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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
It\'s what you learn, after you know it all, that counts.
Two, maybe three millimeters. Since it was hard to even look at it, I'm pretty sure it was too hot too touch. ;-) I'd have to say that was the most smoke I ever let out of a TO-220 case.
Two dead US 5$ IGBTs, a roasted solderless breadboard and a damaged gate driver later, I've got it working. One mistake was picking the microchip TC4423 driver. It inverts the input signal on both drivers for maximum hassle. The TC4024 has two non-inverted outputs and the TC4025 has one of each. I just ordered five of the TC4425.
With 100V charge into an 820uF cap, the electromagnet coil (stripped Ford starter solenoid - ~4 Ohms) I'm using will launch a computer case screw all the way to the ceiling. 8-)
Major design flaw uncovered. The SCR won't turn off because there is a DC path from the 12V battery to the storage cap. This won't allow the storage cap to drop (much) below 12V, therefore the SCR stays on continuously. I will now have to include a mosfet or relay to switch the 12V battery off after charging the cap. This is really a good idea anyway since I don't want to leave 12V applied all the time. It's just another i/o pin on the PIC to add it.
So far everything can withstand voltages in excess of 250VDC and the associated high currents, including the SCR.
I dropped one on my foot. The cord works to turn it pointy end down. The fine tip punches right through the top of a running shoe. I don't bring an iron out past the edge of the bench any more.
These days, I use a Metcal and it has a short cord. I also solder less. The prototype has 0402 components in it so I try to get others to do the soldering.
On 23 Feb 2006 13:59:35 -0800, snipped-for-privacy@yahoo.com wrote in Msg.
It's actually burnt epoxy resin.
The stench potential is amazing. Once I was called to look at a $100k piece of laboratory electronics that had literally gone up in smoke. A guy had tinkered with it at a spot that he (somewhat rightfully) thought was safe, when suddenly thething stopped working and stenched up an entire laboratory tract. He was of course terrified that he had just terminally ruined a tenth of a million worth of equipment -- and to add insult to injury, colleages, attracted by the odor of catastrophe, came nosing in.
Turned out that he had accidentally shorted out the (short-circuit proof) +/- 18V supply rails from which the inidividual boards of the unit were powered. Each board was subdivided into several subcircuits, and each of those was powered by their own indvidual 78L15/79L15 pair, but the designer had omitted the output-input protection diodes.
Took me about an hour to replace about one or two dozen of the damn things and add the diodes. I didn't give too many details as to what the trouble had been, I just fixed it and saved the life of the lab unit and that of the guy using it. He worships me to this day.
About a year later the same thing happened to myself with another unit of the same model. Man was I glad that I knew right away what was wrong otherwise I would just have dropped dead.
There are a number of ways to turn off a SCR; one way is to pull current out of the gate, but make sure to limit the reverse voltage (below 5V) so you do not zener the gate...
Robert is speaking of the GATE voltage relative to CATHODE.
Current below the holding current spec, or ANODE negative relative to CATHODE will turn it off.
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
It\'s what you learn, after you know it all, that counts.
It's possible, under some conditions (especially at low anode current) but the characteristics are not specified.
Devices which are specified for this kind of operations are called GTO thyristors. I believe they are pretty much extinct, for a variety of reasons.
Best regards, Spehro Pefhany
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"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Funnily, they are still common, but only in relatively large devices. They are used in things like welding supplies, and traction drives for locomotives, electric shovels etc., often with ratings in excess of 3000A.
My Mom was fond of telling about the time when I learned to keep my hand off the top of a hot stove. I wasn't big enough to actually _see_ the top of the stove, but she said it left a mark. It was only a couple of years later when I learned to keep my thumb out from between the prongs of a wall plug, so I guess I had an advantage by the time I got to the really dangerous stuff, like soldering. ;-)
Hold the back of your hand close to the potentially hot element. For some reason there are a pile of "hot" sensors on the back of the hand. ...no need to get blistered at all.
I dropped mine, and caught it in midair. once. For a few seconds I was rather pleased with myself. For a few days, I was somewhat cross :) now I leap backwards, and ensure I only pick it up by the handle.
A place I worked at used to make these. a few years back a customer returned an 80A 400Vac drive for service, after 28 years of continuous operation. One SCR had failed, but alas no low-Tq SCRs could be had that would fit in the available hole. The McMurray-Bedford commutator coils were air-cored, wound on 8" dia. formica formers. The entire unit was in a box big enough to fit 3 very tall engineers. We offered to replace it with a UD3-70, about 18" x 24" x 6". The customer said "no thanks, we want it back, its so reliable". In the end we bought it off them for our museum. At the time, our 660A drive was about 1/2 the volume of the old
80A unit.
A GTO is basically an SCR with a monstrous gate-cathode FET....
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