At 08:08 AM 12/20/2005 -0500, you wrote: >Ian Lesnet wrote: > > Specifically I would like to know 'whats coming out of the base'.... > >Just 'current' :-) > >Specifically, a fraction (1/beta) of the current flowing through its >collector. > > > I don't know if I can eliminate 2 & 3 because I don't know what comes > > out of the base of 1 - if it can be handled directly by the PWM (17V, > > 120ma per channel). > >No, you need to keep all three transistors -- the collector of Q5 needs >to swing up to the HV level when the tube is off, and the output of the >PWM won't be able to handle that. > >However, you can eliminate R3; it's completely superfluous. R2 serves >to limit both the collector current and base current of Q5. > >-- Dave Tweed Personally, I'd put a 1/4W leaded resistor in the collector of the NPN that drops around 50-100V at the operating current, and a resistor about 1/10 that value between base and emitter of the MPSA92. You can make the current about 1/10 of the current you want to go to the neon. The series resistor will let the NPN run cooler, and will probably prevent any disaster if a short happens (think of 180V on your +5 rail), and the base-emitter resistor eats the leakage current when the NPN is off and prevents it from being amplified by the MPSA92. If you look at the schematic you can probably see that the leakage current in Q6 is amplified by the leakage current in T1 (why the inconsistent naming?), more leakage added, and that is amplified by Q5, so you could otherwise have the neon partially turning on (and Q5 getting warm). The base of MPSA92 should never be more than about 0.7V below the 180V rail, so "off" is +180V, and 'on' is +179.3 or whatever. The MSPA92 will turn on hard (saturate) typically when the base current is a percent or two of the collector current, so I suggest using 10% to provide some margin. Eg. neon current is 5mA, so use 500uA for base current. R2 = 4.3V/0.5mA ~= 8.2K. Rs = 75V/0.5mA = 150K. Rb-e = 0.1*Rs = 15K. That way, even if you accidentally get a short across T1, the current is limited to 1.2mA. >Best regards, Spehro Pefhany --"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 ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist