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.
