Nixie cathode driven by under-specks BJT - Page 1
Forum section: electronics / projects

author: ppTRN
author_rank: Regular Contributor
timestamp: January 24, 2024, 04:52:32 pm
content: Hi everyone,I have a couple of old french made nixies (TAF1093A)- Minimum supply: 170V- Mantaining voltage: 140V- Cathode current: 2mAIn the original PCB their cathodes were driven by some NPN BJTs (BSW69)- Maximum collector-emitter voltage: 150V- Maximum collector current: 50mANow, I have no clue about the original value of the supply voltage because the supply circuit was in another section of the PCB, and I do not have it.But, if the tubes needed at least 170V to ignite, and the BJT support up to 150V, what do you think it used to happend in the circuit? Maybe just a 170V (or more) spike and then the voltage settled to 140V or so?If then, how? Mind it was a 1970ish circuit.Or maybe they just did not care about the well being of the BJTs and forced them to work at 170V or more?Last option, they were supply the nixies with 150V or less hopeing for ignition.I wanted to use this transistor with this tube again but I am afraid to destroy them.Cheers!

author: Gyro
author_rank: Super Contributor
timestamp: January 24, 2024, 05:28:29 pm
content: This is one of those non intuitive ones. You're failing to take account the fact that the tube doesn't illuminate [Edit:(conduct)] unless there is around 90V across it, so when a cathode is off, the tube is not conducting so the transistor doesn't see the 170V anode supply voltage.

author: brucehoult
author_rank: Super Contributor
timestamp: January 24, 2024, 05:37:06 pm
content: Slightly off topic, but have you seen this?

author: ppTRN
author_rank: Regular Contributor
timestamp: January 24, 2024, 05:47:05 pm
content: Quote from: Gyro on January 24, 2024, 05:28:29 pmThat's what I tought. But I tryed supplying the anode with 170V DC through a 82k resistor and measured the voltage from one cathode to GND, and it was 170V.Quote from: brucehoult on January 24, 2024, 05:37:06 pmthank you, it was really informative actually, not for my actual problem but gave me some ideas.

author: Gyro
author_rank: Super Contributor
timestamp: January 24, 2024, 06:07:13 pm
content: Quote from: ppTRN on January 24, 2024, 05:47:05 pmI'm not sure how to explain that, unless you were using a really high impedance bench meter. A 10M input DMM should cause a significant discharge voltage across the tube, even if dark (Townsend) discharge. It might be less than the minimum glow discharge voltage, but would still be 10s of volts. Are you sure of your connections and unloaded anode supply voltage?

author: ppTRN
author_rank: Regular Contributor
timestamp: January 24, 2024, 06:13:02 pm
content: Quote from: Gyro on January 24, 2024, 06:07:13 pmYou know what, I think I am just going to try if it everything works driving the cathodes with this transistors. Worst that can happend is that I destroy them, but if I don't use them for this project they are pretty useless, new transistors does not cost that much.So, cathode with low side driving NPN,should I use 170V or 150V?I would go straight for the 170V.PS. It was a basic DMM 10M input impedence

author: Gyro
author_rank: Super Contributor
timestamp: January 24, 2024, 06:20:16 pm
content: I think you will be fine with 170V. Traditional Nixie tube driver ICs like the SN7441 managed with output transistor off-state voltage ratings as low as 60V.

author: CaptDon
author_rank: Super Contributor
timestamp: January 24, 2024, 07:02:07 pm
content: If your display is non-multiplexed there is a workaround that was used many years ago. On the anode side you still use the normal 18K resistor or some value in that range. you connect the cathodes to your transistor driver collectors in the normal way, BUT from each cathode you also connect a 100K resistor to a zener stabilized 100vdc supply derived from your 170 volt supply. This requires 10 resistors per tube. The circuit is described in the Nixie application notes. It provides a quicker turn-off of the lit digit and insures your collectors never see more than 100 volts. This very circuit can also help eliminate ghosting in multiplexed nixie displays. If it were my choice I would just use 7441, 74141, or the Russian driver chip. Don't use less than 170 volts for the anode supply. Old nixies want around 190 to look good but keep that segment current to about 1.5 milliamps via the proper anode resistor. The difference in nixie life between 2ma and 1.3 to 1.5ma is at least double at the lower current!!