Electronics > Projects, Designs, and Technical Stuff

Vacuum Fluorescent Display Driver

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I have enough multimeters, so I'll monitor both, but, since I'm at it, I'll use a third to keep an eye on filament current.

Ironically, I actually performed the measurements opposite of your suggestion. I took a perf board, soldered twelve 20k resistors on it, and then soldered a 20k across each individual one to get the 10k (I had a large amount of 20k resistors so I used them). After I put a clip on a SEG line (looking at my spreadsheet, you can probably tell the order I went - left to right on both rows), and touched each G line resistor with a clip lead.

I tried thinking of the most efficient method to make it go faster and to reduce the amount of times the VFD gets touched. This way I was only touching the VFD twenty times not including connecting all the G lines initially.

On a side note, I realized I had 82ohm resistors for the filament and not 64ohm like I originally stated, and I had five in parallel making it (as it was measured) 16.1ohms. To eliminate the chances of a resistor disconnecting and causing inrush current issues, I also soldered the resistor leads.

Also, I checked earlier, the decade boxes (that's the name I'm familiar with, but they are also called substitute boxes and resistor boxes) are 1/2W resistors - thought I had the -2W version originally. Either way, 1/2W should be more than enough. At one time I had to replace some of the resistors (they were open the day I acquired them), so even if they go up in smoke, I should have some spares.

Unfortunately I didn't expect the need to perform more measurements, so I disconnected everything. Earlier I took some time to gather everything, but most likely I can't commit myself to testing until late this week or the weekend.

Edit: did you (or anyone) look at the scope capture of the +38V taking >12s to drop? Seems odd it should take that long, almost like the zener is open.

Thinking ahead (as I did previously), I'm wondering if it's worth laying out a PCB. Since the HV518 is fed by six voltage/data lines, maybe if the VFD is found to be fine (i.e. no shorts), rather than risk damaging the PCB by soldering more HV518s, a breakout board with an IC socket would be safer.

I can jumper the six lines to the breakout board and either power the filament separately, or jumper those two lines too. Also, I could use pin sockets (whatever they are called) to fit the VFD in rather than solder. The issue would be the cost for a PCB, and, each time an IC blows, it's $8 down the drain.

For the current tests you don't really need to solder the VFD leads; just need to touch Gn and SEG_m with resistors on crock clips. But VFD leads are robust and can take re-soldering many times.  Just make sure the leads don't move (ie mounted on a perfboard) and don't fatigue at the glass interface. 

See attached for a SPI-VFD driver I made a couple of days ago (2*74HC595+2*ULN2003+14*10K pullups, 2.8Vrms add-on secondary filament winding+16VDC "air wired" power supply). Note the VFD taped to a foam block and strip that constrains lead movement. You can do similarly using a 0.1" thru-hole 44-PLCC socket, mounting the VFD on perfboard, and temporarily soldering leads for the SPI+DCpower+FILpower from J1.  Ultimately, it would be nice if you can manage to mount machined sockets for the VFD pins and a low-profile PLCC socket on the display board.

Yeah, I saw that it took >12s to drop.  But that was without any VFD attached so no load but the zener and a 31K resistor.
As the +5V input to DC-DC dies down, the filament is still warm and the last grid and segments that where on should still draw a few mA which should discharge the 470uF cap faster than 12s. But how fast? I dunno.


--- Quote ---Yeah, I saw that it took >12s to drop.  But that was without any VFD attached so no load but the zener and a 31K resistor.
--- End quote ---

That same slow discharge rate I'm almost certain I also saw with the VFD installed and the new-new VFD Driver IC.

After installing the second new IC, I clipped onto the +38V and Vcc on two separate channels (this is why I had and still have fly leads soldered to both points). Not thinking about needing to measure discharge rate, I just looked at the voltage (probably at a fast rate) and thought the slow discharging +38V was odd after powering off the unit.

Afterwards I turned on power again and saw the +38V was now approx. +12V along with nothing on the VFD.

So don't quote me, but I'm 99% sure that slow discharge was also with the VFD installed. Also note a previous scope capture where it shows the slow discharge rate but with the blown HV518; it starts at +12V because the chip was blown, but shows the VFD didn't help the discharge rate because it was still slow.

Wanted to let anyone following this that I haven't had a chance to perform follow up measurements.

I think Thursday night I began setting up, but I haven't had time for anything more. Possibly I'll have time tomorrow.

Earlier I performed tests with 10k resistors; this time noting the current on the G lines.

Although noting all the measurements wasn't asked, thought it would make things easier to have all of them. Entering all the G line currents did get a bit tedious and decided to take the SEG measurements from my previous test due to them being very close.

Also note that all the filament currents are the same in my spreadsheet. They varied very little and I gave up taking notes for each measurement. I noticed it would start at maybe 117mA while switching G lines (i.e. nothing lit) and then drop to about 116.5mA when the G line was connected (i.e. a segment was lit).

I know and understand the trick is to sneak up on 25mA, but is it worth trying to jump from 10k to 100ohms (or 100R as it seems to be used on here) to save steps or is it too risky?


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