audio equipment.As primary and secondary are isolated via the relay, there shouldn’t be any problem.But still you are softstarting the entire control circuit as well - if that creates a problem, you also could place the circuit in the secondary power rail before the filter capacitor.

author: bsikar
author_rank: Contributor
timestamp: October 12, 2024, 06:35:56 pm
content: The primary is isolated via the relay, but the neutral will allow for the possibility of shorting to earth in some cases. At the breaker box I think neutral is tied to earth. If I implement their design and probe with my oscilloscope, I will have the possibility of shorting to earth which can cause damage right?My understanding could be 100% wrong though, please let me know if I am mistaken.

author: inse
author_rank: Super Contributor
timestamp: October 12, 2024, 06:39:43 pm
content: I don’t get it, you don’t have to ground the secondary of your power supply.The lab supplies I own have an additional clamp that allows to do so if necessary.I always prefer to have it floating

author: bsikar
author_rank: Contributor
timestamp: October 12, 2024, 06:42:39 pm
content: Maybe my understanding about how transformers are wrong, but I was thinking that I would use the top of the secondary winding as let's say REF+ and the bottom as REF-. Then my load would flow only through those references on the secondary winding (never in contact with the primary windings). In their schematic they are using the neutral reference from the primary windings, how would I do this same circuit, but without using that neutral primary winding reference?

author: inse
author_rank: Super Contributor
timestamp: October 12, 2024, 06:52:28 pm
content: Do! Not! Ground! It!

author: bsikar
author_rank: Contributor
timestamp: October 12, 2024, 06:56:21 pm
content: I don't understand, what do you mean by dont ground it? I don't want to ground anything; this is why I was confused with their schematic. When they are using the neutral line I was thinking that it would allow for a path to ground since neutral is tied to ground(earth):https://diy.stackexchange.com/questions/157478/why-is-the-neutral-tied-to-ground-in-my-homes-electrical-panel

author: inse
author_rank: Super Contributor
timestamp: October 12, 2024, 07:24:29 pm
content: Are we talking about the circled symbols?Or did I get it totally wrong?Green circle is advisable, red circle not.But you would have to wire it up a little differently as you are combining both transformers.Your power transformer would be hooked to the SA and N terminals and you probably would have to experiment with the delay capacitor as the softstart circuit itself is being softstarted.As well you will have to determine the right resistance and power capability of the resistor bank.I made some softstart circuits myself, one for the electric chainsaw which blew up three times until the right and final dimensioning had been found.The other for a shop vac that started with the table saw - this one worked faultless from the beginning.

author: bsikar
author_rank: Contributor
timestamp: October 12, 2024, 07:30:11 pm
content: I wasn't referring to the green or red circles, but rather SA and N.I only have one transformer, so I am unable to connect SA and N to another transformer. I was wondering if its advisable to either:1) make this without a transformer (since I only have 1 power transformer which i need for the psu)2) make this with only 1 transformer which I can also use for my psu.

author: inse
author_rank: Super Contributor
timestamp: October 12, 2024, 08:26:09 pm
content: In principle you could do it with something as simple as showing in the drawing.The relay coil voltage would have to match the secondary voltage.Problem is that the hold voltage of a relay is rather low, meaning if you do only a short power cycle, the relay may stay energized and not softstarting.You can also add the circuit you found but might need to adjust the timing as the softstart circuit itself is being softstarted.Using a PTC as soft start resistance is a good idea as in case the relay won’t energize in time, the resistor would protect itself.You can do a lot of calculations on this, determining the energy the resistor has to accept.

author: bsikar
author_rank: Contributor
timestamp: October 12, 2024, 08:28:38 pm
content: I see, I will try and simulate/build and test this. If you or anyone else has other thoughts let me know.Thank you for the help

author: inse
author_rank: Super Contributor
timestamp: October 12, 2024, 08:31:32 pm
content: The manufacturers (e.g. TDK) have graphs /tables to find the right PTC for the capacity to be charged.I changed my constantly exploding inrush current limiter to a PTC array, but this has to swallow a couple of kW for ms

author: Benta
author_rank: Super Contributor
timestamp: October 12, 2024, 09:20:59 pm
content: Yeah, that kind of circuit works most of the time.It doesn't work with an unstable mains supply or a child playing with the power switch (takatakataka...).

author: bsikar
author_rank: Contributor
timestamp: October 13, 2024, 01:18:23 am
content: What circuit are you referring to? And in your opinion, what circuit do you think would be best for my use case and why?

author: Andy Chee
author_rank: Super Contributor
timestamp: October 13, 2024, 02:35:52 am
content: Quote from: bsikar on October 12, 2024, 05:32:17 pmElsewhere on the site, there is this circuit, powered from the existing secondary, no auxiliary transformer required:https://sound-au.com/project39.htm

author: bsikar
author_rank: Contributor
timestamp: October 13, 2024, 02:58:54 am
content: I see, there is the warning of:WARNING: In the event of an amplifier fault at power-on, the fuse may not blow immediately with this circuit installed, since there may be no power to operate the relay. The current is limited to 200% of that at normal full power, so the fuse may be safe for long enough for it to destroy the resistor(s)! The ballast resistors will overheat very quickly, and if you are lucky they will fail. If you don't like this idea - Use The Auxiliary Transformer.Since I would have to do this without an auxiliary transformer I will have to test it.I do have one question about it, in this configuration I see Vcc which would be the Hot line from the wall, would the ground symbol they put be the neutral line? A and SA are the inputs to the transformer right?

author: Andy Chee
author_rank: Super Contributor
timestamp: October 13, 2024, 04:22:01 am
content: Quote from: bsikar on October 13, 2024, 02:58:54 amNo!!!!Vcc is your low voltage secondary @ 50 volts DC or whatever. Ground symbol is also connected to the secondary ground system.If the secondary is still unsuitable, then below is the direct mains powered version:

author: inse
author_rank: Super Contributor
timestamp: October 13, 2024, 04:31:20 am
content: This is only a delay circuit, the softstart resistor would be placed between A and SA.The circuit is powered by the main transformer as in my sketch.Ground and earth should be kept separate on a lab power supply.

author: jonpaul
author_rank: Super Contributor
timestamp: October 13, 2024, 05:32:19 am
content: simple:thermistor or Amperite thermal time delay relayj

author: bsikar
author_rank: Contributor
timestamp: October 14, 2024, 02:52:50 pm
content: I am having a hard time simulating this. Could someone please help me out by providing a circuit from ltspice (a .asc file).I found on this forum:https://groups.io/g/LTspice/filessearch?p=name%2C%2Csoft+start%2C20%2C1%2C0%2C0&jump=1the attached .zipThere is so much happening and their schematic is really confusing for me, but if someone is able to help, I think its a good starting place.

author: bsikar
author_rank: Contributor
timestamp: October 14, 2024, 02:56:42 pm
content: I dont want to use thermistors because if a bypass relay fails and excessive current flows through the thermistor, it may explode. Additionally, thermistors can be problematic in applications without consistent high power draw, as continuous thermal cycling can reduce their lifespan. Thermistors also have a wide resistance tolerance, making their behavior less predictable than resistors.

author: inse
author_rank: Super Contributor
timestamp: October 14, 2024, 08:52:25 pm
content: The take PTCs, they will protect themselves in case of relay failure.

author: Benta
author_rank: Super Contributor
timestamp: October 14, 2024, 10:06:59 pm
content: Designed this a couple of years ago. It works, and will take care of kids and unstable mains supply:It uses a phase-angle triac control, which makes it independent of the load.

author: bsikar
author_rank: Contributor
timestamp: October 14, 2024, 10:08:58 pm
content: At first glance, that looks really good. Thank you so much for the circuit. I will spend some time testing and simulating it

author: Benta
author_rank: Super Contributor
timestamp: October 14, 2024, 10:23:24 pm
content: Thinking back, I think it was a bit of a nightmare simulating it, but I'm no expert there.I think I separated the circuit into two sections:1: the left part up to the output of the optocoupler, replacing the relay coil with a resistor.2: the right part, which is really simple.Both sections simulated correctly (but independently), and after building the circuit it worked as expected.

author: inse
author_rank: Super Contributor
timestamp: October 14, 2024, 11:12:02 pm
content: This circuit is the negation of the KISS principleSorry, could not resist

author: Benta
author_rank: Super Contributor
timestamp: October 15, 2024, 12:05:30 am
content: Quote from: inse on October 14, 2024, 11:12:02 pmIn that case, suggest something better. Up until now, you've been unable to do so.We're all looking forward to your super-simple, works-always circuit.Your move.

author: inse
author_rank: Super Contributor
timestamp: October 15, 2024, 03:57:27 am
content: Come on, don’t feel offended by a little poking in the side.Despicable me was overwhelmed by the complexity of the circuit.Posting #9 is the most simple approach.As long as no toddler plays with the switch of a lab power supply 😁

author: jonpaul
author_rank: Super Contributor
timestamp: October 15, 2024, 04:56:51 am
content: Power thermistors have been used for decades, either direct in series with mains or in a simple relay ckt for time dealyj

author: Benta
author_rank: Super Contributor
timestamp: October 15, 2024, 06:25:59 pm
content: What needs to be clear is, that we have to deal with two issues here:One: charging the secondary-side storage capacitors.Two: transformer core saturation, which can be far worse.The primitive secondary-side control circuit from inse will deal with One, but not with Two.The thermistor will fix both. but only if you ignore the unstable mains or kid playing issue. It has to cool down first.My circuit takes care of both.

author: inse
author_rank: Super Contributor
timestamp: October 15, 2024, 09:46:57 pm
content: Sorry to dissent on the primitive circuit from posting #9Why wouldn’t it cover both cases? It all depends on proper dimensioning of resistor and relay coil.If the relay is matched to the nominal secondary voltage, maybe with help of a zener diode to energize at 80% of voltage, all should be covered.Your circuit will work fine with wider variations of the secondary voltage.Take an array of PTCs as limiting resistor and all should be fine.

author: Benta
author_rank: Super Contributor
timestamp: October 15, 2024, 10:16:43 pm
content: Quote from: inse on October 15, 2024, 09:46:57 pmBecause as long as there's voltage on the storage capacitors, your soft-start is out of action.It will not protect against core saturation on the transformer.It might be seen as a marginal problem, but it's not. I've tested several 1 kVA isolation transformers, and they tripped the 16 A breaker 9 out of 10 times when switched on (without load).I recommend a soft-starter on every transformer larger than 300 VA these days.The problem is twofold: the resettable breakers are much faster and more sensitive than the classical fusible ones; and the transformers you can buy today are so close to their operating limit that there's no magnetizing margin at all.

author: bsikar
author_rank: Contributor
timestamp: October 15, 2024, 10:37:46 pm
content: I agree completely with Benta. I also feel that KISS is good only when it solves all problems. I would like to always design/build things that don't have cases where you have to allow the user to make sure they don't make a mistake. If someone turns this on and off too fast, I would like that the soft start to function and not relay on them. Although I am the end user and I might be aware these edge cases, I would like to not have to think about it and know I am safe regardless.

author: inse
author_rank: Super Contributor
timestamp: October 15, 2024, 11:18:55 pm
content: Agreed, the switch back on issue.

author: Benta
author_rank: Super Contributor
timestamp: October 16, 2024, 10:41:21 pm
content: @bsikar,I dug out my old design files, here are a couple of simulation plots.The first shows the current through a 30-ohm resistor as load (red trace) and 120 VAC supply (blue trace) before the triac is shorted by the relay.The second shows two delay circuit traces.Green is the voltage across C1 = supply voltage for the circuit. By implication it reflects when mains power is on or off.Red is the current through the optotriac (BRT22, Vishay) LED, meaning power for the relay coil.What you see is a power-on, followed by a power interruption at 0.4...0.5 s. As you can see, the soft-start behaves correctly and initiates a new delay (where the triac limitrer is in control) after the event.Cheers.

author: bsikar
author_rank: Contributor
timestamp: October 17, 2024, 06:17:53 am
content: Wow thank you so much, if you don't mind or if it's not too much effort would you mind sending me your ltspice asc?

author: Benta
author_rank: Super Contributor
timestamp: October 17, 2024, 03:32:53 pm
content: Don't have 'em. This is KiCAD with Ngspice.

author: bsikar
author_rank: Contributor
timestamp: October 17, 2024, 03:34:40 pm
content: Oh wow, I never knew you could simulate in KiCAD. I have only touched that software maybe once. I will check it outMaybe I am thinking of another software, but can KiCAD also do PCB design? I might have used it for that a while back.

author: Benta
author_rank: Super Contributor
timestamp: October 17, 2024, 06:21:42 pm
content: Here's where to look:https://www.kicad.org/IMO a fully professional EDA design suite. I've used it the last 5 years and am impressed with it's features.Lively discussion here:https://forum.kicad,info

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 18, 2024, 04:19:48 pm
content: Quote from: bsikar on October 14, 2024, 02:56:42 pmNTC-thermistors are made to withstand large current and power spikes. And you normally absolutely don't need accuracy in a soft-start circuit.

author: Zero999
author_rank: Super Contributor
timestamp: October 18, 2024, 05:11:44 pm
content: Quote from: Benta on October 15, 2024, 06:25:59 pmThe thermistor won't fix both, because it takes time to cool down and if power is reapplied too soon #2 will occur.A timer relay bypassing a resistor will solve both #1 and #2.

author: Benta
author_rank: Super Contributor
timestamp: October 18, 2024, 09:27:10 pm
content: Quote from: Zero999 on October 18, 2024, 05:11:44 pmWhich is what the circuit does.. Except I chose a triac limiter instead of a resistor to make the circuit universal.But yes, a resistor will also do the job.

author: Zero999
author_rank: Super Contributor
timestamp: October 19, 2024, 06:08:15 pm
content: Why complicate matters? Just buy one of those cheap timer relay modules and use it to bypass a resistor.https://www.amazon.co.uk/220VAC-Timer-Switch-Module-0-10Sec/dp/B07N2T77C1?th=1

author: Benta
author_rank: Super Contributor
timestamp: October 19, 2024, 06:21:09 pm
content: Quote from: Zero999 on October 19, 2024, 06:08:15 pmI think you've misunderstood the topic completely. That's a time-out module.

author: inse
author_rank: Super Contributor
timestamp: October 19, 2024, 08:41:49 pm
content: I would not say it that hard - the relay has both NC and NO positions available.So in principle the 10s version, set to 0.5s might kind of work.If it’s the best solution is to be determined in the application.

author: Benta
author_rank: Super Contributor
timestamp: October 19, 2024, 08:45:38 pm
content: I have no more comments.

author: inse
author_rank: Super Contributor
timestamp: October 19, 2024, 10:03:17 pm
content: n/r

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2024, 02:35:56 pm
content: Quote from: Benta on October 19, 2024, 06:21:09 pmA timeout module is exactly what's required. On power on, the resistor is in series with the load, which is the power supply's transformer in this case. It limits the inrush current to the transformer, along with the filter capacitors. After the timeout period has elapsed, the relay will close, bypassing the resistor and delivering full power to the load. Unlike a thermistor, the timeout module will reset fairly quickly when the power is cycled.