OK, I think I understood some of that. You're saying that the amount of current the MPSA42 will let through is defined by some relationship with (Vin-Vdrop)/Rground, or (5-.7)/10000=.00043A. This is the amount of current across base to emitter, yes? But, if the Hfe is 50, doesn't that mean that I get up to 50*.00043A or .0215A across the collector to emitter? How did you get that 1/50th of that goes across the BE and 49/50 of that goes across the CE?
Also, I really can't afford to just "try stuff out". I have a limited supply of parts, which is why I carefully plan everything beforehand. On Jan 24, 1:15 pm, threeneurons <[email protected]> wrote: > No, No, No. > > That's only if there was no MPSA92 in the circuit. Remember, the Base- > emitter junction of a transistor is a simple PN junction, ie a diode, > and will act like one. The forward voltage will be limited to about > 0.6V to 0.7V for your common silicon transistor. (Wiki a silicon > diode) If the forward voltage is below 0.7V (for silicon), the base- > emitter impedance is high, hence the need for that bleeder resistor, > but when the voltage exceeds, or tries to exceed, that 0.7V 'knee', > the impedance drops sharply (to close to zero ohms, ideally). > > Ohms law only works for linear components like resistors (AC or DC), > and capacitors and inductors (AC only). PN junctions on semiconductors > (diodes, including LEDs. and bipolar transistors) and gas filled tubes > (nixies) are VERY non-linear. Think of them all as regulator diodes, > that like one specific voltage to operate at. Of course, that voltage > is different for each device. > > If that logic for 2 10Ks in series works. ie 190V supply and 85V > across each 10K resistor, then with the 1M, there should be 188V > across it and 2V across the other 10K. And it would, if you only had > the 2 resistors, but you don't. > > In Jason Harper's (anode drive) circuit, the voltage across his 1M > (should be 10K, to speed it up) is clamped by the MPSA92 BE junction > to 0.7V. That BE junction is in parallel to the resistor. On the > MPSA42 side, its limited by the maximum excursion of the base > excitation, which is the 5V coming out of the IO port. That again > forms a maximum forward drops across the MPSA42's BE junction of 0.7V > (again silicon), so the remainder or the drop across the 10K emitter > resistor is 4.3V. 4.3V/10K is 430uA. 98% of that (assuming a hfe of > 50) will go thru the collector-emitter path of the MPSA42. Only 2% > thru the BE path. In engineering that's pretty much considered all of > it, in this particular type of app. This part of the circuit is > basically a current limiter, where the current out of the MPSA42 > collector is mostly a function of the voltage across base to GROUND. > NOT base to emitter. That BE voltage is fixed to 0.7V, unless the > total voltage drops below 0.7V, then the collector current is zero. > Remember that non-linear stuff again. When higher than 0.7V, the > difference goes across the 10K resistor, and the current coming out of > the collector is almost the same as the current thru the resistor, > which is set by Ohms law. In reality, its a little fuzzier around > voltages from 0.5V to ~1V. In this app, however, we apply only 0V > (off) or 5V (on), so we're never in that fuzzy zone. > > <rant>In engineering precision isn't always needed, and you round off > when its convenient. Don't believe that crap you see on Star Trek and > on digital calculators. The real world never gives you precision to > begin with, so trying to get precision out of it, is mostly a futile > effort. Yes, sometimes its needed, but mostly its not. In the modern > industrial world we get precision in areas where its really not > needed. That's mostly because of consistency. Its easier for a machine > to make the same exact part all the time. If a square of toilet paper > was another size, would world order break down ? > > Finally, don't believe me. Wire up the circuit for yourself, and take > some measurements. All this writing is mostly toilet paper fodder. Its > dogma, which is mostly crap. The essence of the 'Scientific Method' is > the experiment. TRY IT OUT, and SEE WHAT YOU GET. That's why I get > along with the likes of Westdave. He could care less about all this > paper work. He just builds it and we all get to see the end results. > Of course, in that process, a few parts get toasted, but that's all > the fun part. Go out there and build something ! > > The magic of modern science, is not the body of knowledge that it has > revealed, but the method upon which it was acquired.</rant> > > > > > > > > > Wait... if I have a 10k resistor on the high side and a 10k resistor > > on the low side, won't the voltage at the base of the MPSA92 just > > hover around 85v? Is this low enough to turn it on? Also, 20k shorting > > between ground and +V and ground will draw 8.5mA, won't it? > > > > > Also, I have a 1 megohm resistor between my base and emitter. Look at > > > > the bottom of page seven at: > > > > >http://nixietube.info/NixieTransistors.pdf > > > > I'm well aware of that document. It was written by Jason Harper, a > > > long time member of this group. I like his work, > > > That 1M should be reduced to 10K. The voltage drop is limited by the > > > maximum forward drop across the MPSA92s B-E junction, so it will only > > > draw away 70uA at 10K, & 0.7uA at 1M. ... -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
