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Delay on-instant off with 555 timer

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    #21
    Re: Delay on-instant off with 555 timer

    so what the hell are you actually doing?
    i dont like "science theory", tesla had some things to say about that!

    what's your load current, inrush if applicable, voltage etc. ??

    Comment


      #22
      Re: Delay on-instant off with 555 timer

      Close: an inrush limit circuit. 10000uF caps on 48v DC. I'd have the relay before the bridge rectifier, with the COM terminals on one of the AC wire coming from the transformer's secondary and the NC contacts wired to some power resistors before the caps. When the thing first powers on, the resistor(s) take away the inrush current and when the relay flips over, the NO contact bypass the resistors entirely....it's not built yet, but I figured it'd be a good thing to have in there. It's for THIS project of mine BTW, which you're probably familiar with.
      Wattevah...

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        #23
        Re: Delay on-instant off with 555 timer

        if you want to limit inrush then there is an established method and ready-made products.

        you have an NTC or a group of them to limit the inrush,
        then you have a relay to bypass the thermisters once they reach temperature, or a specific time has elapsed.
        (so you arent just burning the thermisters all day)

        big clive did a teardown of one.

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          #24
          Re: Delay on-instant off with 555 timer

          That is what I'm trying to achieve, except my idea was to use a resistor instead of NTC, plus you know me, I always like the harder DIY route
          Wattevah...

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            #25
            Re: Delay on-instant off with 555 timer

            Now I'm confused, didn't that youtube video in your first post explain what you wanted to do, and this thread was meant for something else? I mean, no pushbutton is needed at all which is confusing now, thought this thread was for a pushbutton based design thought experiment.

            As for the inrush solution that doesn't need a pushbutton, just need an RC delay and a reverse biased diode across the R to reset when power goes off or flickers off. Traditional POR circuit, easy peasy. Let me know if you need circuit diagram...

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              #26
              Re: Delay on-instant off with 555 timer

              Ehm...yeah, it sort-off got off track, sorry, but that's what happens and it's a perfect display of how my mind operates: I start talking about one thing, then I start talking about something else, confusing the hell out of people, even though my mind keeps it perfectly organized and KNOWS what it's supposed to do

              To back up a bit, the original idea was NOT for the inrush current limit, nor was it related to GreatScott's video in any way. It was meant to actuate a step relay after a button was held for a set time, instead of doing it immediately (since you might be asking why not just use the button to drive the relay directly). I began researching the 555 delayed-on idea and just as I was doing that, GreatScott's video popped up in my subscriptions where I saw that inrush current idea of his and thought the two circuits go hand in hand perfectly. The original step relay was NOT part of this inrush current idea - I'd use a normal relay for that. Then I got off track again and dropped the 555 entirely in favour of a time relay and here we are....typical Danny
              Wattevah...

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                #27
                Re: Delay on-instant off with 555 timer

                okay, I think my circuit will work... pretty sure. Of course a microcontroller does it all in software though I think it may still need a driver transistor, though you lose that ugly 100uF timing capacitor of which the circuit is not totally consistent from build to build due to component (especially capacitor and mosfet) variability.

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                  #28
                  Re: Delay on-instant off with 555 timer

                  Here's what I concocted using the relays I was mentioning: RL1 closes immediately after the On/OFF switch is turned, thus sending power to the primaries of T1 and T2. RL2 is a time-delay relay which turns on after a few seconds, thus allowing C1 and C2 to charge slowly through R1 and R2. After a while, RL1 flips over, connecting the secondaries directly to the bridge rectifiers. This is the slightly more complicated version, since I could just omit RL1 entirely and have the primaries of the two transformers controlled directly by the switch, but unless it's a particularly beefy one, I wouldn't quite trust it for this application. Not sure if this design would pass any safety standards with 230v for RL1 straight on the power switch the user comes in contact with, but hey
                  Attached Files
                  Wattevah...

                  Comment


                    #29
                    Re: Delay on-instant off with 555 timer

                    the relay contacts are wrong,
                    the resistor / NTC(much better) should be inline at all times, and the relay should short across it.
                    the way you have it the relay will disconnect from the resistor allowing the cap to discharge before it connects again to the transformer.

                    the first relay will have to be a huge octal base job - you would be better just using a 20A 2pole rocker switch.

                    Comment


                      #30
                      Re: Delay on-instant off with 555 timer

                      Inrush or soft-start circuits are usually two relays, one to turn on power and another to short out a NTC thermistor or resistor after a few seconds. They always have a small aux power transformer to provide relay power say 12V.

                      I made this one using a ULN2002 Darlington array but I forgot if it is a push-on/push-off power switch (flip flop) I had made. I eventually went MCU and used an ATtiny85.
                      There are 555 timer ones from china I can dig for the schematic - wait -found it and here is pcb pic. It sorta works, kinda cheap though.

                      You could add a NTC inrush limiter from a scrap power supply and something to switch on a relay when raw DC comes up close to your target voltage, and short out the NTC. Like a transistor and zener would work.
                      Attached Files

                      Comment


                        #31
                        Re: Delay on-instant off with 555 timer

                        or just buy one, they sell units in some EU country's for use on stuff like tumble dryers.

                        Comment


                          #32
                          Re: Delay on-instant off with 555 timer

                          Originally posted by stj View Post
                          the way you have it the relay will disconnect from the resistor allowing the cap to discharge before it connects again to the transformer.
                          Agreed, and I was aware of that to be honest, though I imagine the time it takes for the contacts to flip over is very short and with such large capacitance, it wouldn't lose much voltage.
                          I redid the schematic though like you suggested, with the timed relay shunting the NTCs. BTW, what value NTC should I use for such a setup ?

                          Originally posted by stj View Post
                          the first relay will have to be a huge octal base job - you would be better just using a 20A 2pole rocker switch.
                          True, a switch is the simple version and I WAS going to go with one, which I already have in my parts bin, but it's not as aesthetically pleasing, so I went for one of THESE to which I added RL1. Is a contact rating of 5A not enough ? Again, I haven't got much experience with transformers to know their particularities and design accordingly, hence why I do it by ear and then polish the design up a bit with a little help from our friends here
                          Attached Files
                          Wattevah...

                          Comment


                            #33
                            Re: Delay on-instant off with 555 timer

                            NTC value would have to be calculated based on the load current so it operates at a reasonable speed and doesnt just sit there not heating up, or worse yet - go nuclear on you!

                            as for switch and relay contacts, overrate them by atleast 2x because it's an inductive load.

                            Comment


                              #34
                              Re: Delay on-instant off with 555 timer

                              For two of 10,000uF caps to 48VDC, that is about 23 Joules of energy to charge up the filter caps plus some for the transformer. Compare to a PC PSU with 680uF cap is almost the same amount.
                              A 10R (cold) PTC would limit inrush 240VAC/10R to 24A. The ~12mm dia. parts are OK for that.

                              The PTC does not see steady load until you turn on the DPS modules, so you have forever to pull in the relay.

                              Comment


                                #35
                                Re: Delay on-instant off with 555 timer

                                Just so I don't stay dumb forever, how do you calculate that ? Like THIS ?
                                Also, you sure it's not NTC ? Doesn't a PTC's resistance increase as it heats up ? Or is that what we want actually ?
                                Wattevah...

                                Comment


                                  #36
                                  Re: Delay on-instant off with 555 timer

                                  24A is too much, european outlets are only about 12A.

                                  the resistor has to start high - to limit the inrush, then slowly go down in value so the voltage rises to the full level.
                                  so you need an NTC

                                  Comment


                                    #37
                                    Re: Delay on-instant off with 555 timer

                                    Originally posted by stj View Post
                                    24A is too much, european outlets are only about 12A.
                                    16A actually, but I don't expect to see THAT amount of current being pulled at the primary side anyway. Still, better to keep it down as much as possible, so I would go higher in resistance on the NTC.

                                    Originally posted by stj View Post
                                    the resistor has to start high - to limit the inrush, then slowly go down in value so the voltage rises to the full level.
                                    so you need an NTC
                                    That's exactly what I was thinking.
                                    Wattevah...

                                    Comment


                                      #38
                                      Re: Delay on-instant off with 555 timer

                                      schuko is 16A?? or are you using CEE/IEC60309 plugs??

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                                        #39
                                        Re: Delay on-instant off with 555 timer

                                        Yes, THESE things are rated for 16A. Apartments in old buildings such as mine will not have an earth wire running to every outlet, so the earth pin of the outlet is simply not connected or, most commonly, an unearthed (Europlug ?) outlet is installed which omits the earth pin entirely. Only two outlets in my whole apartment actually have a true "earth" connection - one of them where a washing machine is most likely to be installed and the other one in the kitchen for an electric stove perhaps. When the building was erected 50 something years ago, electricity wasn't even a "thing" yet, so I doubt they had microwave ovens or electric stoves in mind when they earthed that kitchen area, especially since in 99.9999999% of households here LPG is used for cooking (either in cylinders or piped into the building), never electricity, since it's hella expensive to do that, except for maybe very large restaurants and malls, but even those have a huge-ass gas pipe running into them from what I've seen. When I renovated the kitchen, I tapped into that earth wire and ran it to the adjacent room for the A/C as well, so I'm pretty close to having earth everywhere in the house now.
                                        Wattevah...

                                        Comment


                                          #40
                                          Re: Delay on-instant off with 555 timer

                                          Originally posted by Dannyx View Post
                                          Just so I don't stay dumb forever, how do you calculate that ? Like THIS ?
                                          Also, you sure it's not NTC ? Doesn't a PTC's resistance increase as it heats up ? Or is that what we want actually ?
                                          Yes that calculator works, energy in a capacitor is 1/2*C*V^2 so I got 2x 10,000uF to 48V for 23J. The NTC inrush limiters need to be rated for the energy they might have to limit. Too little and they don't warm up much and too much and they get hot and take forever.
                                          You can use a light bulb to limit inrush a bit, I was saying 10 ohms is what I see all the time plus wiring and transformer resistance but that might be too low for 230V systems. Look in your junkbox and see what you have.

                                          I get mixed up NTC or PTC and that website I gave somehow uses either to limit inrush current.
                                          Attached Files

                                          Comment

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