On 2/7/2013 10:39 PM, Erik Christiansen wrote: > On 07.02.13 19:04, Leonardo Marsaglia wrote: > >> The basic thing that I need is a Square wave generator, that outputs 2 >> signals one at 180ยบ from the other, and with a duty cycle less than 50%. >> This is to avoid complications with the switching transistors. I'm almost >> sure that hal could do this, even read the signals to adjust the frequency. >> > If a frequency of twice the desired output rate is used to clock a JK > flip-flop, the result is two antiphase 50% MS ratio clocks, from Q and > Q' respectively. The "less than 50%" duty cycle that you refer to is > called "dead time", and typically ensures that totem-pole connected > power devices do not conduct at the same time, shorting out the power > supply. While that can be added by simple diode/resistor networks when > driving power MOSFETs, it is already included in many gate drive chips, > such as the IR2111, which provides a deadtime of 650 ns, typical. > (Others are adjustable, IIRC.) > > All that would be required then, is a 2x clock from LinuxCNC, or just a > DC enable, if a gated oscillator were simply turned on and off by > LinuxCNC. > > That would be much more robust than generating the antiphase clocks in > software, where a software hiccup could lose the deadtime, destroying > the power devices, or melting other gear. (Just my $0.02. :) > > It sounds like a very interesting project, and a lot of fun, but > susceptible to the wrong things glowing red hot. > > Erik > >
You guys are braver than me. I did some service on the machine I retrofit later when they blew a scr in the power cabinet. I swapped out the entire rectifier/scr stack with identical parts from another machine. The semi conductor devices were about the size of hockey pucks and there was a stack of them probably 16" tall with several interconnections to buss bars and trigger circuits. I got the machine running again for a while, but then something else failed and they called in Inductoheat to find the cause of the problem. I was fine with that. That machine has a "walk in" power cabinet with a large 480 volt disconnect switch (I think it was 400 amp) that feeds the cabinet. The machine has its own cooler on the plant roof to keep the power circuits and the liquid quench tank cool. (Many of the buss bars are hollow and coolant flows through them to keep them from melting) The machine brings a 1.25" diameter steel bar from room temp to red hot in a couple of seconds. Yep, better be sure that power control is correct or else you might have a lot of melted copper in the bottom of the power cabinet! Dave ------------------------------------------------------------------------------ Free Next-Gen Firewall Hardware Offer Buy your Sophos next-gen firewall before the end March 2013 and get the hardware for free! Learn more. http://p.sf.net/sfu/sophos-d2d-feb _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
