On Tuesday 02 March 2010, Erik Christiansen wrote: >On Mon, Mar 01, 2010 at 02:22:06PM -0500, Gene Heskett wrote: >> But probably right at the motor supply terminals of the board, not 3 feet >> away in the psu. > >I'm with you on the "stop interference at the source" approach, but >having the transorb on the board will catch fast spikes induced in that >cable. > However, I get the impression you are saying they should be right on the motor cable. Nyet. No, Nope. At least not until I had determined the effective capacitance of the device to be on a par with the inter wire capacitance of the cable to the motor. Any capacitance at that point adds to the inrush currents when the h-bridge switches states, and can silently but most assuredly over stress the bridge and kill it. Jeff is well aware of that which is why his instruction sheets warn against probing the motor lines. I have, but my scope probes are on 10pf probes, and I 'got by with it', but there are 10x probes out in the wild whose std input capacitance is 47pf or more, and if the switch is in the 1x position of a switchable probe, the popping of epoxy will probably be coincident to the probes contact.
That will ruin your whole day. >> It's a pi section filter, with 75,000 uf on each side of a choke that >> probably has 3 to 4 ounces of decent silicon steel in its E core but I >> have not measured its true inductance. The hf noise isn't much greater >> at the xylotex terminals than it is at the psu, another 50mv or so. >> Barring failure of the caps, I think I am in pretty decent shape. Those >> 75,000 uf caps are also beyond my ability to measure, but were top >> quality stuff 45 years ago, in humungous screw terminal cans. Even I, as >> acutely aware of the ESR requirements for such duty as I am, was >> pleasantly surprised at the seemingly zero ESR those old caps have. > >Ah, I didn't realise you had a pi filter. With any decent inductance >between two of those caps, and good ESR, your setup is a very serious >spike killer. > >However, H-bridges can generate their own spikes, given the partially >inductive load, and a transorb on at least the supply rail can be >life-extending. That was where I would put them if I felt I needed them. The scope probes at least are telling me I don't. >> >You'd want something with tighter tolerances than the old 5Z27 devices >> >floating at the bottom of my junk box, for lower headroom. >> >> No doubt. How are these transorbs thingies for long term stability? >> Only true zeners are stable over time, and those stop at 4.7 volts, >> anything above that is actually an avalanche diode, and they will drift >> low over time, rate dependent on how much its average power dissipation >> is. > >Haven't seen drift in the datasheet, even for a slightly more recent >device. A couple of those old 5Z27s checked out within their barn-door >specs, and they've been lying about for about 30 years now. Even >in-circuit, they're only conducting when there's a spike. If they are >very high energy spikes, then some cooking might go on. I think this is like asking if freshly poured concrete is going to crack. Wrong question, it should be when, there is no 'if'. ;-) >Erik > -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The price of greatness is responsibility. ------------------------------------------------------------------------------ Download Intel® Parallel Studio Eval Try the new software tools for yourself. Speed compiling, find bugs proactively, and fine-tune applications for parallel performance. See why Intel Parallel Studio got high marks during beta. http://p.sf.net/sfu/intel-sw-dev _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
