Electronics > Projects, Designs, and Technical Stuff
Need help understanding VFD Filament
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dom0:

--- Quote from: janoc on September 15, 2019, 12:31:58 pm ---How do you do that with an H-bridge that isn't floating with regards to the rest of the drive circuit?

--- End quote ---

It has been a couple years since I've done this, but IIRC the ~1.5 V drop on the low side of the filament driver was sufficient to avoid ghosting. (These were relatively low voltage VFDs witha nominal anode voltage of I believe around 24 V).
Ian.M:

--- Quote from: dom0 on September 15, 2019, 01:16:21 pm ---
--- Quote from: janoc on September 15, 2019, 12:31:58 pm ---How do you do that with an H-bridge that isn't floating with regards to the rest of the drive circuit?

--- End quote ---
It has been a couple years since I've done this, but IIRC the ~1.5 V drop on the low side of the filament driver was sufficient to avoid ghosting. (These were relatively low voltage VFDs witha nominal anode voltage of I believe around 24 V).

--- End quote ---
Alternatively, capacitively couple the H-bridge to the ends of the filament, and provide a DC path for the cathode current via a pair of clamping diodes from the filament ends to your logic supply, or toa pre-biassed (via a pullup to HT) Zener to provide the desired cathode bias voltage.   You can also use a resistor divider across the filament to get a neutral point, but as I mentioned above that's a trade-off between wasting power and 1's being noticeably brighter than 8's.
janoc:

--- Quote from: Ian.M on September 15, 2019, 03:52:10 pm ---
--- Quote from: dom0 on September 15, 2019, 01:16:21 pm ---
--- Quote from: janoc on September 15, 2019, 12:31:58 pm ---How do you do that with an H-bridge that isn't floating with regards to the rest of the drive circuit?

--- End quote ---
It has been a couple years since I've done this, but IIRC the ~1.5 V drop on the low side of the filament driver was sufficient to avoid ghosting. (These were relatively low voltage VFDs witha nominal anode voltage of I believe around 24 V).

--- End quote ---
Alternatively, capacitively couple the H-bridge to the ends of the filament, and provide a DC path for the cathode current via a pair of clamping diodes from the filament ends to your logic supply, or toa pre-biassed (via a pullup to HT) Zener to provide the desired cathode bias voltage.   You can also use a resistor divider across the filament to get a neutral point, but as I mentioned above that's a trade-off between wasting power and 1's being noticeably brighter than 8's.

--- End quote ---

All that is a lot more complicated (how big would need those caps to be to pass the heating current of the filament?) than a small transformer that you can wind in a few minutes yourself. Or even buy, these are fairly standard parts.
Ian.M:
Yes. Way up thread I said "TLDR: Not having a filament supply secondary center tap is a P.I.T.A.".   Assuming you dont have a negative rail available to let you take inactive anodes and grids right down to a good cutoff voltage, you'd have to have a dammed good reason to try to do the filament supply without magnetics.
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