On Tuesday 19 May 2020 14:39:28 Chris Albertson wrote: > The usual method to limit inrush that is used in almost all > electronic devices is to place an NTC thermistor in series with the AC > mains supply. If you take apart any TV, stereo amp, or PC power > supply that was built this century you find them inside. > > They are really easy to use. They act just like a resister and > limt current just like a resistor but the resistance changes with > time. When you first hit the "on" switch the thermistor might have a > resistance of 100 Ohms, then after a second it falls to about 2 ohms. > Ametherm is a major manufacturer of these and will give anyone a > handful of samples and there is a ton of information on their web site > about how to select and use these devices.
> https://www.ametherm.com I'll do some reading. The first schematic shown there basically uses one of those in place of my fixed R, but the 2nd SSR still shorts it, exactly as my timers do. So it is not an obvious cost saver. > The trick to using these is to match their specs to your needs. Yu > need to use one that has the correct resistances, and time constant. > It is like building an RC lowpass filter. > > Yes, you could simulate a thermistor with some switches, a timer, and > load resisters or just ask Ametherm and they give you a dozen samples. > > There is a technical article on the above web site that is titled > "limiting in-rush current in DC motors". They work out a problem > where you place an NTC Thermister in series with a large DC motor. In > this case the resistence starts at about 1 Ohm and falls to about 0.01 > Ohms when the motor is running full speed. > > With motors they place them in series with the motor. In a power supy > they place them in series with the AC mains or in series with the > transformers winding that has the most current. > > The best thing is that these things are so simple that the can't fail > and they are way-cheap. > This might be true, but the truth is that they are also one of the more troubling failures in home electronics/ They are not new by any means, so my history with them goes back about 40 years. The failure is generally that the NTC fades over time, until they either start a fire, or simply turn the center section into ash which is so weak the connections at their soldered joints pulls them open. And they do it with no drama until the customer brings it to me saying it doesn't turn on anymore. Finding the one with suitable characteristics can be fun too because the maker usually flat lies about the actual power drain. So you put in the recommended one, but the tubes are tired so it workls but doesn't draw the target power, causing the device to run cold and temporarily at a higher resistance, further hurting the performance. My cobbled up timers and SSR circuitry may cost 50x the price one of those, but so far the failure rate has been zero. But doing that softer start to a switcher makes me ask for other peoples experience since I have only relatively ancient history to go by. > On Tue, May 19, 2020 at 10:51 AM Gene Heskett <ghesk...@shentel.net> wrote: > > Greetings all; > > > > Having destroyed one 40 amp SSR by turning on the switching psu;s > > for the two running the X&y motors and the builtin supply of the > > even bigger drive running the Z axis, already, I am now > > contemplating how to do a soft start to limit the initial in-rush of > > charging up those 3 power supplies. > > > > And doing it in a lower powered version of how I am doing the huge > > analog supply that runs the spindle motor. > > > > There, I have two timers which are controlling two 40 amp SSR's with > > a 50 ohm 200 watt resistor, so it starts charging that supply with > > its thousands of u-f's by applying the power thru that big resistor, > > then the 2nd SSR comes on applying it directly 3 seconds later. > > This has worked flawlessly for about 4 years now. > > > > Now I have obtained two more used genuine crydom 40 amp 480 volt > > SSR's, and am considering doing the same thing in essence, only with > > a much smaller ohmage R to limit the in-rush this batch of switchers > > in a similar manner. > > > > But switchers, I have heard, need that initial bump to get started > > correctly. To that end I've also ordered a pair of 10 ohm 70 watt > > resistors, which if need be can be paralleled for 5 ohms in series > > with these supplies for the first 1/2 to 1 second. The heating pulse in the R during the turn on is significant, but very short in duration. That is why the relatively large mass of the R. > > > > But I intend to use one of these in another analog supply, starting > > the spindle psu in The Little Monster too, putting all its heavy > > power under LCNC's control. But that is a different project. > > > > These switchers are, or s/b all fused to protect them in the event > > they don't start. So I don't expect a start failure to do more than > > blow the fuse. > > > > Has anyone else any experience with this, that can add gotcha's to > > watch for in such a current limited startup? 10 ohms in series with > > the power to protect the SSR's is the basic idea. 10 ohms would > > limit at 12.5 amps of in-rush, which seems reasonable. > > > > Thanks all. > > Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> 0 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
