Your MPSA44 transistor is fine for 3 reasons 1- You are below BVceo 2- You are within the safe operating area (SOA) with 4meg of resistance that limit the max current below 1.5mA at any voltage (in your case, it's 85uA) 3- Bipolar devices, unlike MOSFETs, can actually sustain voltages above BVceo as long as your circuit limits the current. At higher voltages, there is reverse-junction breakdown resulting in current, but it's not destructive as long as the current is limited. MOSFETs, however, will have permanent oxide destruction at any current.
Those IN-28 boards are cool !! On Wednesday, April 15, 2026 at 7:48:37 AM UTC-7 Richard Scales wrote: > [image: 7SegIN28.jpeg] > > On Wednesday, 15 April 2026 at 15:30:51 UTC+1 Richard Scales wrote: > >> Hello, >> >> I have a number of these '7 segment' panels which are made up of discreet >> groups of IN-28's. >> [image: 7SegIN28.jpeg] >> >> I am in the UK and am driving them with bridge rectified UK Mains (238V >> before rectification). >> >> Each 'segment' has all it's grids connected to VCC via a 3M, a 1M >> resistor is also connected so when the end of that is pulled to ground, the >> grid voltage is reduced to about 85V (assuming 340V peak). >> >> If I ground a segment via the 1M resistor, the tubes turn off, if I let >> it float, the tubes turn on - all good so far. >> >> [image: IN28.jpg] >> >> My question is this, what safe and reliable mechanism might I employ to >> make that switch? I was thinking of using MPSA44 NPN transistors which have >> a Vceo max of 400V, is it as simple as that or is there more to it? >> >> Right now I am using a hand made bridge of x 4 UF4007, I would use a >> proper bridge if this project ever gets anywhere near the finish line! >> >> Many precautions are being taken during testing to ensure that the >> rectified does not go anywhere near me (or anyone else for that matter!). >> >> - Richard >> > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion, visit https://groups.google.com/d/msgid/neonixie-l/5aacbc78-ce69-43bd-93ad-7b604c574eebn%40googlegroups.com.
