just wanted to say I think its pretty dang cool that you take the time to make these videos and share your knowledge and personal perspective with those of us floundering through the forest in search of lupins..

Yet another educational and entertaining piece of work that you did Tony. Many teachers could learn from you how to combine these two elements when explaining a subject, whatever that subject is, not only electronics. What I personally find very useful in your videos is a logical structure behind what you present. Starting with explanation of the idea and theoretical background, then diving into a practical application and finally demonstrating certain number of tests that not only prove what was said before, but also helps to better understand that theory. We know you don't use script. You simply don't need it. Excellent job.

Such a simple circuit for such an important job!👏👏👏👍

Thanks for your point of view Professor. Enjoyed your session.

A much more elegant and easy solution for inrush, is to use a NTC thermistor device on the line. We utilized this method on all of our high current amps at Altec-Lansing, which included the EV line as well. - K5DBX

Excellent! I just subscribed after seeing this elegant solution. I build tube amps and I typically heat the early stage filaments with DC to limit AC hum. To do this you need a lot of capacitance because 4 tubes can draw a couple amps and you need that capacitance to keep up with the ripple. But this does present a big inrush strain on the filament winding of the transformer. For 6,3 volt filaments maybe I'll experiment with a 5V or 6V relay. If I can find a sweet spot, I'll definitely add this to all my projects. Many people use a 555 timer or such, but this is downright simple with far fewer points of failure. Thanks!

Thank you for the added information displayed in yellow text. That is always helpful - especially when dealing with numbers, values, and equations. I usually take a screenshot of that so that I can study it at the end of the video. Details matter. 👍

HI Tony, that is an elegant way to do the job and will easily work on anything. Rich

I had to make something similar for my truck. I attached a seven pin trailer connector to a 1500 watt power inverter. As soon as I would plug it in, it would blow the 40 amp fuse for the 12V auxiliary power tap. The 40 amp circuit doesn’t allow me get anywhere near the rating of the inverter, but it’s great for things like emergency & work lighting, small tools, a PA system, ham radio gear, etc. When I have larger loads, I either connect the inverter straight to the battery, or bring out the 3,500 watt Yamaha generator. (The quiet, inverter type. Very nice!) For my application, I used a 555 timer, to make a time delay relay. When I plug the inverter in, it charges up through a large power resister. Then, after about 10 seconds, it closes the relay, bypassing the resistor. I used a 40A Bosch automotive relay, which is more than enough to handle the load. It’s been working great for several years, now. The only *potential* problem I can see with your method, is if you use a relay with self cleaning/wiping contacts. (Which is pretty much any relay.) If the relay closes, just as soon as it barely has enough voltage to do so, it will close much slower and with much less force. This will lead to arcing, contact pitting, etc. If the relay opens too slow, it can also cause arcing, which will further degrade the relay contacts. Then again, it might work just fine, but if it doesn’t, you can always make a time delay relay for it. The only other option that I can think of, is to wire a relay in parallel with the speaker protection relay and use it to bypass your inrush limiting resistor. By the time that the speaker protection relay closes, the power supply should be at full voltage. Great video! Inrush current limiting is very useful, for many applications. 👍

Hi Tony. If you have a relay with an extra set of contacts (DPDT), you can wire in an extra resistor (whose value will have to be determined by experimentation) in series with the one already there, and put this resistor in parallel with the normally-closed contacts so the resistor is shorted on initial power-up. Once the relay pulls in, the N.C. contacts open, and the 2nd resistor is now in the circuit so the relay only needs the hold-in voltage/current. This should extend the life of the relay coil. You may need a small capacitor to prevent relay chatter, though, across the N.C. contacts/resistor. Maybe .01-.1 uF disk capacitor.

24:54 If you want, you can add a small neon bulb in parallel with the relay coil. During normal operation, the breakdown voltage of the neon bulb is way higher then the voltage of the protection circuit itself and the neon bulb acts like a open circuit. When back EMF is generated by the relay coil, the voltage will be clamped down once it reach the breakdown voltage of the neon bulb.

Thanks really appreciate your tweak.

Poor man's soft start is an NTC thermistor.

Great video. I'll be applying this at some point.

Hell yea Tony! UKposts was lacking a good video on this subject.

I made a similar in-rush limiter on my DIY variable DC power supply. The input voltage of the supply is 50V DC, rectified from a 36VAC toroidal transformer. I put a 330ohm 5W resistor in series with the 4x4700uF parallel caps, so when powered on, they charge slowly. The relay coil (48V) is connected directly to the caps, the voltage increases slowly as the caps charge up thru the 330 ohms, and when they reach around 38-40V, the relay pulls in and cancels the resistor. Works well, takes around 3 seconds from a cold start. 48V relay coil has no objections to 50V so I did not use a dropper resistor there. Now I see this doesn't help with the in-rush of the transformer, so I need to re-think the whole thing. Thanks for the video.

Even without caps, the inrush is huge because of that transformer.

Thanks a lot. I am planning to build a power amp using a 625W, 30v 20A (x2) toroidal transformer and 2 large capacitors (22000uF). I was considering using an Inrush limiting NTC thermistor but I realized that during quick on/off cycles may not work well. I also realized that some high end vintage amplifiers (like Nakamichi 620) are using relays as you are suggesting. Unless somebody has a good reason to use Inrush limiting NTC thermistor, I will go with your suggestion. Thanks again.

A lot of great info I just learned

PeaceJoyHappines, thanks for video(s) Tony. _Morning hours across pond(scandinavia) here, at about 08am ..Coffe & XTB-video ..ThumbsUp!

Very good!

learnt a lot...cheers.

That a great circuit. I do a lot of power supply's and Tube RF amps that have large capacitors . Them values of inrush resistors can change with size of transformer and voltage of line. with that amp in video you could use 48V relay coil. 24 DC is very common. Also that inrush current is hard on diodes . I had so many go bad . even broadcast transmitters use soft start . just 3 resistors for 3 phase with a contactor across. and the output tubes filament supply has step start . taps on primary of filament transformer. There is a outher soft start circuit that is similar and works great I use on tube RF amps. ham radio type. using a 240V or 120V relay coil across transformer primary. then the series resistor is 20 to 30 ohm 240V 10 ohm 120V 50 watt or so with a 2 or 3 amp slow blow fuse in series with resistor. If you have a diode short or tube short the 2 or 3 amp slow blow fuse blows saving the big surge. I copied the soft start from 3 phase and made it for smaller units. mind you running 480V with 277V to natural. 277V contactor. 500 watt resistors in a cage . 3 cartage fuses 600V . does same thing. large or small that soft start really works. Thank you,😀

Nice. Thanks to you i just discovered that my amp has something similar already installed ( Onkyo M 509 ). But it just uses 5*0.22r5w and no resistor before the coil but a cap and a diode across the coil. Now im thinking about changing the 5*0.22r ones against one 4r50w because it has new and bigger filtercaps now too ;) (from 4*30000 to 33000) Oh and its one relais for both transformers.

You make very good instructional videos as your explanations are very thorough. Regarding the Marantz design, I am surprised that 60Volt caps are being used with approximately 50 volts applied across them. 50/60 would be an 83% stress level on those caps, and if it's 57 volts, as you stated later in the video, that would represent a 95% stress level. Could be why one of them failed open. Mil Spec defines high reliability, at stress levels of 50%. I'd imagine the form factor of the caps, available at the time, was the major consideration, but newer caps have smaller form factors and if it were my amp, I'd try to get 100 Volt caps. Also not to nit pick, but you didn't mention that the inrush current also stresses the caps, you mentioned that it could hurt the power switch and x-fmr, (you probably just forgot to mention). Just an observation, not meaning to be critical for the sake of being critical, just providing some feedback. All in all I enjoy your videos, and find them very practical. Thanks for taking the time for the tutorials, as they are probably very time consuming and difficult to pull off.

Very cool.

Current limiting is essential .the inrush current will damage both the transformer and the capacitors .And since audio quality can be greatly affected by power supply it makes sense to protect your power supply and your equipment connected to it

The 510 power uses the relay with primary only and needs a cap across the relay’s coil Pulls in almost instantly

I've done similar using a scr that slowly (over 10 cucles shorts out a resistor that is in series with the input of the transistor. The only thing agaisnt this is even wen it's full on it will still rob a volt of the incoming votge because the acr does not go to zero.

I like this solution better than the thermal ones. Quick cycling can defeat those. Whether it's someone playing with the power switch or just quick power-outs, it's bound to happen.

The min max button on the Fluke would be useful to capture that full momentary voltage drop.

I got to say my halfler DH 200 is a heavy hitter my lights dim turn it on so I just leave it plugged in and leave it on all times it is a nice and warm at all times

I'm wondering if an inrush current limiter (like Amphenol CL series) would be an adequate alternative solution. I know that it's a tiny space heater, which is not ideal, but with so much empty space inside it shouldn't be a problem. Oh, and thank you so much for sharing your wisdom, yet again! Please know, it's appreciated beyond words.

The idea is good, but I would strongly recommend to use a solid state relay.

A straight forward & simple circuit. I.M.H.O. wiring with relay across the entire dc (+/- ....& not +/ ground ) with appropriate resistance and suitably rated zener....this ensures we see sufficient voltage buildup on " both " caps.. instead of only one.. ( eg for +/-24 dc , a 22/27V zener) .. in series with the resistance and relay coil...the coil must have its f.w.d. across it... Relays can pick up @70% rated volts and will drop out when voltage falls below 30% rated volts.....A24 v relay,once energised... will reliabily remain energized between approx. 17 & 9 volts......

I'd probably just wire a NTC thermistor in line with the mains winding, if I could find anything durable enough for the job. When turning the amp on, this thing's resistance is a few ohms, decreasing as it heats up under current. It will eventually reach a steady state temperature and resistance, dissipating some heat. BTW, the dim bulb is a PTC thermistor.

Hello! You always have such great videos! I am learning a lot! Concerning the control of the capacitors and their charging, why not just add a 30 ohm, 100watt resistor on the positive side on each capacitor to limit the inrush of current when powered on?

Tony, what is the current rating of the relay contacts? That needs to be considered also. Very good video, thanks.

Great circuit addition, however I like to be more conservative and put a capacitor across the relay coil to increase the delay time, and increase the power resistor to limit the in rush current more. I have found that this is less stressful on the rectifiers and main filter capacitors and increases their life.

Tony, Great information. I repair welding equipment for my living and all of the welding inverter power supplies I repair employ a similar "pre-charge" circuit for the buss caps. I deal with caps at least as big the two here. How do you measure them? I cannot find a meter over 10k. Thank you so much!

I thought you would like to know that I’m going to put this soft start circuit you show into an amplifier I am recapping for a guy. I don’t support this necessarily but he has a Hafler DH-120 power amp and he bought o capacitor upgrade kit for it which among other things increases the main filter caps from 6,800uF to 22,000uF. It seems to be a common thing for owners of this amp to do. I advised him not to do it but recommended that if he really wanted it done, he should at the very least add the soft start circuit and upgrade the rectifier. I still couldn’t take responsibility for what this modification might do. I don’t recommend it but it’s his call. Without looking at a schematic, other than start up, would you have any other potential concerns with caps this large put into a circuit that wasn’t designed for it? Anyway, I just wanted to let you know that the videos you put out are a great educational tool for people like me. You are a priceless asset for the community. Thank You

A good, practical build for a soft start, thanks for sharing it! One thing I wonder about relays that are "soft started", such as ones used for your circuit here or for DC protection on amp outputs, is, does the "slow" build up of DC voltage on the coil to close the contacts make a difference over time to any "self cleaning" properties that a hard "0 to working voltage" application may cause. I'm wondering that, if contacts close a bit harder, will they "self clean" any better.

I would have put the resistor in series with the B+ and use a P-channel MOSFET wired across maybe a 10 ohm resistor, but either way it's a good idea.

Can I use soft start from (or designed for) power tools? They seem to be cheap and made in small nice packages easy to fit to audio amplifier. Not sure if I should be trying such devices for audio amplifiers.

Wonder if it's a good idea to soft-start a decently sized SMPS like this. Proper ones will have an NTC on the input, but if that's already hot after it's been in use for a while, it can very well act like a piece of wire, passing full current to the main cap(s) and causing arcing across switches/relays. I WAS thinking of using this method, but I heard it's not recommended to "starve" a SMPS of current for too long by placing a resistor/light bulb in series with its mains input, so I guess the resistor value and "hold" time would have to be pretty precise so as not to reduce its lifespan. If it's got a PFC boost circuit it's probably even worse...

excellent video really helpful! could someone maybe help me out a bit? 1) the wiring to ground from the relay coil as depicted in the schematic drawing; is it wired to B-? my amp's got a split rail voltage, there's 46V difference between B+ and B-, and a ground in between (23/23). i reckon i CANNOT just go wire the relay coil to the mains ground... where *do* i wire it to? 2) i live in Europe, 230Vac on the line, shouldn't the big resister be rated, like, over 1kW in power dissipation? thanks in advance, all

Tony, I am curious about your choice of using the 2k ohm resistor to drop the 57 volts to 24 volts for the relay. If one plugs those in a voltage divider calculator with 850 ohms for the relay the voltage across the relay is 17 volts. I’m trying to figure out what dropping resistor I will need to use with the very same relay with a B+ of 70 volts. Using a voltage divider circuit calculator, it comes up with a 1667 ohm resistor. I thoroughly enjoy watching your videos Larry

Why is a relay preferred over a MOSFET or SCR in inrush limiters?

That big inrush can damage all in its path. A 6 volt relay has a 30 ohm coil if your initial charge is through the relays coil when the difference in voltage is less than 2,5 volts the relay drops out and the Cap is directly connected to the source through NC contacts

Hi Tony…..I’m wondering why you put the 4 ohm resistor on the mains instead of on the output of the transformer. Wouldn’t it operate the same way and still protect the transformer? Just curious. I’m adding one now and would like to know. Thank You

Could you replace the 4 ohm resistor with an NTC and then instead of using just a resistor inline with the relay coil, add a zener diode. The zener will give better control of the relay switching point and increase the drop-out voltage.

I was thinking about a fly back diode may increase the time of the kick out when turning the power off. Isn’t the caps taking the charge built from the magnetic field of the coil? Curious if the relay is actually causing the capacitors to bleed out it’s charge. Am I asking right?

Спасибо! Было интересно 😊

I’m afraid this won’t work in Europe or anywhere else where the grid AC is 230V. The inrush resistor’s power capacity will have to be prohibitively high; we’re talking quadruple-digit wattages here.

As the amp in question has a split rail supply did you put the resistor on the 0v/ground rail? Thanks for the upload.

excellent work, quite educational. does the loud 'thump' some amps produce when plugged in or coming out of sleep is also the effect of a large inrush current? my bass guitar amp makes one when turned ON and also OFF. as i recall the power caps are way smaller though, i think 2 caps @5600uF. i could try powering it thru a light bulb like shown here.. :}

I would be interested in a similar circuit that uses an Solid state device in place of relay. Maybe SCR

Your bench looks like mine.

Tony, What is the voltage drop across that 4.7 ohms resistor?. Is that enough to have that same voltage drop in parallel with the relay coil, so that the relay turns off after the capacitors are charged? This way the relay will not need to stay on all the time when the amplifier is on. Would this be a suitable alternative way to do this?

Came across this video as it randomly popped up….whilst looking for a way to stop my 3hp router from throwing me over the bench every time I switch it on….. way over my head but very interesting all the same.!! Why was there no current limiter built in from the factory you ask.? Because like every manufacturer of every machine be it machanical, electrical or both, reliabillity, longevity and reputation all come a poor second to repeat business and profitability..!!!!👍

Will the soft start save the speakers, especially the tweeters?

Might your soft start circuit be even more simplified, by using a Mains VAC Supplied STDP Relay? The resistor is wired across the Normally-Open contacts of the Relay. The Line side feed goes through the resistor, to limit the in-rush to the relay and the load. Once the relay and load reach 80% of the mains voltage (80% being the typical voltage at which a relay engages,) the relay engages, thereby bypassing the resistor, and the load finishes charging (but much less violently.) Not my idea = Credit goes to james_s, on the EEVBlog Forum. Thanks, james_s for (originally) suggesting this to me.

preferring transistor than relay for realizing smoother and safer soft start function, and hoping the circuit diagram fixed on the table and viewed by your camera with smooth panning and enlarging in order for watcher to view the circuit exactly, best...

Another nice video and explanation but since I don't have a test setup, how much does that 4 ohm resistor reduce the inrush current? And more out of curiosity, how much is 4 ohms (percentage wise) in relation to the power x-fmr primary resistance?

Todderbert i like your utube videos

A little off topic, but how about using a soft start with a SCR voltage regulator, a 100A rated bridge rectifier and 10,000uF filtering caps driving a DC motor?

Couldn't a zener be connected in series with the relay coil instead of a resistor to make the threshold more decisive?

How long is time delay before the relay "kicks in" without the dim-bulb ? From what I've read , you need at least a 250ms relay turn on delay. In addition the larger the trans output voltage and caps on the supply the greater the "turn on delay" should be. I have about 1 sec delay with an effective bleed resistance of about 50 ohms (4 * 200 ohm at 5 watts ea), but the soft start circuit is more complex (complete with a relay turn-on LED) in its own small case.

Hi, Tony. A related question, please... I am building my DIY AC PSU chain. It consists of: 1) Emergency All Stop Button to 2) Isolation Transformer to 3) Variable Autotransformer to 4) Dim Bulb Current Limiter (Multi-Bulb) to 5) DUT. Each piece is independent, so that it can be used separately, if need be, for other purposes. Might it be helpful to include a NTC Current Limiter? Advantages/Disadvantages? If so, which component should I add it to? The input on the Dim Bulb Current Limiter? It sort of seems redundant, unless all the bulbs are bypassed... But, I won't always be using the Dim Bulb, in the chain. Thank you for your help.I posted Disclosure> I posted this question on a forum, but I never got a response.

isn't it better to use like 100 w SMD resistor smaller should get less hot, imagine that gets quite hot that 4 ohm resistor??

Question is a tungsten long tube bulb a good substitute for an incandescent for a Dim Bulb current limiter ??

Wouldn't it bei easier to parallel the relay to the speaker protection relay, at least If it's circuit can provide enough power and it kicks in even with the reduced current?

Would a PTC thermistor used as an inrush current limiter work in that?

Hello, where can i contact you about the product sponsorship?

Want to comment on the use of thermistors for the same reason? Pros and cons?

👍🙏

But how to calculate value of R1? How can we know what current will flow through it? P.S. Can't find current value for my relay....

Could you use a varristor?

Wouldn't a NTC thermistor be much less complicated and more reliable? Fewer components, no moving parts.

When you say, “you can add fly back diodes if you want, it won’t hurt anything,” I’ve read (and can dig up a hard copy of) an article by a relay manufacturer that said that reflexive use of fly back diodes can and do harm the contacts of relays. The manufacturer was objecting to the very thinking you are commenting on, that is, that many feel that if the relay is DC, they need to include a fly back diode. I guess what I’m trying to say (and not doing a very good job of it) is that while in the case the circuit presented, i.e., the soft start circuit, the use of fly back diodes won’t hurt anything; it is not true in every case that the use of fly back diodes won’t hurt anything. If I recall correctly, I think the relay manufacturer was advising the addition of a resistor in series with the fly back diode to cut down on the rush of ‘fly back current and resulting arc flash destroying the relay contacts. I need to go back and re-read the article. As others have stated, I found the entire video very interesting and informative. I have a tube amp with soft start. The manual is at pains to say never to turn the amp off and then immediately back on or damage to the power supply may result. Because I managed to inadvertently perform that maneuver a few times, I put a label on top “1 Minute” to remind myself. I’m not sure how their soft start circuit is constructed, but you analysis helps me understand better how and why such a delay is recommended. I do like how you break everything down, your passion is contagious. Going to dig that article out now.

Ok man, this is a fantastic idea, but when you push hard the amp, your rail go down and this can cause the relay's disengaging. Probably the best way is a timer to set a treshold time to engage the relay and if you want, with other components, create a sort of protection at start up if there are problems in the amp stages!

😂😂😂 meteor shower

I have the Atlas ESR Gold that claims that it can measure up to 90,000uF, but so far I did not try measuring a capacitor larger than 1,000uF. My Siglent SDM3045X can only go up to 10,000uF and again I never tried over 1,000uF.

The amplifier doesn't appear to have a mains ground connection, is it double insulated?

You should NEVER use the "aux" AC outlets on a pre-amp to feed a Power Amp. The Phase Linear 400 didn't have a power switch either.

Noooo...a poor mans soft start is just a Dim Bulb setup thats easy, simple ) problem is incandescent bulbs are banned in EU at least in favourbof LEDs

Math's not math.

This is becoming The Capacitor Channel. I wish you’d focus more on other components. Otherwise, it’s becoming like all the others that do little more than shotgun capacitors. It’s really boring.