Soft-Start Circuit for Isolation Transformer - Review and design help. Testing - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2023, 08:34:24 am
content: My BackgroundI took up electronics, when I retired, to keep my brain learning. I am a self-taught EE hobbyist. Meaning, there are large gaps in my knowledge base and understanding. Therefore, please explain things in detail and give directions with very specific instructions. I am fortunate to have very nice lab equipment and I can test most things that you need me to test.==================================================================Hello, EEs and Gurus!I need to add a soft-start circuit to my isolation transformer. Your engineering help is greatly needed, because1) I do not know so much about Mains Level VAC (and it is dangerous,)2) I want to make sure that my circuit design will do what it needs to do, safely, and3) I have married various circuits from other makers (we all know that that can be problematic.)I could use particular help with- The mathematical relationships, between all influences and factors. I just used the values stated in the open source circuits. Things like- The thermistor (TH1) Ohms rating verses the dropper capacitor charge up period.- The zenner’s protection resistor value (R2.) The relay data sheet does not state its current draw.(DS attached) Should I just use its Contact Resistance (100M) at 5V and do the math?- The type and ratings for the relay’s free-wheeling protection diode (D5.) Plain 1N4007, other fast Silicone, 1N4148 Germanium, etc?- How to add a LED to the dropper capacitor’s bleeder circuit to create a dimming warning indicator. See following.So, please dig around in the details and let me know where I have made errors. Or, maybe there is just a much simpler way to get things done... As always, I greatly appreciate your help! Thank you!==================================================================DesireTo have the fuse on the mains supply of my DIY isolation transformer correctly limit said supply to 3 Amps. See this prior project here:https://www.eevblog.com/forum/projects/diy-isolation-transformer-project-back-to-back-method/msg3174352/#msg3174352IssueThe large inductance of the iso-trans is causing excessive in-rush current, thereby causing the 3 Amp (slow-blow) fuse to fail immediately at power-up, without any load. A 6 Amp (slow-blow) fuse is required to prevent said immediate fuse failure. However, a 6 Amp fuse is excessive for continuous operations and could allow the iso-trans and any downstream equipment to be damaged. My entire DIY VAC Power Supply System is based on a maximum 3 Amp/360W rating: Emergency Shutdown Switch to Iso-Trans to Variable Autotrans to Dim Bulb Current Limiter to DUT.Consideration of SolutionsA simple NTC thermistor would cure the problem. However, it would add significant heat to the iso-trans case, if it was allowed to run throughout continuous operations. And, it would effect the operations of the downstream Dim Bulb Current Limiter. A more advanced relay-based soft-start circuit could reduce the heat issue, by bypassing the thermistor after it has done its job of limiting the initial in-rush of current.The relay could be powered with:- A simple SMPS wall adapter configured within the case. However, this would add significant noise to the system.- A simple transformer wall adapter configured within the case. However, this would add heat to the system.- A capacitor dropper circuit. It is acknowledged that this circuit (as well as the two above) will add a burden current to the protection fuse. However, the additional current draw of this current dropper circuit should be so minor as to be considered insignificant. (Is this statement true?) This solution is selected, to solve the fuse problem and for the sake of learning.Theory of Operation- The soft-start circuit will be mounted within the iso-trans case. Its power will be taken from the mains, after the case switch and fuse.- A bridge rectifier adapts the relay’s supply from VAC to VDC.- A dropper capacitor and zenner diode are used to reduce the relay’s supply voltage from mains level to 5V.- The initial in-rush of current to the iso-trans is moderated by the thermistor. As the iso-trans is charging up, so is the relay circuit’s dropper capacitor. Once the dropper capacitor is charged, the zenner voltage (5V) is reached and the relay is activated. Thereby, the thermistor is bypassed and its resulting heat is adverted throughout continuous operations.- A free-wheeling diode has been added across the relay to manage any reverse inductance that it might cause at shut-down.- A bleeder resistor has been added to drain the dropping capacitor, after shutdown. For clarity, the current’s path is from the cap, back along the main’s Line trace to the Normally-Open side of the relay, to the bleeder resistor to Earth Ground. As this attachment to Earth Ground is before the iso-trans, the iso-trans output remains isolated. (Is this statement true?) I am considering using the remaining energy for the dropper cap to illuminate a warning LED, which would glow, until the power has dissipated from dangerous levels.Cheers!

author: wasedadoc
author_rank: Super Contributor
timestamp: July 06, 2023, 09:00:32 am
content: Your D5 is unecessary.Your ZD1 is unecessary. The relay coil load is constant. Just make a single series resistor to get required current through it.Put the bleed resistor permanently across the dropper capacitor.Do not have a connection to Ground anywhere in this circuit.I think you will need a much larger capacitor to get a long enough delay for the thermistor to change resistance appreciably.

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2023, 12:04:48 pm
content: Thanks, wasedadoc! I replied within your quote.Quote from: wasedadoc on July 06, 2023, 09:00:32 amI am so grateful for your help! Most gracious of you! Cheers.

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2023, 12:22:29 pm
content: Here is a link to the dropper portion:https://www.digikey.com/en/maker/projects/capacitive-dropper/965d2328b35e43079e4eb99cf717137fQuote from: wasedadoc on July 06, 2023, 09:00:32 amIt appears that the formula for the bleed cap does not include the R1 resistor value. Meaning, the resistor does not (significantly) influence the needed cap value, however wouldn't it be what influences the length of time to fill the cap? What I am trying to say is this... Don't I need to vary the resistor value, not the cap value, to change the timing?Thanks, again.

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2023, 12:51:05 pm
content: Here is a Dropping Cap Calculator:https://www.allaboutcircuits.com/tools/capacitor-Charge-and-time-constant-calculator/According to it... Given that USA120VAC = 170V Peak... And, a Joules of say 0.0144... For each 1M of series resistance, you get a 1 second delay to reach a 63.2% capacitor charge level.This type of math is where things get muddled, for me... I believe we have several things interacting... Capacitance charge, time, resistance, Joules/energy, wattage on the resistor... Actually, I would love an intentional delay of say 3 seconds, if that does not let the Joules get out of hand... I am off to remind myself of the definition of a Joule, yet again...

author: wasedadoc
author_rank: Super Contributor
timestamp: July 06, 2023, 12:57:11 pm
content: Quote from: t1d on July 06, 2023, 12:22:29 pmI was referring to the electrolytic on the output of the bridge rectifier when I wrote about it needing to be much larger.The dropper capacitor should be sized to provide enough current (plus a margin) to operate the relay.Concerning the time constant for the voltage rise across the relay coil, the coil resistance is in parallel with the source impedance of the capacitor. The coil resistance will be dominant.

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2023, 01:05:49 pm
content: Quote from: wasedadoc on July 06, 2023, 12:57:11 pmAhh... Sorry that I misunderstood. Yes, correcting that value is easy enough... I had roughed in D6.3mm_P2.50mm of board space. I think I will up that to D8mm_P5mm, just to have some leeway. I know that will accommodate a 1KuF cap. The better thing to do is to learn the math and actually calculate what is needed, of course. Ahh, again, I am still missing the point... R2/C2 determine the relay timing, not the dropper cap and its series resistor; yes? Older and no sleep = tired and confused. I will try to leave it alone, until I rest. But, stopping is so hard to do, when you are having fun!

author: Ian.M
author_rank: Super Contributor
timestamp: July 06, 2023, 01:06:29 pm
content: Transformers are resistant to short time overload, and starting from cold can usually run at 200% of rated power for minutes at a time without damage. Therefore if you fuse the primary at 6A to provide gross protection against primary side shorts, and fuse the secondary at your desired 3A (or if you want a faster and more consistent trip, use a DC rated magnetic MCB) your transformer will be well protected, and providing you've got a >6A circuit available to power it you wont *need* a soft start circuit.You *NEED* soft start for large transformers with inrush current capable of tripping a 16A breaker, typically in the >1 KVA range.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2023, 01:14:46 pm
content: I agree about fusing the primary, as well as the secondary.Another option is adding thermal protection to the transformer, in the form of a normally closed bi-metalic switch. Choose one with a cut-off temperature, just below the maximum winding temperature rating.

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2023, 01:19:06 pm
content: Quote from: Ian.M on July 06, 2023, 01:06:29 pmSuch excellent information, Ian; thank you. Your suggestion is much more simple/better.Quote from: Zero999 on July 06, 2023, 01:14:46 pmThanks, Zero999. Hmm, I wonder if I have one in stock... I rather think that I do... Or, something similar... Maybe from repairing a coffee maker... Must sleep, now. Cheers to all!

author: Ian.M
author_rank: Super Contributor
timestamp: July 06, 2023, 02:16:16 pm
content: If you go the thermal trip route, (and its worth having one even with the suggested fuses) use a manual reset or non-resettable one not an auto-reset one that will keep the transformer simmering! Also note the surface temperature will be well below the deep winding temperature so you want a generous margin between its trip temperature and the transformer's insulation temperature rating.

author: james_s
author_rank: Super Contributor
timestamp: July 06, 2023, 05:33:06 pm
content: There's an easier way which I've used on a very large transformer. Just get a relay with a mains voltage coil and connect that coil in parallel with the transformer. Put a resistor in series with the primary (and relay coil) and wire the N.O. contacts to bypass the resistor. Now when power is applied the resistor limits the current until the voltage across the primary rises and the relay closes cutting the resistor out of the circuit. So far it's been working great for me.

author: bdunham7
author_rank: Super Contributor
timestamp: July 06, 2023, 06:15:14 pm
content: Quote from: james_s on July 06, 2023, 05:33:06 pmDoes that work if there is already a load on the transformer output when you turn the power on?

author: james_s
author_rank: Super Contributor
timestamp: July 06, 2023, 11:23:16 pm
content: Quote from: bdunham7 on July 06, 2023, 06:15:14 pmYes, but it's going to depend on how big of a load and at what voltage the relay will engage. The idea is to limit the enormous inrush, you can still size the resistor such that enough current flows to support the load, and the power rating doesn't have to be massive as it will only carry a load for a fraction of a second.In my case the application was powering a 4800V "pole pig" distribution transformer backwards, the output then being used to power the primary of a constant current series streetlighting regulator for a test rig. Previously we were having problems with the inrush tripping the 50A 240V breaker despite the load only being around 600 VA. I don't remember what rating the resistor we used was, it was a huge old wirewound ceramic thing I salvaged out of a 1970s machine tool drive along with the contactor used to bypass it. It was just a case of trying what we had on hand and it ended up working perfectly so I never attempted to analyze or improve it.

author: t1d
author_rank: Super Contributor
timestamp: July 07, 2023, 04:45:03 am
content: Thanks, folks, for all the great suggestions and information. I am learning a lot!Quote from: james_s on July 06, 2023, 05:33:06 pmJames, this is clever... But, let's make sure that I understand... When I don't fell well, it is very hard to think. So, I drew your circuit, to help...IIUC, you said the resistor is wired to the N-O side of the relay... It's the Normally Open part that is giving me trouble... We want to use the N-O connection to bypass the resistor, so that the relay is not doing any work, during the long periods of use/operation... I get that...However, let's see if I get the circuits operations correctly... At rest, the relay is open. Power is applied. Power bypasses the resistor and flows to the relay and the iso-trans, until sufficient power is achieved and the relay is engaged. Then, power flows through the resistor, until the iso-trans is fully charged. It seems that the iso-trans would be more or less fully charged, before the relay engages the resistor?This would not be an issue, if the resistor was on the Normally-Closed side of the relay. However, the relay would have to be doing the work of keeping the contacts closed, for the duration of the operational period.What am I missing, here, please and thank you...

author: wasedadoc
author_rank: Super Contributor
timestamp: July 07, 2023, 07:23:19 am
content: [quote author=t1d link=topic=383941.msg4947343#msg4947343 date=1688705103However, let's see if I get the circuits operations correctly... At rest, the relay is open. Power is applied. Power bypasses the resistor and flows to the relay and the iso-trans, until sufficient power is achieved and the relay is engaged.What am I missing, here, please and thank you...N.O. means the resistor is not bypassed until the relay operates.

author: t1d
author_rank: Super Contributor
timestamp: July 07, 2023, 07:41:18 am
content: Quote from: wasedadoc on July 07, 2023, 07:23:19 amThanks, wasedadoc, however I still do not get it; I'm sorry. Please write a corrected function statement to replace mine: "At rest, the relay is open. Power is applied. Power bypasses the resistor and flows to the relay and the iso-trans, until sufficient power is achieved and the relay is engaged." Much appreciated.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 07, 2023, 11:00:26 am
content: Quote from: t1d on July 07, 2023, 07:41:18 amMy previous post was clear. The N.O. contacts are across the resistor. At power on the relay is not energised. Current flows through the resistor. The voltage across relay coil and transformer rises. At some point it is high enough that the relay operates and shorts the resistor.Are you confused by the completely opposite meanings of "open" and "closed" in the contexts of electrical circuits and doors? An "open circuit" lets no current flow whereas an "open door" lets people through. And vice versa for "closed".And for relays the "Normal" state is unenergised.

author: james_s
author_rank: Super Contributor
timestamp: July 07, 2023, 06:10:28 pm
content: Quote from: wasedadoc on July 07, 2023, 11:00:26 amYes, this.In practice it happens almost instantaneously, a fraction of a second, all the resistor has to do is limit the magnetizing current to something reasonable for a few cycles so the breaker doesn't trip, then the relay closes and remains closed, bypassing the resistor until power is shut off.

author: t1d
author_rank: Super Contributor
timestamp: July 07, 2023, 11:00:02 pm
content: Thanks. I've got it, now.Any advantage/disadvantage to using a SSR, for this application?

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2023, 02:08:18 am
content: A SSR will have higher steady state losses compared to a mechanical relay and may require a heatsink, and it's also likely to engage at a much lower threshold, and it will cost more and likely be physically larger. Really I can't think of many advantages SSR would have in this application.

author: tanveerriaz
author_rank: Contributor
timestamp: July 08, 2023, 03:25:43 am
content: https://sound-au.com/articles/soft-start.htm

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2023, 05:38:51 am
content: @ james_s... Thanks for the pros and cons. Very helpful!@ tanveerriaz... Wow, that link appears to be very comprehensive. I will enjoy reading it (completely.) Thank you!Edited:Here is a drawing of james_s solution. I did not mean for the symbol for the Normally Open position of the relay contact to be in any particular state. However, as is, I believe it shows the relay as engaged and bypassing the resistor. Meaning, it [the iso-trans] has been powered on [and been on long enough for the relay to become engaged.] HTH someone.

author: jonpaul
author_rank: Super Contributor
timestamp: July 08, 2023, 06:21:21 am
content: Bonjour just saw this now....Been a power electronic EE since 50 yrs, this problem is easy to solve and the various limiters are unnecessary or prone to fail.1/ Inrush: The iso trsf like any other mains will have a magnetizing current inrush of 10..30x rated for a few mS. A properly rated slo blow fuse protects. The mains breaker must be rated eg 15..25 A.2/ PTC thermistors have a drop of perhaps 1..3 V when hot an dare not an issue if the load is high enough. But bad for a lab use.3/ Solution for lab bench is use a Variac (TM) or Powerstat (TM) variable transformer BEFORE the iso trasf. and always turn up the volage slowly.4/ Even on household circuits we have run a 2 KVA Varia and 2 kVA 240v/240V Signal isolation transformer for decades with no issues.Mains>>switch, slo blow fuse 5..15A>>Variac>>1:1 iso traf >>mains socket5/ Some variable transformers have a built in isolation transformer after the variac.Just a few tips from a very old retired EEBon Chance,Jon

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2023, 06:35:35 am
content: Quote from: jonpaul on July 08, 2023, 06:21:21 amHi, Jon! How very excellent to have you join in. Thank you!A good while back, you helped me with microphone preamp circuits... The That 1512. So, regardless of age, I know your skills. By your simple solution, it is clear that those skills are still razor sharp. Kudos!

author: jonpaul
author_rank: Super Contributor
timestamp: July 08, 2023, 07:24:42 am
content: T1D, Many thanks for your kind note!See the IEC and UL info on power line transients.Look out for iso trsf and Variacs at ham Fleas or firms that relocate/close in your area. Also ham radio silent key.Easy to make a low cost isolation trsf just two identical LV power back to back, eg 120V trsf>>24V>>24V>>120VWe have several Yodogawa diff probes, all 1500V and 100x with dedicated safety probes, ~ $400..1000 each, 9V batt and ext wall trsf PSU powerNEVER USED! Too delicate, no need!Bon Chance !Jon in ParisPS:::::In my twilight years I want pass on my accumulated experience , EE since 1968!!I was consultant, manufacturer. in medical, avionics proaudio, cinema, lighting..I see threads where the less experienced in power electronics are making a complicated or odd solution to an old problem.I had experienced huge motors /generators short ckt faults,75 kV 1 kW arc lamp ignitor blowups,SMPS complete blowups ,MOV caused lab fires....

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2023, 08:27:41 am
content: Quote from: jonpaul on July 08, 2023, 07:24:42 amYes, I am building up a VAC Power Supply in the manner you suggest... I made an Emergency Shutdown Switch. I built my Iso-Trans from two, identical, UPS transformers. I bought a used Variable Autotransformer in great condition... I am trying to finish my Dim Bulb Limiter, but I need a custom metal panel to mount the switches in the case.Creating the panel is troublesome. I am aware of the tricks to obtain one - using PCB proto boards as the panel, etc. However, because of the size of the panel, it still is not cheap.This thread is about the fuse for the Iso-Trans. The system is to be rated at 3 Amps. But, 3 Amp (Slow Blow) fuses pop at power up.I had designed a circuit to solve the problem and posted it for review. Thankfully, other folks and you have come up with much simpler/clever/elegant solutions.I have settled on james_s's solution of a current limiting resistor across a Normally Open Relay and I have selected the relay on Amazon. So, I can move ahead to its installation.Accordingly, I am going to mark this thread as "Solved." I extend my heartfelt gratitude to everyone that participated!. You make the EEVBlog forum a great Community. Cheers.

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2023, 03:51:07 pm
content: Oops... I forgot to ask how to calculate the value for the current limiting resistor.I would like for it take about one second for the relay to engage, if this would not cause problems. That should really tamp down the in-rush. But, I just like the smooth and easy sound of the relay coming in. Can you be too old to be a geek? <grin>Hmm... For that matter, I have an assortment of NTCs. I like the idea of using one of those, if I could. I realize that it might not be necessary...Thanks for the additional help!

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2023, 06:45:02 pm
content: Quote from: t1d on July 08, 2023, 03:51:07 pmThere is absolutely no reason for a delay of a full second, you said you want to reduce the inrush, the inrush event is a small fraction of a second. A longer delay just results in more heating of the power resistor and may cause problems with some loads as they will sit in an undervoltage condition until the relay pulls in.The resistor value is not critical, but you can easily apply Ohms law here. Assume worst case the transformer behaves as a short circuit initially, so if you want to limit the inrush to an absolute maximum of say 20 amps RMS just calculate for R, so R = 120 / 20, or 6 Ohms.The reason I don't like using NTCs for something like this is that they take time to cool off and reset before they will work again. Also they run hot in normal use.

author: t1d
author_rank: Super Contributor
timestamp: July 09, 2023, 02:05:11 am
content: Thanks, James.Quote from: james_s on July 08, 2023, 06:45:02 pmGood reasoning.Quote from: james_s on July 08, 2023, 06:45:02 pmGood ol' Ohms - a great guy... <grin> Well, I had thought of that, but, as I said, I don't know so much about AC. My intuition would have been to use the peak voltage to make the calculation: 120 x 1.414 ~ 170. A lot of things can go very wrong at Mains levels.Quote from: james_s on July 08, 2023, 06:45:02 pmYes, those are good points. JTBC, I was meaning to use the NTC, in the position of the resistor, in the circuit. There is no real reason to do this, I suppose. It was just an idea.Now, I think we have the subject well covered. I will mark the thread as "Solved," again. Thanks, to everyone!

author: jonpaul
author_rank: Super Contributor
timestamp: July 09, 2023, 11:59:19 am
content: Rebonjour T1D, BRAVO to finish the job please post pix when its ready.Just a few thoughts...."Dim Bulb" is useless in many SMPS loads, and may give deceptive results. With a fuse and variac is it not needed.Delay with thermistor seems odd, you need perhaps 10 mS delay for mag current spike.With loads under 10A and proper fusing, such a delay relay /inrush is not usually needed.Panels: We used to use a Dymo hand plastic lable maker, later we used to laser print on thick glossy paper and cutout.You can also laser print on clear transparency plastic sheet.Easier and more visible than PCBBon weekend.Jon

author: t1d
author_rank: Super Contributor
timestamp: July 09, 2023, 12:27:28 pm
content: Hi, Jon,Quote from: jonpaul on July 09, 2023, 11:59:19 amOh, that is a good suggestion!Quote from: jonpaul on July 09, 2023, 11:59:19 amAgreed.Quote from: jonpaul on July 09, 2023, 11:59:19 amI was mostly considering using them, because I have them in stock, whereas I would need to order a high-watt resistor.Cheers!

author: james_s
author_rank: Super Contributor
timestamp: July 10, 2023, 11:08:03 pm
content: Quote from: t1d on July 09, 2023, 02:05:11 amYou could use the peak level if you want, as I said, it's not critical, there's enough wiggle room that calculations using either peak or RMS will probably work just fine. In my own case I just wanted to prevent the breaker from tripping, and it takes quite a large amount of current to instantaneously trip a breaker.

author: Bud
author_rank: Super Contributor
timestamp: July 11, 2023, 10:56:10 pm
content: There should be numerous practical mains power delay circuits in ARRL Handbooks. This problem is/was very well understood by Ham radio amplifier builders.

author: t1d
author_rank: Super Contributor
timestamp: July 12, 2023, 09:53:43 am
content: Testing =I made various resistor value calculations and tested them. Meaning, I thought a 1/4-1/3A supply would be a reasonable current value to power the relay and charge the transformer. If I interpreted the data sheet correctly, the relay only requires ~ 10mAs, to operate the contacts.What I discovered is that a resistor value rated only high enough to supply the relay did not work. The relay contacts did not fully close. I was using a 60W bulb to simulate the system no-load draw at 1/2A. So, I changed the resistor for a low value, as james_ s suggested, and things improved/worked.Then, I thought to substitute the resistor with a 20W/120VAC Donor Appliance Bulb, as this would have the same benefits as a Dim Bulb Limiter - soften/limit the current and provide a visual indicator of the system's status - and still be a temporary/removable limiter, as it is on the relay contacts. This worked.Having this success, I cobbled the test rig into the actual VAC Supply Rig power chain. Meaning, I- Changed the fuse in the Emergency Shut-Down Switch to a 3A/Slow-Blow. (Reducing this fuse from 6A/Slow to 3A/Slow is the whole point of the project.)- Connected the DIY Soft-Start Circuit (which still had the limiter bulb) to the ESD Switch.- Connected the Iso-Trans to the Soft-Start.- Connected the Variable Autotransformer to the Iso-Trans. I left the output of the Iso-Trans switched on, to also simulate the Variable Autotransformer start-up influences. I left the output of the Variable Autotransformer in the off position.This will be my normal power-on test starting position. Meaning the downstream Dim Bulb Limiter and DUT will not be included in the Iso-Trans initial current draw. Those downstream factors will be switched in/out, after power-up.Powering up, in this configuration works. Meaning, the relay contacts close, the Iso-Trans fan comes on and the 3A fuse does not blow. It happens so fast that there is not a discernible flash on the bulb in ambient light.There is a 15A electronic reset-able fuse in the Iso-Trans case. I left it in place, because it could not hurt to leave it in. When I trip it, the Soft Start bulb illuminates. Neat.Next Steps- Install the Soft-Start Circuit in the Iso-Trans case.- Make and post photos.- I would like to do additional testing:a) Operate the system with a full/precise 3A/120V/360W/Continuous load.b) Observe the system's characteristics on my scope. This is where I could use your (very specific and detailed instructions. The Forth of July has come and gone; no need for more fireworks.) I look forward to doing this testing with you. Thank you for your continued help and support.Note: Ian.M's statement still stands:Quote from: Ian.M on July 06, 2023, 01:06:29 pmMeaning, all of this effort may seems as much ado about nothing. However, confidence in your test equipment and your protecting yourself are requisite. Meeting those requirements are slightly different for each individual. And, there is also just the plain old fun and learning from doing a project to factor in, as well.RELAY = Omron LY2-UA-006244 Relay, DPDT, General PurposePurchased on Amazon/USA Pack of 2 = $9USD

author: t1d
author_rank: Super Contributor
timestamp: July 12, 2023, 11:06:18 am
content: Hmm... Does anything useful, or fun, come to mind for the second set of relay contacts?

author: james_s
author_rank: Super Contributor
timestamp: July 12, 2023, 07:02:50 pm
content: I suppose you could try using the second set of contacts to switch the output load, that way the transformer will magnetize and come up and the load will not be connected until the inrush limiter is bypassed. Whether that offers any real world advantages I don't know.

author: bdunham7
author_rank: Super Contributor
timestamp: July 18, 2023, 08:10:01 pm
content: Quote from: t1d on July 12, 2023, 09:53:43 amSince I had the transformer dug out and on my bench in your other thread, here is the turn-on input measurements. I had to do this capture many times before I got this high of an input surge. I was using a mechanical switch and it was just random chance where the primary waveform would start. I was using a current clamp (Fluke i1010) with a x10 splitter (Klein) for current, a MicSig 50X Differential Probe for the primary voltage. The maximum input surge here was about 100A with a 20V drop in the peak primary voltage on that first cycle. That's not enough to trip a breaker, but it did make my UPS on this circuit squeak once in a while.

author: t1d
author_rank: Super Contributor
timestamp: July 18, 2023, 11:06:00 pm
content: More super-great information = Thanks for the extra effort!Quote from: bdunham7 on July 18, 2023, 08:10:01 pmI could cobble together some form of most all of the needed gear, except the Diff Probe. I tried to build one, long ago, but it did not pan out. I may come back around to that project.Quote from: bdunham7 on July 18, 2023, 08:10:01 pmWell, IIUC, you said that my transformers might be half-again as big. If all the math equates (and I am sure that it does not,) then I might have been bumping 150A. Whatever my actual amps were, I was instantly blowing a 3A/Slow fuse. A 6A/Slow could survive, repeatedly/long term.

author: t1d
author_rank: Super Contributor
timestamp: July 18, 2023, 11:19:24 pm
content: As most projects go, I ran into a good bit of project creep for the Soft-Start install. Here's the new plan...- Add the soft-start.- Change the 20A onboard reset-able fuse for a fuse holder. Move it to a new location.- Move the LED Power indicator to a new location.- The soft-start adds lots of new connections. Install bus bars for the soft start and to tidy up all of the wire work.First step... Gut the pig... Right down to the transformers.The tear down/out has consumed the whole day. I also- repaired the loose output power socket.- prep'd the fan = rewired it.- cut a new mounting plate for the fuse holder and LED.I researched wiring options for the transformers. bdunham7 and Ian helped me with that. I will be changing from the black 110VAC coil wire to the Yellow 120VAC coil wire. You can read about that, here:https://www.eevblog.com/forum/beginners/diy-back-to-back-isolation-transformer-multiple-secondaries-question/new/#newThe bus bars are due to arrive, tomorrow. The pictures should get more interesting, thereafter. Cheers.

author: t1d
author_rank: Super Contributor
timestamp: July 26, 2023, 02:34:39 am
content: Progress update within quote...Quote from: t1d on July 18, 2023, 11:19:24 pmI made the VAC LED Power Indicator board to be rather universal...- I used a RGB LED, so the color can be chosen/changed. The color is selected with a jumper. It is nested it in between the resistor sockets. The LED is mounted on the back of the board. The resistor sockets give ample headroom for the LED pin stubs.- The dropping resistor is doubled up, to spread the heat. The two resistors are on sockets, so the LED intensity can be easily adjusted.- The Neutral rectifying diode is on a socket, as it is the most likely failure point.- With the use of sockets, the unit can be configured to any type of voltage source, or amplitude.My guess-T-ment on the number is as follows...- 120VAC/RMS/USA- Neutral rectifying diode drops RMS to 60V- 60VAC x 1.414 = 84.84Vp- Two 22K dropping resistors are configured in parallel = 11K- 84.84/11,000 = 0.0077AI used my DC PSU to test the green LED. At 0.0077A, it produced a significant glow. I am fairly confident that it will be sufficient at 30 cycles/sec. If not, it is easy to adapt the board to whatever I like.I am adding the LED Board pictures, here, for your enjoyment. If we need to discuss it in detail, we might want to open its own thread.

author: igor78
author_rank: Contributor
timestamp: July 26, 2023, 05:20:33 am
content: see page 8 herehttps://www.metrel.si/assets/Metrel/Navodila_instrumentov/Instruments/MA_4804_Power_Supply/Ang/MA_4804_Variable_transformer_ANG_Ver_3.0_20_750_280.pdf

author: t1d
author_rank: Super Contributor
timestamp: August 17, 2023, 09:30:45 am
content: Quote from: igor78 on July 26, 2023, 05:20:33 amThanks, Igor. Here's the schematic:I believe this circuit is very much what has been suggested and comparable to what I am creating... A small value resistance, to soften the in-rush current, with a relay configured to automatically switch it out of the circuit, after the startup event. A couple of differences in their approach...- They are using a 8K2 resistor and a rectifying diode to create a power supply to power the relay coil. I have negated the need for a separate coil power supply, by using a VAC/Mains powered coil.- They are using a 47uF cap on the coil to (likely) prevent switch bounce and/or relay chatter. My relay does not state this as a requirement, but it does seem to be a good and easy addition. Of course, it does add a small bit to the in-rush current.Thanks, Igor; this was a good contribution.

author: t1d
author_rank: Super Contributor
timestamp: August 17, 2023, 09:40:11 am
content: I am plodding along with this project. My VAC LED Power Indicator Board had a rather significant design flaw... The pin spacing on the Mains L/N connector were unnecessarily close. This led to some sparks, when probing those pads during testing. I was not sure, if my wobbly hands crossed the probes, or if there was a true jump. It did not matter, because the matter remained - the connection points needed to be separated farther. So, I have a new board design being manufactured. More delays, but I am in no hurry and this was a significant safety issue.

author: t1d
author_rank: Super Contributor
timestamp: August 17, 2023, 10:16:04 am
content: Here is a new consideration with which I need your help. I know that we a dealing with VAC, but do I need to align the phase of the elements... Mains Input, Iso-Trans Output and Case Fan? (The Case Fan is in parallel to the Iso-Trans.)I would think that the Iso-Trans Output should surely be in phase to Mains to provide the DUT with power properly oriented to Mains' Line and Neutral. I would think that the Fan might need to be aligned to help reduce noise in the system?I injected a signal through the Mains power plug and read each with my scope in series. They are not all lined up. The Iso-Trans Output and the Fan are in phase. But, they are out of phase to the Mains cable plug.Hoping to maintain the phase of the Iso-Trans Output to the Mains Input, I matched all of the wiring colors, when I connected the two transformers back-to-back. (There are no duplicate colors on any individual set of coils, primary, or secondary. So, I could not have crossed up any set of wires.)Curiously, this did not prevent the Iso-Trans' phase problem. I am glad I took the extra step of scoping things and caught the problem. However, it occurs to me to scope two transformers's phases, regardless of them coming out of identical model UPSs, to make sure that their phases are truly identical. I'll do that.The Case Fan remains out of phase, even when I reverse its wiring. I believe that it has internal rectifying circuitry. So, when lining up the three elements, the Case Fan will set the orientation required of the others.If orientation is needed, at what point should I flip the phase of the Iso-Trans? At the- Input to the first Transformer,- In between the two Transformers, or- At the output of the Set?I look forward to you teaching me what to do and why to do it. With much appreciation, please and thank you.

author: jonpaul
author_rank: Super Contributor
timestamp: August 17, 2023, 11:29:39 am
content: T1D, just revisitng this now,Seems you are killing flys with an A bomb.Phase of mains trsf is needed for use in series or parallel.Thermistors/NTC are time sensitive so will NOT limit in case of rapid turn OFF><ON sequence as the thermistor is still hot and low R.Maintain Line><Neut relation and earthUPS and other consumer power transf are optimed for lowest cost, have a high iron and copper losses ~ 15..20% and are noisy. We avoid themSignal, Belfuse and other trsf mfg have a huge seoection of 120/240 V isolationUse a Variac/Powertrans variable mains trsf BEFORE the isolation, then NO transient limiters needed.Since 1960s (!) we use this very simple setup for mains/power developweent/troubleshoot:Mains >>fuse/switch>>Variac>>1:1 islation trsf>mains socket and metersHope this is useful,Bon ChanceJon

author: t1d
author_rank: Super Contributor
timestamp: August 17, 2023, 11:48:01 am
content: Thanks, JonPaul! Always great to have your input.Is this much ado about nothing. Yes and readily admitted earlier in the post. But,- I am learning a great deal about things that effect my personal safety. With learning comes confidence. With confidence comes better performance. With better performance comes better safety.- I am making some small improvement in my VAC PSU System.- I am having fun.Each and all being sufficient reward for me.Cheers!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 17, 2023, 12:14:13 pm
content: I did not read all the posts, but wonder where you get the power for the relay from. I see a capacitive dropper on mains?Why not take it from the low AC voltage from in between the back-to-back transformers?Also, buffering the voltage of the RC timing circuit by adding an transistors (simple emitter follower) decouples the relay current from your timer current, so you can use much smaller capacitor and resistor.A relay with a higher coil voltage is also advantageous, because it needs less current to operate, and you can put more voltage into your timing capacitor before the relay operates.And when you bridge your NTC anyway, there is no real need to an NTC and a power resistors also works swell. Especially the white "cement" types are very robust and can be grossly overloaded for short durations. Do not use the (more expensive) gold colored aluminium power resistors. These have thin resistor wire that tend to work as a fuse on big overloads. But a PTC works too, use whatever you have in the parts bin.

author: t1d
author_rank: Super Contributor
timestamp: August 17, 2023, 03:35:34 pm
content: In the body of the thread, I abandon the original power supply idea in favor of a relay that uses an AC coil. Now, there is just the relay and a resistive element... Just two components.

author: t1d
author_rank: Super Contributor
timestamp: September 19, 2023, 11:30:32 am
content: I have completed the project. Here are some pictures and final thoughts...I used a 40W Halogen Bulb for my final choice of resistive elements that I tested. I like it very much. Below that, you can see the connector for the LED.Though the case has lots of extra room, that room was in the wrong location for what I needed to install. There was a large gap in the sidewall of the case and, of course, that is where I needed to mount things. So, I made a power panel for the Line leg. Here, you see the bar supplied by Mains/Line, the Relay, the LED Power Indicator PCB and the Output/Line Bar.Here is the Neutral Bar.Top Down View.That's it, except for finalizing the burn-in and buttoning up the case.Thanks to everyone for help! I learned a lot and had a great time. Cheers!