VFD tube driving with transistors - Page 1
Forum section: electronics / projects

author: IO390
author_rank: Regular Contributor
timestamp: January 31, 2014, 10:10:59 pm
content: So I'm trying to drive a VFD tube from an Arduino. This particular tube requires 30V for each segment that you want to be lit. Clearly, I can't use the 5V output pins from the Arduino, so a transistor switch would make sense.The problem is, each segment draws about 70uA when lit, which is not much. The transistor switches I have experience with have the load placed between the positive rail and the collector, with the emitter going to ground. The problem is, in this instance I have the transistors connected with the collector to the 30V rail, and the emitter connected to the segments. Problem is, the segments seemingly don't draw enough current to get the transistor to turn on properly. As such, the voltage on the emitter is only about 10V when the base current is applied, so the segments are far too dim. I'm using BC337s.Should I use a pull down resistor to ground after the segments?Hope this makes sense, and thanks in advance.

author: dannyf
author_rank: Super Contributor
timestamp: January 31, 2014, 10:17:21 pm
content: VFD displays typically have their own high voltage generator.In case that you are driving individual tubes directly, you will need to turn on / off the segment and gates on the high side. It is very rare to have any bjt that would NOT turn on / off 70ua.To help yourself better, you should post the schematic so others can provide more specific suggestions.

author: IO390
author_rank: Regular Contributor
timestamp: February 01, 2014, 01:15:30 am
content: Here's a simple schematic.The box represents one of the segments in the tube. Essentially, I need around 30V going to the segment when the 5V o/p of the Arduino is on.I'll add some more detail, in case it helps. The tube is an IV-3A (the tiny one), the heater is 0.85V at 30mA, the grid runs on 30V and each segment lights up when something around 20-30V is applied. I'm using a multiple output power supply for the separate 30V and 0.85V outputs, but of course I connected the ground of both outputs as the only ground connection to the tube is on the heater.

author: dannyf
author_rank: Super Contributor
timestamp: February 01, 2014, 01:20:19 am
content: QuoteThere is no way that's going to work. Switch from the high side - read other vfd schematics to see what that means.

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2014, 07:33:07 pm
content: The VFD will only get 5-0.6V, look up emitter follower circuit.You need to use a PNP transistor and switch the base with an NPN transistor.

author: IO390
author_rank: Regular Contributor
timestamp: February 01, 2014, 08:00:15 pm
content: Quote from: Hero999 on February 01, 2014, 07:33:07 pmThat's exactly what I did. I feel clever now haha

author: shebu18
author_rank: Frequent Contributor
timestamp: February 03, 2014, 05:59:22 am
content: This is another method to drive the tube. You could use a dedicated chip like aHV5812.picture fromhere

author: dannyf
author_rank: Super Contributor
timestamp: February 03, 2014, 11:40:00 am
content: -This is another method to drive the tube-that will not work, for the same reason the first schmetic didn't work. Switch on the collector.