Driving neon lamps - Page 1
Forum section: electronics / beginners

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 10, 2022, 08:07:09 am
content: HiI'd like to drive four INS-1 neon lamps (https://tubehobby.com/show_det.php?det=9) for my Nixie clock project using a single output of a DIP MCP23017https://www.farnell.com/datasheets/12179.pdfI have this circuit in mind -Should I just parallel up four of those modules or would I be better using a single resistor with the four MPSA42s ?I've previously been advised not to share one transistor between four neon lamps. I'd prefer it if I could though, especially as it's a retro-fit to an existing PCB design. Would this be ok ?The DC supply is 180v.Thanks

author: AndyBeez
author_rank: Frequent Contributor
timestamp: July 10, 2022, 08:22:22 am
content: Use an opto isolator. The high voltage side must *always* be kept isolated from the low voltage side.Google for opto isolator triacs. Vis:http://lednique.com/opto-isolators-2/opto-triacs-zero-cross-work/

author: Colt45
author_rank: Contributor
timestamp: July 10, 2022, 08:37:01 am
content: If you want all four neons to do the same thing (colons on a clock, eg?) just duplicate the 220k + neon, one transistor is fine.It's piddly current, the transistor won't care at all. Are you sure you weren't advised not to use a single 220k with 4 tubes? That is a problem (only one tube will strike).

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 10, 2022, 09:13:27 am
content: Quote from: Colt45 on July 10, 2022, 08:37:01 amI suggested one 150K resistor.Thanks.[ Attachment Invalid Or Does Not Exist ]

author: CaptDon
author_rank: Super Contributor
timestamp: July 11, 2022, 01:07:04 pm
content: Opto triac won't work with a D.C. high voltage supply. It will turn on and stay on. With the 220k limiting resistor there is little chance of any high voltage causing damage to the low voltage driver section. I think NE-2 once lit show around 80 volts across the lamp. If your +HV is around 150vdc minus 80vdc leaves 70vdc across 220K or around .3ma current. if you rig 4 lamps, each with their own limiting resistor and connect all 4 to the single transistor you would only have 1.2ma total current. Certainly in the safe operating area of your MPS A42.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: July 11, 2022, 01:38:47 pm
content: You're right, DC does not do triacs - I assumed the HV was an AC transformer tap, not a DC2DC conversion. Nevertheless, it is good practice to keep the HT away from the side with parts that fingers can touch - even if a collector-base failure mode is one in a zillion. Maybe a 4N25 opto-isolator would do? Thought, what happens to all the neons if one neon starts to flutter, as they can do?

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 11, 2022, 07:12:55 pm
content: Quote from: AndyBeez on July 11, 2022, 01:38:47 pmAs it happens, I've got a bag ofhttps://docs.rs-online.com/3bd8/0900766b80f9a391.pdfHowever, it looks to me like the transistor in there isn't going to be able to switch 180V on it's own.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 11, 2022, 07:16:54 pm
content: Quote from: CaptDon on July 11, 2022, 01:07:04 pmThanks for the reply. I'm going to give that a go.

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2022, 08:21:39 pm
content: If you want to reduce the collector-emitter voltage of the NPN when the bulb is "off", you can add a voltage divider from the positive HV supply, and a diode to the collector from the divider.You can also slightly re-draw the circuit to accomplish the same thing, while still limiting the bulb current in the "on" state.The design requirement is that the voltage across the bulb in the "off" state is below its "lit" voltage (roughly 80 V).

author: AndyBeez
author_rank: Frequent Contributor
timestamp: July 11, 2022, 08:42:39 pm
content: I note this article achieves a similar outcome:"(4) Neon Lamp Player at 150VDC"https://create.arduino.cc/projecthub/whitebank/neon-lamp-player-at-150vdc-54bcae

author: Brumby
author_rank: Unknown
timestamp: July 12, 2022, 12:46:01 am
content: Quote from: netdudeuk on July 10, 2022, 09:13:27 amIn the circuit you have given, one resistor will mean only one tube can light at a time.This is because a neon will require a given voltage across it before it will strike (start conducting) but this voltageimmediatelyfalls to a significantly lower voltage while it is conducting. By sharing a single resistor, once the first tube lights, the voltage will fall to a value whereno other tubes will beableto strike. If you only want one of the tubes to light at a time (if, say, you were multiplexing), then this is fine - but if you want more than one to light at the same time, you will need separate resistors.