> 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 > > A 10k resistor goes between the MPSA42 emitter and ground. > > I've been looking around craigslist for an old scope to buy, but > nothing's really appealed to me. That new DSO quad digital mini-scope > is actually really pretty nice (36ms/s) but I wish it was <$100 like > its relatively mediocre counterparts. > On Jan 22, 4:25 pm, threeneurons <[email protected]> wrote:
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. The 10K on the emitter leg of the MPSA42 limits the current from emitter to collector to Vcc-0.7V/ 10K, or ~430uA assuming a +5V supply. Most of that (360uA) will go thru the BE path of the MPSA92. Multiplied by the minimum hfe of 25, that means a max possible current to the nixie of 9mA. With a anode resistor limiting the actual current to under 3mA, this means the MPSA92 will be well into saturation, and voltage across its emitter to collector should be in the 0.5V territory, when ON. Also, the hfe will most likely be ~50, so it will even be more into saturation. The drop across the CE path of the MPSA42 will always be close to the full supply voltage, but the current will vary as a function of its BE excitation (the uC IO port signal). Max power dissipation will be ~80mW with a 180V supply. A TO92 case can handle around 350mW. My first scope was a Dumont 304R. I still have it. Dumont was the big name in o-scopes, before Tektronix arrived. Mr Dumont didn't think people needed the extra features a Tek scope had, so he didn't change his offerings. The 304 only has a 30KHz bandwidth, but it served me well. I upgraded to a 400 series Tek scope in the late 80s, which is my workhorse scope (40MHz). I also have a small Vellum handheld scope, which is always at ready, and handy for quick checks of signal integrity. The Dumont has been retired, but its too big to stick on eBay. -- 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.
