On Tuesday 21 May 2013 16:54:40 Peter C. Wallace did opine: > On Tue, 21 May 2013, Gene Heskett wrote: > > Date: Tue, 21 May 2013 10:50:27 -0400 > > From: Gene Heskett <[email protected]> > > Reply-To: "Enhanced Machine Controller (EMC)" > > > > <[email protected]> > > > > To: [email protected] > > Subject: Re: [Emc-users] Superglue, or Loc-tite for that motors > > flywheel? > > > > On Tuesday 21 May 2013 10:26:05 Peter C. Wallace did opine: > >> When using bipolar transistors as high power current sinks, make sure > >> you are not violating their secondary breakdown specs (a lot easier > >> to kill than you might think) > >> > >> > >> Peter Wallace > >> Mesa Electronics > > > > Despite my confidence that I knew what I was doing, it turns out that > > the SOA ratings on the bugs I bought and which arrived while I was > > trying to get some sleep on a dairy farm in NYS, turns out to suck > > dead toads through soda straws, Peter. > > > > It appears that the only way I can make a 10 amp sink, would be to > > make >15 identical circuits and stack them. I'm not all that fond of > > semi's in series. > > > > Page 5 of this link > > > > <http://www.st.com/web/en/resource/technical/document/datasheet/CD0000 > > 0005.pdf> > > > > defines the bad news. Operation in parallel, so they get the full > > jolt from the motor, says I'd have to use an emitter R high enough to > > keep the individual bug under .350 ma. So a 10 amp sink would need > > 30 of them. Sooo, I am back to wondering how much pure R I can find > > around the place that is not already in my EDM lashup. That has a > > pair of 50 ohm 200 watters, and those, even in parallel, won't do > > much more than slow it at about half the rate it was accelerated at > > by the controller. 10 ohms would be a more serious load. Using the 5 > > ohm I have from full speed, it could exceed 20 amps at the initial > > contact close an I can see demagnetizing the PM fields. > > > > Do you have any better ideas for a semi choice? These $2.25 each > > ignition coil switchers sure aren't it. > > > > Thanks Peter. > > > > Cheers, Gene > > MOSFETS are better but for toughness when you are dissipating KW for > seconds, a big wirewound resistor can't be beat. So I would try your > ignition transistors in switch mode into a say 16 Ohm resistor. > Humm, is there an IC that will function as the switch driver, and whose output could be controlled with a PWM mode? Something that would start at 15% duty and taper to 100% when the applied voltage had dropped to say 20 volts. Then the Q is how fast can these be switched, I get the impression I'm still looking at hexfets I can steal out of dead computer PSU's. These things speed (lack thereof) will cause their own SOA self destruction I expect.
> If you want to get fancy and do constant current (so you get a faster > more linear speed drop instead of exponential), I would use a PIC or > something similar driving a MOSFET in switch mode into a low value > brake resistor at say 30 KHz and vary the PWM as the voltage drops. See my question above. Something that could be powered by a 8 pack of fwd biased 1n914 type diodes? For this of course, the load R would go in the collector. A good google search term would probably help. > PS: we blew a couple 75A 1200V Darlintons trying to make a constant > current dynanometer load, so the lesson about secondary breakdown (or > FBSOA) kinda sticks with me :-) I did the same thing in a 275 volt bias supply. 600 volt hexfets slowly faded to pure resistance, which rather played hell with the operation point of an 85kw water cooled triode final in our old NTSC transmitter. I liked how it worked, making definite improvements in the video linearity, for about a week, but was forced to put the 7094 tubes back in after toasting the 3rd bag of hexfets. I even called IR, but was flat told they were never intended to be used as an analog pass device. Sigh. Thanks Peter. 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) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! My views <http://www.armchairpatriot.com/What%20Has%20America%20Become.shtml> When all else fails, read the instructions. A pen in the hand of this president is far more dangerous than 200 million guns in the hands of law-abiding citizens. ------------------------------------------------------------------------------ Try New Relic Now & We'll Send You this Cool Shirt New Relic is the only SaaS-based application performance monitoring service that delivers powerful full stack analytics. Optimize and monitor your browser, app, & servers with just a few lines of code. Try New Relic and get this awesome Nerd Life shirt! http://p.sf.net/sfu/newrelic_d2d_may _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
