Electronics > Projects, Designs, and Technical Stuff
Need help understanding VFD Filament
romhunter:
I'm trying to work with some IV-3A VFD Tubes from Russia. In the datasheet, the filament is specified to be driven with 0.85V (?). So I just google around for a solution to use this with a 12V power supply, and come accross those ideas:
1. Drive it with DC since it's a short tube: 0.85V regulation with 6 filaments as load, it's a scary thing to do. I'm thinking about a LM317, set the output to its minimum, and drop the output voltage using a few diodes. I might get close if the temperature of the diode does not fluctuate too much.
2. Drive it with AC voltage: I don't quite understand it right here, so I might need some help to clear my head. Anything will be of use. I'll be refer to the Noritake VFD guide:
The output of this "DC-AC Converter" is a square wave? If that's true, can I drive the filament using a micro and a few opamps? (with an output BJT).
3. What's the filament bias voltage? On the Noritake website, it's said to be applied to the center-tap of the transformer. So how can I do it using the forementioned method (MCU and opamps).
janoc:
I have used this driver for the filament of a salvaged VFD:
http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/
Worked a treat.
The filament voltage/current varies, you will need to find the correct one for your VFDs in the datasheet. Normally the filament wires should only barely glow, anything more than that will seriously degrade their lifetime.
romhunter:
--- Quote from: janoc on September 12, 2019, 07:58:16 pm ---I have used this driver for the filament of a salvaged VFD:
http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/
Worked a treat.
The filament voltage/current varies, you will need to find the correct one for your VFDs in the datasheet. Normally the filament wires should only barely glow, anything more than that will seriously degrade their lifetime.
--- End quote ---
As far as I understand, the transformer in this circuit "worked like" a low pass filter, therefore filter the square wave and output a near sine wave on the output. I'm trying not to use a center-tapped transformer in the design (lower space constraint, lower BOM), but it might serve the purpose. I just don't understand the needs to bias the center-tap to some voltage.
Bud:
It is done to prevent inactive segments from glowing.
Zero999:
AC is required to keep the segments at the same brightness. It doesn't need to be a sine wave. A square wave will work perfectly.
A basic inverter can be made using a transformer with a centre tapped primary and a couple of transistors. Obviously higher value resistors (try 2k2) and smaller transistors, than the 2N3055 can be used e.g. BC338, PN2222A.
EDIT:
The filament bias voltage can be generated by connecting one side of the transformer secondary to a potential divider. The bias current is tiny, so fairly high value resistors can be used.
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