On Wednesday 06 January 2016 15:10:55 Bertho Stultiens wrote: > On 01/06/2016 01:54 PM, Gene Heskett wrote: > >> When you turn off the machine and the SSR starts to bleed, and then > >> turn on the machine before the caps are completely empty, then you > >> will have the bleeder load on there permanently. > > > > Exactly the reason for the forced time delay in the turn on as I > > described. That, and the low voltage AC in series with the load > > resistors to encourage its turn off when the DC volts gets down to > > the 6 volt range if it does not recover from lack of sufficient > > holding current, but that may be several T=RC's depending on the > > minimum holding current of the individual device. This applies to > > the spindle psu only, and other initializations of the machine, like > > homing & touch offs, can continue while this is taking place. > > I'd do it differently and use an active bleeder setup. > > See atached diagram and simulation result. > > The setup uses the voltage from the cap an energy source for bleeding > the cap. The bleeder kicks in at about 40ms after the power is turned > off and is disabled instantly when the AC source is enables. > > The cap is discharged using a constant current bleeder. This reduces > the need to have a high-power resistor in the circuit. Most of the > power is dissipated in the mosfet, which should be mounted on some > cooling fin to dissipate the instantaneous heat (but it is not too bad > or much). > > This setup is probably also cheaper than using SSRs.
Perhaps, but your parts count is many times what I have in mind. Some History: My experience in using hexfet's for analog pass transistors in a 275 volt bias circuit, where they had to sink the surplus grid current, up to several hundred milli-amps during vertical synch, was that they had a fairly short lifetime, even though I was buying 1 kilovolt, 200 watt rated hexfets, individually heat sinked and driven in parallel with current sharing resistors in the source leads. They did not get "can't touch the heat sink" warm either, perhaps a 20F rise over ambient. I was eventually forced to put the 1955 design power supply that used a pair of vacuum tube 7094's for the pass element back into our old GE transmitter. Because they did not well handle the surge during vertical synch time, there was about a 5% sag in peak power during that time. The hexfet circuit only sagged perhaps 1% when they were fresh. But since those internally are actually made off many junctions bonded out in parallel, they would punch thru and blow the bond wires removing themselves from the circuit, gradually removing gain until it was time to replace them. As that was about an every 45 to 60 day need, I eventually ordered some fresh sockets to fit the 7094's and put the 50 yo supply back in. The tube sockets heater contacts were oxidized and had been overheated so many times only fresh sockets would make it dependable again. That was in about 2002, and AFAIK they were still in and working when we pushed the off button the last time at midnight June 30, 2008 for the digital switch over here in the US. I don't know if there is a major diff in internal construction between a hexfet and a mosfet, but while hexfets make fantastic digital switches, they are not recommended for use in an analog circuit by IR as I called them on the phone and discussed my high failure rate with their engineers. If they had a corresponding mosfet that could have done the job, I am sure they would have mentioned it, but they did not. This failure mode is similar in nature to the SOA ratings of normal bipolar junction transistors. So if after a time, it seems your circuit is getting slower, that is probably the failure mode. A slower rate of failure at 130 volts might be expected, but I have doubts its completely hideable by the passage of time. If the 100 ohm r was replaced by an inductor and a current sensor of an ohm or so, driving a comparator, essentially converting it into a switch mode current regulator running at 20 some kilohertz, I would expect close to infinite life from a hexfet used as the pass switch. I have no clue where the heat energy (law of conservation of energy) would go given good switching drive and a low ohms inductor. Might be interesting to build such a critter and see just how fast it could be made to work by raising the current regulation point. Since I have all sorts of hexfet's in BIG packages laying around in failed computer psu's that could do that rather nicely, and inductors from the same source, its a should be investigated idea. I am sure there are circuits for switch mode current regulators out in the wild. Thank you Bertho, you made me think... :) 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) 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
