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I am working on a project with Nixie tubes which require a high voltage supply. I have some "indicator dot" tubes that require 90V DC, and I want to turn them on/off using a microcontroller control line.

I am thinking a transistor is the right tool for this, but I am not familiar enough with the different types to know what will work (BJT vs MOSFET, NPN vs PNP, high-side vs low-side switching).

schematic

simulate this circuit – Schematic created using CircuitLab

Please forgive this schematic. It is probably wrong; it is merely indicative of what I want to accomplish.. that is the question I am asking. What is the right way to do this, and what type of transistor should I use? Specific part numbers would also be appreciated.

To be clear, I don't need to amplify any voltages, I already have the voltage supply I need for the 90V line.

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  • \$\begingroup\$ Make sure you use transistors rated for the voltage. You would normally consider using a PNP for switching the high side. But are you planning on multiplexing these? (I wouldn't if it were me.) \$\endgroup\$ – jonk Oct 15 '16 at 19:26
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    \$\begingroup\$ NPN is find as sjhon BUT load MUST be in collector. That way low V switches base and collector switches HV. Transistor must be rated for usefully more than 90 V see www.digikey.com selection guide. MPSA42 was the olde tome favourite at about 300V rating. With load in collector cct invertes - Vin h = on. This depends on your need. igh \$\endgroup\$ – Russell McMahon Oct 15 '16 at 19:35
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NPN is correct transistor type BUT load MUST be in collector.
That way low voltage at Vbe drives the base, and the collector switches HV. The transistor must be rated for usefully more than 90 V.
See www.digikey.com selection guide.
MPSA42 was the olde time favourite at about 300V rating.

With load in the collector the circuit inverts - ie Vin high = on. Whether this suits depends on your need.
This can be altered to Vin high = off with additional components.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Not only that, but nixie tubes have a common anode (the positive electrode / wire mesh) so you cannot switch the cathodes independently (the negative electrodes / digits) with high side switches. \$\endgroup\$ – jms Oct 15 '16 at 21:46
  • \$\begingroup\$ This is perfect! MPSA42 is still available, too. @jms in this case I'm dealing with INS-1 indicator dot tube (one anode, one cathode); I have a separate working solution for the multi-cathode numeric tubes (using 74141 IC). \$\endgroup\$ – Chad E Oct 15 '16 at 21:53
  • \$\begingroup\$ @ChadE The INS-1 is basically just a neon bulb, so you could indeed use a high side switch if you wanted to. If I were you I wouldn't want to, as using a high side switch gets more complicated for no benefit. \$\endgroup\$ – jms Oct 15 '16 at 21:57
  • \$\begingroup\$ @jms hmm, well the overall goal is to control the INS-1 (many of them eventually) with a microcontroller, and this seemed like the simplest way... is there a simpler solution? \$\endgroup\$ – Chad E Oct 15 '16 at 22:33
  • \$\begingroup\$ @ChadE Sorry if I confused you. Russel's answer exhibits a low side switch, and you can't go simpler than that. As the schematic in your question has a (incorrectly driven) high side switch, I reminded that a high side configuration is incompatible with a full-blown nixie tube. You then stated that the driven device is actually just a neon bulb, so I corrected my comment; you actually could use a high side switch in that case (but not with a full-blown nixie tube), however a low side switch (as in this answer) would still be simpler. \$\endgroup\$ – jms Oct 15 '16 at 22:46

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