Hi Gene, Since I'm running DC Servos I wanted the ability to clamp the back emf from a decelerating motor. I built a small circuit that sensed both high voltage and AC power. It puts a resistor across the DC rail to pull it down and the resistor is connected with both a relay and a FET in parallel.
When power vanishes the relay is de-energized and the 105VDC power supply is quickly drained through the NC connection on the relay. The FET does the same but at high speeds to avoid that over voltage condition. Since the little PIC processor senses AC voltage it can also run a relay on a second small board that switches across an inrush resistor. The advantage over a high power thermistor is no heat and you can cycle power without worrying about the inrush thermistor still being way too hot and therefore too low a resistance. Schematics attached. John Dammeyer > -----Original Message----- > From: Gene Heskett [mailto:ghesk...@shentel.net] > Sent: May-19-20 10:49 AM > To: Enhanced Machine Controller (EMC) > Subject: [Emc-users] hypothetical, maybe not, switching psu question > > 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. > > 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> > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users
SoftStart1.10.pdf
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DrainControl1.20.pdf
Description: Adobe PDF document
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