mini h-bridge for driving VFD filament very warm to the touch

[tiltit]

Hi all,

I have been having fun with a small calculator-like VFD display, I thought I should do things right and put together a small h-bridge the drive the filament. It works but gets in my opinion overly warm. To me transistor datasheets are still very hard to read and I am not quite sure how to choose a base resistor to simply use the transistor as a switch.
It would be greatly appreciated if someone could look and the schematic and let me know how to improve the circuit.

Note that on the schematic below +5V is in fact +3.3V, and so is the input.

Thanks all.

[Andy Watson]

You will need to drive the voltage at the base(s) to within 0.6 of the emitter(s) in order to turn the transistor off. At present both transistors in the same leg are ON simultaneously. Note that the base-emitter junction behaves like a diode - when it is biased on, it will only allow the forward voltage drop of a diode (0.6-0.7V) to occur between base and emitter. The transistors in the left-hand leg will never turn off. Only the lower, NPN transistor in the right-hand leg will turn off.

[tiltit]

Now I am really confused 
I am currently probing R6, on the right side I do see my signal but on the left side I see a constant 2.5 Volts. But the circuit as a hole seems to be working. I am lost as to what is happening.

[Andy Watson]

Consider Q2, what is the lowest voltage you expect to see on its base?
Then consider an input voltage of 0V. The resistor from the base of Q2 (560R) and the input resistor (470R) form a potential divider. It should be apparent why Q3 will never turn off.

Only Q5 can be turned off - this is probably the reason that the heater appears to be working.

[tiltit]

I think I am starting to get it.

With an input voltage of 0V the voltage between R4 and R1 should be:
(3.3 - 0.6) / 1030 * 470
Witch equals to ~1.2V

Am I correct?

So the solution would be to decrease the value of R4.

[tiltit]

Or I can I tie the bases of Q2 and Q3 together?

[amyk]

VFD filaments should be warm...

Remember that BJTs will pass current through the base, so you'll need to take into account these currents when you're designing them to switch on and off. This is very different from the behaviour of MOSFETs.

So the solution would be to decrease the value of R4.

No, that would just increase the voltage on the base of Q3. Get rid of R1 to fix that side of the bridge - that way you can pull the base of Q3 all the way to 0V.

Also this is one of those instances where running a simulation can give you a good idea of whether your circuit behaves as you'd expect it to.

[tiltit]

Ok, so I will remove R1 and R3 and replace Q1 with a pnp transistor. Maybe that will work. I will also try to find some sofware to do simulations. Hopefully something that easy to use and works on Linux.

As a side note does anyone know of a sot-23 jellybean logic level Mosfet?

[tiltit]

So I have revisited the schematic, which I am going to put together tonight. Does anyone see anything obviously wrong with it?

[tom66]

Shouldn't the emitters be connected to a common point on the H-bridge? Been a while since I've used this, but it looks like a class AB amplifier driven with a square wave.

Better to convert the PNP inverter to an NPN type (more common) or draw it so the Vcc is at the top of the device, at least...

If you have the available Vcc, it would be easier to make an ac-coupled half bridge or two-capacitor mid supply half bridge design.

[Zero999]

It still has the same problem as before. All the transistors will turn on simultaneously.

You could try swapping the NPNs and PNPs so they're emitter followers but then you lose 1.2V to 2V, depending on the current draw.

[tiltit]

So I will try and emitter follower configuration. I can see now why they would be all on. I think that I keep forgetting about the current flowing from the emitter to the base.

I have also tried before to use ac-coupling to drive the filament using two electrolytic capacitors back to back. But I got such a voltage drop that the digits on the display where barely visible. The filament should draw about 100mA, If I measure it's resistance with my multimeter I read ~50 Ohm.

[T3sl4co1l]

When you do that, you MUST have base-emitter resistors, and you must have the pull-up (R1 and R3 in the first schematic) much smaller than the series base resistors drawing from it.  Or, you need some sort of rail-to-rail buffer or logic gate to supply 0V and +5V to the driving node (the node between R4-R5, and the node between R6-R7).

Here's another way to do it,



Assuming you need full wave operation, connect your input to A and use an inverter to drive B.  Typical values might be Rb = 100 ohms, Re = 220 ohms (for 5V CMOS logic levels at A, B) and supply anywhere from 5V to 30V (one advantage of this drive method is, it is independent of supply voltage).

Tim

[Zero999]

One way of doing this is to use a complementary common base buffer to switch the transistors.

Note if you do use this for a motor, you'll need to add in some protection diodes but they're not needed for a resistive filament.

Does it actually need to be AC? It's just a heater so perhaps it'll work on DC.

[tiltit]

Thanks,  I think I will try out those schematics, they are not as simple as I would of hoped, but then I wouldn't learn anything.
The heater does work on DC but there is a noticeable gradient in the luminosity of the digits.
My end goal with this project is to have a small and cute SMD only USB powered desk clock that also has HID-Raw interface for setting the clock and displaying information like the temperature outside. I hope to learn more about circuit design with this project and that is why I do not want to take shortcuts like powering the filament with DC and call it a day.

[Zero999]

I've had another idea. Since the frequency isn't important. It's possible to use an astable multivibrator to drive it. Two additional transistors can be added to the typical astable circuit to form a self oscillating h-bridge.
http://en.wikipedia.org/wiki/Multivibrator#Operation

[tiltit]

Thanks, I just tried that last circuit on a breadboard and it works rather well.
I replaced the 1uf caps for 0.1uf ones and the frequency of the circuit is ~6KHz, I hope that the heater doesn't mind. I also don't know how well the circuit will work with the bc847 and bc857.
The good thing about this circuit is that I wont have to worry about toggling a GPIO pin anymore and that it only has 4 transistors. It just works.

[c4757p]

Does it actually need to be AC? It's just a heater so perhaps it'll work on DC.

The filament also acts as the cathode - DC on the heater causes a voltage gradient across it, which results in a quite noticeable brightness gradient.

[Zero999]

Thanks, I just tried that last circuit on a breadboard and it works rather well.
I replaced the 1uf caps for 0.1uf ones and the frequency of the circuit is ~6KHz, I hope that the heater doesn't mind. I also don't know how well the circuit will work with the bc847 and bc857.
The good thing about this circuit is that I wont have to worry about toggling a GPIO pin anymore and that it only has 4 transistors. It just works.

I'm glad it works. R3 and R4 also control the frequency but they need to be low enough to provide sufficient base drive for Q1 and Q2. The Wikipedia article I linked to in my previous post provides more information on the calculations, if you're interested.

The heater should work at any frequency, within reason and 6kHz should be fine.

What transistors did you use for your breadboard prototype? The problem with the bc847 and bc857 is they're only rated for 100mA and it's not a good idea to run components continuously at their maximum ratings. The gain of a transistor also falls significantly as the current rises and is very low at the maximum rated current so the voltage drop could be high and it may not oscillate reliably.

How about the BC817 and BC807 which are SMT parts and electrically the same as the BC337 and BC327?

[tiltit]

I used BC327-16 and BC337-16 just like in the schematic, they are my goto transistors for breadboarding, so I had a few of them in my junk box.
I also added two 100Ohm resistors in parallel from the 3.3V rail to the emitters of the two PNP transistors. That is to drop the voltage a little since the filament requires only 2.4 volts.
I am still looking for another simple and efficient way to bring the voltage down a little but for the time being resistors seem to be the simplest solution.

[Zero999]

Oh, I didn't realise you wanted 2.4V, why not just connect a resistor in series with the heater.

[Telek]

Terribly sorry for the necropost, but just wanted to thank Zero999 for this ingenious idea. I found a collection of 15 starburst 20x2 VFDs in storage and have been trying to get them to work. I'm using HV518 for the segments (perfect as it provides 32 outputs) + HV5812 for the grid columns (perfect as it provides 20 outputs). Only thing I was struggling with was how to properly drive the filament since there is noticeable fade on DC.

Some preliminary tests seem to indicate that this works perfect! I'll add some pics in another post. I don't have 1.5kOhm resistors, so I used 1k+440 and a 1uF 400V cap (it's what I had), all other components as you described. I am getting about 600Hz under load with 4.68VDCin and 4.40VAC (cyc RMS) out and 9.72V pk-pk. Filament draw is 71mA AC. According to the datasheet, typical at 5.6Vac 60Hz RMS is 73mA, so that tells me that I have it dialled in nice.

The waveform has a little inconsistent frequency, but it seems to work! At 5.0VDC in the frequency is noticeably more stable than 4.65VDC in.

But question - the datasheet for these VFDs says that the ideal filament voltage is 5.6Vac RMS 50-60Hz, with 6.2Vac max. I know DC pretty well, and I know that overall power from AC to DC is about sqrt(2), but I don't know this situation enough to know what voltage I can apply. Since this is pretty close to a square wave, which means from a power perspective it's a lot closer to DC calculations, I've tweaked it to 6.2/sqrt(2) or 4.4VAC. With the supply at 4.68V that's within range of my 5V logic (4.5 .. 5.5) so that way I don't need another PSU, but I'd really prefer to use 5.0V for the logic. However, that brings it up to 4.77Vac - is that going to be too high? At that voltage I'm detecting about 76mA (under the maximum of 80mA that the datasheet says), so that should be ok?

Running the calculations on the mentioned wikipedia article for frequency, I get 527Hz expected, and ~600Hz observed. I assume the resistances are going to be slightly lower in-circuit due to the second set of transistors which, while on, will have significantly less resistance than the R1+R5 combo.

If I wanted to bring down the frequency a little, could I just increase the capacitors to maybe 4.7uF or even 10uF? That should bring me down closer to 60 or 100Hz without changing R3/R4.

In terms of productionizing this - what do we think of MMBT4403 - BJT 600mA 40V PNP SC-70-3? Specs look very similar (5V VEBO -0.75V CESV).

[Telek]

Here you can see the VFDs as I originally started to play, then the breadboard with the HVs and this H-bridge, with the VDCin at 4.69V and VACout at 4.45 RMS, and you can see the inconsistent frequency. That stabilizes quite a bit if VDCin is 5.0V.

[Zero999]

The RMS AC voltage and current are the same as the peak for a squarewave. The output voltage will be a little lower than the supply voltage, due to the voltage drop across the transistors.

None of the resistor values are critical. You can use a lower, or higher frequency if you like. I doubt it makes much difference for a VFD.

The circuit isn't perfect. It has one flaw: both the upper and lower transistors simultaneous turn on, when it changes from one state to the next, drawing high current spike from the power supply. The h_FE of the transistors limits them to some degree.

[Telek]

Good to know about the spikes - thank you! Should we add a capacitor or inductor to the input then to try to mitigate this?

So does this mean that if the datasheet says 5.6Vac typical, that I can do 5.6V output with this circuit without worry? Is paying attention to the current draw the more critical factor?

You mentioned that we can't really increase the resistor values that much because it needs a certain amount to drive the base, which is why I was wondering about the capacitors. I'll try it as is and see how it works out.

What do you think about surface mount 600mA BJT PNP transistors (for example, MMBT4403)? Anything special that I should look out for? I'm trying to minimize footprint on the board so would prefer to stick away from through hole transistors.

Thanks again for all your help!!

[Zero999]

Good to know about the spikes - thank you! Should we add a capacitor or inductor to the input then to try to mitigate this?

So does this mean that if the datasheet says 5.6Vac typical, that I can do 5.6V output with this circuit without worry? Is paying attention to the current draw the more critical factor?

You mentioned that we can't really increase the resistor values that much because it needs a certain amount to drive the base, which is why I was wondering about the capacitors. I'll try it as is and see how it works out.

What do you think about surface mount 600mA BJT PNP transistors (for example, MMBT4403)? Anything special that I should look out for? I'm trying to minimize footprint on the board so would prefer to stick away from through hole transistors.

Thanks again for all your help!!

An inductor in series with the positive connection to emitters of Q3 & Q4 (try 10µH) will limit the current spikes. Current limiting can also be added to reduce the surges.

The transistors aren't critical, as long as they can pass the required current.

I don't know much about driving VFDs, as it's not something I've done before, but it's just a filament, so the frequency won't be important. I designed this circuit to make a loud, low voltage buzzer.

The resistor values can probably be increased a bit, with no problems.

[Ian.M]

OTOH if your MCU can generate PWM synchronous to your multiplexing frequency it might well be a better choice (and probably more compact) to use a MOSFET H-bridge IC driven by 50% duty cycle PWM from your MCU at the multiplexing frequency or a multiple of it.  You can fine-trim the filament RMS voltage by adding deadtime to the PWM.

Must Read: Noritake's A Guide to Fundamental VFD Operation, or PDF here

Another compact approach to VFD filament drive is to use a dedicated VFD filament driver. Unfortunately the LM9022 is NLA, but a substitute is readily available https://www.eevblog.com/forum/projects/vfd-filament-power-using-lm4871-replacement-for-obsolete-lm9022/

[Zero999]

The BD6211F-E2 is an example of an H-bridge IC which will work at 5V.
http://www.farnell.com/datasheets/2096555.pdf?_ga=2.128000716.1303208652.1596274850-1107606318.1553553996&_gac=1.247072304.1596274987.EAIaIQobChMI-6Sjg9b56gIVWe3tCh13gg4pEAYYByABEgKThPD_BwE

Regarding current limiting for the astable: add an emitter resistor to Q3 & Q4 and clamp the base voltages, with diodes.

[Telek]

Thank you both Zero999 and Ian.M for your input! This is very useful.

It looks like this is way more complicated than I was hoping. I was hoping to stay simple but I want the most longevity and least ghosting, so what do you recommend? I'd prefer to stay with DC in, but I guess I don't have to go transformerless. Ideally 5VDC in as the HV518/HV5812 chips that I'm already using are 5V logic.

Is Rolo's universal VFD PSU with the LM4871 from 7-5-2018 the best bet then? I see there was still some discussion even after that.

Having said that, the SN6501 looks very simple - would that be the best way to go? If so, any hints about the transformer? This is well outside of my wheelhouse. 75mA and 5.6Vac RMS 50-60Hz output is the recommended for the VFD based on the datasheet.

[Zero999]

Thank you both Zero999 and Ian.M for your input! This is very useful.

It looks like this is way more complicated than I was hoping. I was hoping to stay simple but I want the most longevity and least ghosting, so what do you recommend? I'd prefer to stay with DC in, but I guess I don't have to go transformerless. Ideally 5VDC in as the HV518/HV5812 chips that I'm already using are 5V logic.

Is Rolo's universal VFD PSU with the LM4871 from 7-5-2018 the best bet then? I see there was still some discussion even after that.

Having said that, the SN6501 looks very simple - would that be the best way to go? If so, any hints about the transformer? This is well outside of my wheelhouse. 75mA and 5.6Vac RMS 50-60Hz output is the recommended for the VFD based on the datasheet.

The SN6501 is a good idea and works at 380kHz. Don't worry about the frequency. It's just a heater and will work from any frequency.

If you want something very simple, here's a cut down version of my circuit. Although not good practise, it will work without base-emitter resistors, so they can go and the resistor values increased to reduce the surges. It will still need a good decoupling capacitor though.

[Ian.M]

I'm concerned you are designing yourself into a corner here. 

From Noritake's guide: "...  reduction of filament voltage or anode/grid voltage is not recommended because this may cause uneven illumination."  Your filament specs: 5.6V RMS @ 75mA.

However you are looking at BJT H-bridges, which are going to drop 0.5V or even more (to get 2x Vce_sat under 0.5V would require much larger transistors than you are considering and a lot more base drive), so starting from a nominal 5.0V Vcc rail, at best you'll get 4.5V RMS to the filament, underrunning it by 20%.   Cranking up the HT in an attempt to compensate for low emission is *NOT* recommended, as that accelerates ageing of both the phosphor and the filament coating by ion bombardment.

Even with a MOSFET H-bridge powered from your nominal 5V Vcc, the filament voltage will be marginal unless you run your Vcc rail a bit on the high side.

If you only have 5V in from a 'USB' power supply, you've *really* boxed yourself into a corner.  The only ways out are a boost converter to power the filament driver, or to wind a custom step-up filament drive transformer for a Royer oscillator filament driver.  However custom magnetics are expensive and difficult to prototype even with a suitable pot-core development kit and considerable coil-winding experience.

[Zero999]

I'm concerned you are designing yourself into a corner here. 

From Noritake's guide: "...  reduction of filament voltage or anode/grid voltage is not recommended because this may cause uneven illumination."  Your filament specs: 5.6V RMS @ 75mA.

However you are looking at BJT H-bridges, which are going to drop 0.5V or even more (to get 2x Vce_sat under 0.5V would require much larger transistors than you are considering and a lot more base drive), so starting from a nominal 5.0V Vcc rail, at best you'll get 4.5V RMS to the filament, underrunning it by 20%.   Cranking up the HT in an attempt to compensate for low emission is *NOT* recommended, as that accelerates ageing of both the phosphor and the filament coating by ion bombardment.

Even with a MOSFET H-bridge powered from your nominal 5V Vcc, the filament voltage will be marginal unless you run your Vcc rail a bit on the high side.

If you only have 5V in from a 'USB' power supply, you've *really* boxed yourself into a corner.  The only ways out are a boost converter to power the filament driver, or to wind a custom step-up filament drive transformer for a Royer oscillator filament driver.  However custom magnetics are expensive and difficult to prototype even with a suitable pot-core development kit and considerable coil-winding experience.

Well he said:

Some preliminary tests seem to indicate that this works perfect! I'll add some pics in another post. I don't have 1.5kOhm resistors, so I used 1k+440 and a 1uF 400V cap (it's what I had), all other components as you described. I am getting about 600Hz under load with 4.68VDCin and 4.40VAC (cyc RMS) out and 9.72V pk-pk. Filament draw is 71mA AC. According to the datasheet, typical at 5.6Vac 60Hz RMS is 73mA, so that tells me that I have it dialled in nice.

So there must be a fair tolerance on the figure of 5.6VAC, which is given as a typical figure and 71mA is only slightly lower than the  data sheet.

If the voltage is too low, a Royer converter or a transformer driver IC are a good ideas, but there's the added expense of a transformer.

[Telek]

I'm concerned you are designing yourself into a corner here. 

From Noritake's guide: "...  reduction of filament voltage or anode/grid voltage is not recommended because this may cause uneven illumination."  Your filament specs: 5.6V RMS @ 75mA.

However you are looking at BJT H-bridges, which are going to drop 0.5V or even more (to get 2x Vce_sat under 0.5V would require much larger transistors than you are considering and a lot more base drive), so starting from a nominal 5.0V Vcc rail, at best you'll get 4.5V RMS to the filament, underrunning it by 20%.   Cranking up the HT in an attempt to compensate for low emission is *NOT* recommended, as that accelerates ageing of both the phosphor and the filament coating by ion bombardment.

Even with a MOSFET H-bridge powered from your nominal 5V Vcc, the filament voltage will be marginal unless you run your Vcc rail a bit on the high side.

If you only have 5V in from a 'USB' power supply, you've *really* boxed yourself into a corner.  The only ways out are a boost converter to power the filament driver, or to wind a custom step-up filament drive transformer for a Royer oscillator filament driver.  However custom magnetics are expensive and difficult to prototype even with a suitable pot-core development kit and considerable coil-winding experience.

You guys definitely know more than me So I'm happy to change the design based on your feedback. I want to do this right, but with a preference towards simple. If we can do it simpler and still right, that's where I want to go, but if we need to keep it less simple then I'm ok with that.

As Zero999 pointed out, and with my understanding, the current should be the critical factor overall. So I think that 5V in with his circuit should be ok, but I'm happy to change if that's the consensus. Also note that I did measure 5.01VDC in and 4.77Vac out with 71mA.

I know that powering off 5V USB is limiting, so if that's not really feasible then we can switch it up. However, another thought - since I have to boost to ~30V anyway for the segments, I wonder if I can still use the 5VDC in, then from the 30VDC segment use a 1:5 step-down transformer from there could work with something like the SN6501 to give me simple and relatively off-the-shelf without needing custom drivers / transformers? The boost converters that I already have are 3A, and I don't need anywhere near that so there is room there.

The datasheet calls for 35Vp-p (max 42) for the grid/anode but again they're expecting AC. If I provide DC, I'm thinking somewhere around ~30VDC is the correct spot. But I think there's some flexibility there, so that way I can stick with a 1:5 transformer and still tweak it to get the correct Vac out.

[Ian.M]

I notice you haven't provided a link to the VFD datasheet, nor have you stated whether or not your MCU is capable of generating a 50% PWM at the desired multiplexing frequency or an integer multiple of it.

What does the filament look like if you power it from an adjustable DC source and slowly crank the voltage up until you either reach its nominal voltage or nominal current (5.6V or 75mA, if I'm not mistaken)?  DO *NOT* continue increasing the voltage if it starts glowing brighter than a dull red. It shouldn't be bright enough to be noticeable in normal diffuse domestic room lighting.  If its glowing noticeably red before you reach its nominal ratings, you may well be able to get away with a lower filament voltage without issues. 

42V is less than 10x your 5V supply, so a simple single inductor boost converter should be adequate for the HT supply - no transformer required. I'll defer further comments on the filament supply till I've seen the VFD datasheet, and you've seen how bright the filament gets as you approach its nominal voltage and current.

This article on the effect of filament voltage on transmitter tube operating life is rather interesting: https://www.w8ji.com/filament_voltage_life.htm

[Telek]

I notice you haven't provided a link to the VFD datasheet, nor have you stated whether or not your MCU is capable of generating a 50% PWM at the desired multiplexing frequency or an integer multiple of it.

I was (quite surprisingly) sent the datasheet directly from Noritake, I have asked if I'm allowed to share it publicly and they are apparently discussing it, but haven't given me a yes or a no yet.  I'm using an Arduino right now, and will be using an ESP8266 later, so yes to the second part of your question. Let me type up the critical bits in a spreadsheet and I'll share it.

What does the filament look like if you power it from an adjustable DC source and slowly crank the voltage up until you either reach its nominal voltage or nominal current (5.6V or 75mA, if I'm not mistaken)?  DO *NOT* continue increasing the voltage if it starts glowing brighter than a dull red. It shouldn't be bright enough to be noticeable in normal diffuse domestic room lighting.  If its glowing noticeably red before you reach its nominal ratings, you may well be able to get away with a lower filament voltage without issues. 

I am currently using an adjustable DC regulator from 12VDC in, and have done exactly that previously. At 5.6VDC even in a fairly dark room I cannot see the filaments at all. Around 6.5VDC I can see a faint red glow. At 7.4VDC I can definitely see a clear red glow (two photos attached).  This was done a few weeks ago, and I have 15 of these so I wasn't too caring if I burnt one out. I didn't check the currents at those limits though.

At 5.11VDC source right now (tuned to 5.01V with no load) and 4.84VDC measured across the filament legs, I'm seeing it settle to 68.7mA after a few seconds. At this level, the leftmost (brightest) segments on the display are quite bright even in a bright room, with a clear drop-off to the right. I'm sharing the 5VDC with logic chips, so I don't want to bump it up too much past this right now, but I don't think that I really need it any brighter.

I've attached a photo at 5.11VDC source with the fade. It's difficult to really capture it. I wrote a small app that goes through all 96 characters in my font, moving one column at a time, but "building up" the columns from the right.

As per your question, I slowly dialed it up. At 5.61VDC source and 5.44V measured across the filament legs, I have 73mA which is exactly what the spec sheet says should be nominal. I don't want to go above that right now unless I separate out the filament voltage.

I haven't done any brightness testing yet below 5V though. Should I?

42V is less than 10x your 5V supply, so a simple single inductor boost converter should be adequate for the HT supply - no transformer required. I'll defer further comments on the filament supply till I've seen the VFD datasheet, and you've seen how bright the filament gets as you approach its nominal voltage and current.

This article on the effect of filament voltage on transmitter tube operating life is rather interesting: https://www.w8ji.com/filament_voltage_life.htm

I am currently using a cheap ebay/Amazon generic step-up regulator https://www.amazon.ca/dp/B07JNQFV7F/ and it seems to be working well at 30VDC from 12VDC in.

Thank you very much for that article, I will read it, and thank you very much as well for helping me out here! I really appreciate it!

[Telek]

Here's the relevant section of the datasheet

[Zero999]

Here's the relevant section of the datasheet

According to that, 4.8V is below the minimum, so is marginal design. How many are you going to build? If just is just a one off, then it's clearly working, so stick with it. On the other hand, if you're mass producing it, then you should use a higher voltage, as it might not work for all displays. You could add a buck converter and h-bridge, after the boost converter, but it's messy.

Another possibility is to use a transformer for the booster, with another winding for the filament supply.

[Telek]

Here's the relevant section of the datasheet

According to that, 4.8V is below the minimum, so is marginal design. How many are you going to build? If just is just a one off, then it's clearly working, so stick with it. On the other hand, if you're mass producing it, then you should use a higher voltage, as it might not work for all displays. You could add a buck converter and h-bridge, after the boost converter, but it's messy.

Another possibility is to use a transformer for the booster, with another winding for the filament supply.

I have about a dozen that I'll put together for various personal use and gifts. Since my goal is to drive with the ESP8266 I want to make an "alarm clock" type of thing with some buttons on it, and have it auto connect to some wifi points. Then I can control/send them messages remotely. Give some to friends as well.

What if I just used a step-down from the 30V anode voltage regulator? That way I don't have to worry about Vin. Not the most efficient, but easy to do.

Note that I have NOT tried this yet with your filament circuit, I'm just straight DC driving it at the moment. I wanted to get my code working first before trying to play with the filament drive (otherwise I wouldn't have anything to compare it to). I'm still tweaking the code.

I should mention, now that I actually have the code a bit better and not using just a flickering test, the brightness is a lot lower than I would expect. I'm trying to tweak the settings to not get flicker, so maybe I'll figure out how to get it brighter. Note that the photo makes it look worse in terms of glow. In reality it's very clean right now, no ghosting or background illumination... just dimmer than I was expecting.

Edit - ok, this is odd. So I used the Vusb for logic to separate out the filament. Running from 3.0 VDC to 6.5 VDC for filament seems to have no impact on brightness. Maybe I need a higher anode voltage? Bringing that up to 35VDC certainly did make it brighter, but still not as bright as I expected based on 100% duty (all segments and columns on). 25VDC was quite a bit dimmer.

[Telek]

Update, the original astable self oscillating h-bridge is working well. This is now well into the most complicated breadboard system I've put together



Measured, 5.03VDC in under load, 4.87VAC Cyc RMS shows on oscilloscope, with 10.2Vp-p at ~505Hz +/- 10. Hovers right around 75mA under full operation (+/- 0.5mA). The waveform stabilized (less jittery) under full operational load.



I have not added inductors yet, and I don't have any 2Ohm resistors to work with (although I probably have some 1/4W 1 ohm ones, I guess I could tie 2 together to test). I do have some 1812 SMD 22uH though, should I try one each of those in series with the positive emitters to help? Or would you still prefer one of the two later designs? I'm happy with this one so far, but I don't know what it means for the longevity of the VFD.

I had an error in my original code due to misunderstanding some awkward wording in the datasheets. Now, at 30VDC grid, and no awkward delays to avoid the bad ghosting I had before, with no blanking, the brightness is quite acceptable and the background is quite clean. I don't know if I need to do any blanking. There's the tiniest amount of ghosting from the previous column, but I have to look hard to find it.

Noritake did finally get back to me, they have asked me not to share the datasheet (the previous snip was what I retyped myself), but they are interested in the results of my tinkering. If anyone is interested they welcome to ask them on Twitter for the datasheet directly and they will provide.

Oddly, with my oscilloscope attached and the computer USB attached, something strange happens. The waveform gets destroyed and almost entirely negative, and an odd buzzing comes from the 5V switching regulator. The brightness also increases ever so slightly. This only happens if the USB cable is connected, or even if only the USB ground is connected to circuit ground  .



If I disconnect the oscilloscope ground, the perfect waveform returns at only half strength (2.5VAC but the display brightness doesn't change). It's the same as if I probe either side of the bridge to ground, instead of across both sides of the bridge.

I also found that one of my DS3231 boards appears to be a cheap knockoff and defective (not that I expected real ones when buying them on Amazon for $2ea) and my Arduino nano internal oscillator seems to be off by enough to be 0.14sec slow every minute... I'm starting to think that maybe I should separate this out into its own thread 

[Zero999]

Which circuit did you build?

Regarding the oscilloscope: it sound like a ground loop issue.

[Telek]

Which circuit did you build?

Regarding the oscilloscope: it sound like a ground loop issue.

This is the one that I built.

Yeah, I figured some sort of ground loop. I tried to add a few decoupling caps but no dice. I have a UPS so just ran it off there for a few minutes to check that it was ok lol.

[Telek]

If I do decide to go the IC route - what do we think of something as simple as TC4428 (high-speed 1.25 dual mosfet, one inverting). Run it off PWM from the MCU and drive the filament directly from the outputs. If I'm reading all this correctly, it's like a simpler BD6211 without the protection diodes needed for a motor, and the extra control pins. I could just PWM to both IN1 and IN2 from the same pin, since it's one inverting.

With the astable route, is it possible to tweak one side of it to get it so that the negative side is half the voltage and twice the time? As per the Noritake site:



I assume change R4 on the negative side of the capacitor?

[Zero999]

If I do decide to go the IC route - what do we think of something as simple as TC4428 (high-speed 1.25 dual mosfet, one inverting). Run it off PWM from the MCU and drive the filament directly from the outputs. If I'm reading all this correctly, it's like a simpler BD6211 without the protection diodes needed for a motor, and the extra control pins. I could just PWM to both IN1 and IN2 from the same pin, since it's one inverting.

No, because the TC4428 has far too higher output resistance, of around 7 Ohm on each output, so the voltage drop will be too high.

With the astable route, is it possible to tweak one side of it to get it so that the negative side is half the voltage and twice the time? As per the Noritake site:

(Attachment Link)

I assume change R4 on the negative side of the capacitor?

No, getting asymetrical output voltages would need a much more complicated circuit. I don't know why you'd want to drive the filament, with that waveform.

[Ian.M]

The text next to that figure states "However, a 1/2 duty factor should be set  ..."

The *ONLY* reason to use an asymmetrical pulse drive waveform for the filament, is that when AC coupled, it lets you reduce the RMS voltage below that a 50% duty cycle waveform would give, without loosing efficiency.   

LTspice sim of 5% to 95% duty cycle AC coupled PWM with RMS measurement attached.  Right click the error log that pops up and Plot the .step'ed .meas data, for Vrms vs duty cycle.

The disadvantage of asymmetric drive waveforms and waveforms with higher crest factor, is you need a higher HT for the same brightness as you need to increase the filament bias voltage to prevent ghosting on positive going filament supply peaks.

[Telek]

If I do decide to go the IC route - what do we think of something as simple as TC4428 (high-speed 1.25 dual mosfet, one inverting). Run it off PWM from the MCU and drive the filament directly from the outputs. If I'm reading all this correctly, it's like a simpler BD6211 without the protection diodes needed for a motor, and the extra control pins. I could just PWM to both IN1 and IN2 from the same pin, since it's one inverting.

No, because the TC4428 has far too higher output resistance, of around 7 Ohm on each output, so the voltage drop will be too high.

With the astable route, is it possible to tweak one side of it to get it so that the negative side is half the voltage and twice the time? As per the Noritake site:

I assume change R4 on the negative side of the capacitor?

No, getting asymetrical output voltages would need a much more complicated circuit. I don't know why you'd want to drive the filament, with that waveform.

  just going based off the manufacturer's website https://www.noritake-elec.com/technology/general-technical-information/vfd-operation

Ok, so based on how this circuit is working well, do I just add the inductors in-line with the emitters of Q3/Q4?



EDIT: Hmm. Here's what the waveform looks like with no inductors:



and with two inductors (I found some nice 0.57uH 1.5ohm through-hole inductors in my parts bin) from the +5V rail to emitter.



The voltage drop I expected due to the resistance, which pulls it a bit lower than I'd like. But is that expected for the waveform? Is that waveform a problem? I noticed that I can get inductors with much less resistance - should I try for maybe 33uH 0.1ohm?

Interesting thing to note - all this runs perfectly fine off USB power right now, with expected brightness. Is this maybe something that if it's working, don't worry about it?

I don't know why this editor just doesn't let me get those attachments working inline.

[Telek]

The text next to that figure states "However, a 1/2 duty factor should be set  ..."

The *ONLY* reason to use an asymmetrical pulse drive waveform for the filament, is that when AC coupled, it lets you reduce the RMS voltage below that a 50% duty cycle waveform would give, without loosing efficiency.   

LTspice sim of 5% to 95% duty cycle AC coupled PWM with RMS measurement attached.  Right click the error log that pops up and Plot the .step'ed .meas data, for Vrms vs duty cycle.

The disadvantage of asymmetric drive waveforms and waveforms with higher crest factor, is you need a higher HT for the same brightness as you need to increase the filament bias voltage to prevent ghosting on positive going filament supply peaks.

Neat, thanks! I figured it had something to do with not burning out the filament or something. If a 50% duty cycle works, let's not complicate it!

[Zero999]

If I do decide to go the IC route - what do we think of something as simple as TC4428 (high-speed 1.25 dual mosfet, one inverting). Run it off PWM from the MCU and drive the filament directly from the outputs. If I'm reading all this correctly, it's like a simpler BD6211 without the protection diodes needed for a motor, and the extra control pins. I could just PWM to both IN1 and IN2 from the same pin, since it's one inverting.

No, because the TC4428 has far too higher output resistance, of around 7 Ohm on each output, so the voltage drop will be too high.

With the astable route, is it possible to tweak one side of it to get it so that the negative side is half the voltage and twice the time? As per the Noritake site:

I assume change R4 on the negative side of the capacitor?

No, getting asymetrical output voltages would need a much more complicated circuit. I don't know why you'd want to drive the filament, with that waveform.

  just going based off the manufacturer's website https://www.noritake-elec.com/technology/general-technical-information/vfd-operation

Ok, so based on how this circuit is working well, do I just add the inductors in-line with the emitters of Q3/Q4?



EDIT: Hmm. Here's what the waveform looks like with no inductors:

[ Specified attachment is not available ]

and with two inductors (I found some nice 0.57uH 1.5ohm through-hole inductors in my parts bin) from the +5V rail to emitter.

[ Specified attachment is not available ]

The voltage drop I expected due to the resistance, which pulls it a bit lower than I'd like. But is that expected for the waveform? Is that waveform a problem? I noticed that I can get inductors with much less resistance - should I try for maybe 33uH 0.1ohm?

Interesting thing to note - all this runs perfectly fine off USB power right now, with expected brightness. Is this maybe something that if it's working, don't worry about it?

I don't know why this editor just doesn't let me get those attachments working inline.

There have been a few problems with attachments on the forum recently, so Dave has disabled inline attachments. It doesn't matter, as I can see all of the images you've attached, as expandable thumbnails. If you still want inline images, click on the image to expand it, copy the URL to the clipboard, go back to the post and past it into [img][/img] tags.

If it works, then leave it be. Don't worry about the waveform.

The inductor was just to limit the current spike. Only one might be needed, but unfortunately you won't know whether it's required at this stage or not, until you build the entire circuit. My advice is to leave room for it on the PCB, so it can be added, if the MCU is unstable, or replaced with a wire link, if it works. You could also design the PCB for the  current limiting design, with extra pads for the inductor.

[Telek]

If it works, then leave it be. Don't worry about the waveform.

Great! It occurs to me now that the waveform is exactly what we'd expect with two inductors, one per transistor. If I use only one shared inductor then the waveform should be normal as current is constantly flowing.

The inductor was just to limit the current spike. Only one might be needed, but unfortunately you won't know whether it's required at this stage or not, until you build the entire circuit. My advice is to leave room for it on the PCB, so it can be added, if the MCU is unstable, or replaced with a wire link, if it works.

That's a great idea and exactly what I'll do. Does it make sense to look for an inductor with a very low DC resistance so that I don't get the voltage drop? My understanding is that the current limiting capabilities are based on the EMF buildup, so the pure DC resistance shouldn't impact that? So like 33uH and 0.15ohm? Or 10uH and 0.17ohm? Or 47uH 0.17ohm? Since it's pretty good on its own, I assume any of these will be fine. Fun fact: USB from my laptop works fine, but from a 7-port external powered USB hub it has an overcurrent situation. I assume the spike when it plugs in is just a bit too much for the hub to handle. I'm thinking this one: https://www.mouser.ca/ProductDetail/Bourns/RLB0712-330KL

You could also design the PCB for the  current limiting design, with extra pads for the inductor.

What's the purpose of the diodes in that one? I know you mentioned that it clamps the base voltages, but I'll have to admit that I don't know what that means.

[Zero999]

Yes, a low ESR inductor is a good idea.

Regarding the diodes:
The base voltage of a PNP transistor needs to be about 0.6V below the emitter voltage, in order for it to turn on. As the current rises, the voltage across R5 also increases, pushing the emitter voltage on Q3 & Q4 down, with respect to the supply voltage. When the current gets too high, the diodes clamp the base voltages, cutting off the current flow.

I hope my explanation is good enough. If not, I'll post more tomorrow. It's getting late.

[Nifty]

I've had another idea. Since the frequency isn't important. It's possible to use an astable multivibrator to drive it. Two additional transistors can be added to the typical astable circuit to form a self oscillating h-bridge.
http://en.wikipedia.org/wiki/Multivibrator#Operation

Hi! Dear Sir, In the schematics of the cassette deck that I am currently restoring (TEAC V-770), I saw a very similar solution.
Unfortunately, I am not well in analog circuitry.
Can you please help me to understand, how can I slightly increase the voltage at the output of this generator.


https://ibb.co/x1rXDND

Thank you!

[Zero999]

I've had another idea. Since the frequency isn't important. It's possible to use an astable multivibrator to drive it. Two additional transistors can be added to the typical astable circuit to form a self oscillating h-bridge.
http://en.wikipedia.org/wiki/Multivibrator#Operation

Hi! Dear Sir, In the schematics of the cassette deck that I am currently restoring (TEAC V-770), I saw a very similar solution.
Unfortunately, I am not well in analog circuitry.
Can you please help me to understand, how can I slightly increase the voltage at the output of this generator.


https://ibb.co/x1rXDND

Thank you!

That circuit works on a different principle to mine. U16 is the oscillator and Q7 to Q9 from an H-bridge which drives the VFD filament. In my circuit the transistors also form the oscillator.

Why do you want to increase the voltage?

R80 and R85 are effectively in series with the VFD and drop some voltage. Reducing their resistance should increase the output voltage, with the VFD connected.

[Nifty]

That circuit works on a different principle to mine. U16 is the oscillator and Q7 to Q9 from an H-bridge which drives the VFD filament. In my circuit the transistors also form the oscillator.

Why do you want to increase the voltage?

R80 and R85 are effectively in series with the VFD and drop some voltage. Reducing their resistance should increase the output voltage, with the VFD connected.

Thank you! I want to try to increase the VFD brightness a little. It's dim, still usable, but the extra brightness will definitely add to the comfort.

[T3sl4co1l]

Possibly the 47's can be reduced a little, or a turn or two removed from the primary winding (that drives a transformer, right?), but that most likely overdrives the heater, leading to even shorter life.  VFD fade is usually due to phosphor wear, unavoidable.  (Come to think of it, I wonder what temperature it could be annealed at. Probably nothing you could do without an inert-atmosphere oven though, or melting the glass envelope for that matter.)  If it's due to fading cathode emission, raising cathode temperature will work momentarily.  Increased plate voltage may not be possible, due to other limits (driver voltage rating, grid cutoff voltage shift).

Tim

[floobydust]

Dim display I find it can be oxide contamination on the filament giving low emission, so a bake off say careful slow build up to max 2x V_FIL for ~20 seconds helps a lot.
Second, I recently found the control grid is really a screen grid and something builds up there. It makes segments/icons appear dim and washed out at the edges. I did a burn off there of something that went away after a few seconds and it never came back. I have notes on it somewhere.
I find phosphor wear is not common unless they drive the tube very hard, high voltage like 40-60V. The giveaway with phosphor is not all digits/segments are dim - some (rarely used) will light up bright like new, so you get a display with bright and dim.

edit: I would check the +20V and +30V rails, see if the electrolytic capacitors are OK or there's hum/ripple there, as well.

[schmitt trigger]

Before the current Russia-Ukraine war, I was able to procure some Soviet era VFDs.

I went through the same iterations as the OP has, but settled with a fully integrated H bridge, the LB1638MC from Onsemi.

[Zero999]

A bit if background:

Nifty, who resurrected this thread, originally sent me a private message, asking me about this and referred to a circuit I designed. Since I generally don't answer questions via private message and I had no idea which circuit he was taking about (it turns out it was nearly 10 years ago!), I politely asked him to resurrect the thread in question.

Now I know it's different to my circuit, but there was no way of knowing before.

That circuit works on a different principle to mine. U16 is the oscillator and Q7 to Q9 from an H-bridge which drives the VFD filament. In my circuit the transistors also form the oscillator.

Why do you want to increase the voltage?

R80 and R85 are effectively in series with the VFD and drop some voltage. Reducing their resistance should increase the output voltage, with the VFD connected.

Thank you! I want to try to increase the VFD brightness a little. It's dim, still usable, but the extra brightness will definitely add to the comfort.

How old is the VFD? Sorry, I suspect it's just old age and will eventually need to be replaced.

[Nifty]

Possibly the 47's can be reduced a little, or a turn or two removed from the primary winding (that drives a transformer, right?), but that most likely overdrives the heater, leading to even shorter life.  VFD fade is usually due to phosphor wear, unavoidable.  (Come to think of it, I wonder what temperature it could be annealed at. Probably nothing you could do without an inert-atmosphere oven though, or melting the glass envelope for that matter.)  If it's due to fading cathode emission, raising cathode temperature will work momentarily.  Increased plate voltage may not be possible, due to other limits (driver voltage rating, grid cutoff voltage shift).
Tim

No, there is no transformer, the outputs in the schematic diagram are directly connected to the VFD.
I measured the voltage, it's about 5V RMS, 400Hz.

[Nifty]

Dim display I find it can be oxide contamination on the filament giving low emission, so a bake off say careful slow build up to max 2x V_FIL for ~20 seconds helps a lot.
Second, I recently found the control grid is really a screen grid and something builds up there. It makes segments/icons appear dim and washed out at the edges. I did a burn off there of something that went away after a few seconds and it never came back. I have notes on it somewhere.
I find phosphor wear is not common unless they drive the tube very hard, high voltage like 40-60V. The giveaway with phosphor is not all digits/segments are dim - some (rarely used) will light up bright like new, so you get a display with bright and dim.
I would check the +20V and +30V rails, see if the electrolytic capacitors are OK or there's hum/ripple there, as well.

Thanks for the info. I'm still afraid to experiment with VFD, because it still works and I have no replacement. But today I found an inexpensive donor of spare parts, if the display there is in better condition, I’ll try to revive this one.
Voltages seem ok. All capactors is "Nippon Chemicon" branded, and according to the RLC meter, comply with the specs. The voltage on the VFD filament is approx. 5V RMS 400Hz.

[Nifty]

How old is the VFD? Sorry, I suspect it's just old age and will eventually need to be replaced.

I believe this particular device is from 1988.