Re: [neonixie-l] VFD grid current. Any ideas why it's so high ?

['John Rehwinkel' via neonixie-l]

> I just received some ILC1-1/8 VFD's (large-ish 7 segment Soviet era, approx 
> 100mm tall) and did some basic testing.
> 
> I was very surprised to see a large current on the grid, on the order of 
> 30-40mA, whereas the segment current was around 5mA when glowing nicely.
> I havn't dabbled with vacuum tubes since my high school electronics class in 
> the 1970's, but I always remember the grid current was very small (of 
> course....if it wasn't small the tube isn't doing it's job as an 
> amplifier....)
> 
> Anyways, for any of you VFD folks out there who have experimented with VFD 
> tubes that have a grid, were you seeing significant current ?
> I havn't found the official datasheet, but the info I gathered does state 
> there is a grid current in the 100mA range at higher voltage (around +35V).
> My own experimenting found that I get good illumination at lower grid 
> voltages (which means lower grid current) around +18V .
> 
> I thought that perhaps the grid + filament is operating like a rectifier 
> tube, where the grid is acting as an anode. Normally, the grid is biased 
> negatively to shut-down the tube-current; with this particular VFD the grid 
> is positive. I have confirmed that positive grid bias is needed in order to 
> get the tube segments to light-up. I also confirmed that around +12V of grid 
> voltage, the current starts to ramp-up.
> 
> I'm reluctant to drive the segments at a higher voltage and leave the grid at 
> a low-enough bias such that it's current is zero; in the end it's probably a 
> zero-sum for reducing power dissipation.

Yup, vacuum tubes (and VFDs, which are essentially triodes) draw grid current 
when the grid goes positive.  Remember the "space charge grid" tubes designed 
to work with 12V on the plate?  The first "space charge" grid would draw a few 
hundred mA to support a dozen or so mA of plate current.  However, wasting huge 
amounts of current is no big deal in a vehicle.

VFDs are the same way, the grids eat up a lot of current.

- John


-- 
You received this message because you are subscribed to the Google Groups 
"neonixie-l" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to neonixie-l+unsubscr...@googlegroups.com.
To post to this group, send an email to neonixie-l@googlegroups.com.
To view this discussion on the web, visit 
https://groups.google.com/d/msgid/neonixie-l/895D26AC-9091-419E-8EDC-56D747F414B7%40mac.com.
For more options, visit https://groups.google.com/d/optout.