Hi Welcome to part 2 of my mix II Tube Display Driver Project Let's have another little look at just driving these Nixie tubes because there's a couple of issues outstanding Before we actually go into a final white solution that we make into a schematic and a PCB. Now the first issue is: do you actually need a high voltage driver transistor here? Be it within the like, the custom microchip serial array that we looked at or individual driver transistors, or a transistor array like a Uln Two Double-a three. Do you actually need the full compliance voltage of 170 volts or like say, 200 volts nominal to drive these things? Well, that's you know, a reasonable question to ask. and I actually answered this in a separate video on my second Eevblog channel just an hour or two after upload in my previous video.

So those on my who were subscribed to my second channel and I recommend you do, because occasionally I put updates and other miscellaneous videos like that I'll link that in. but rather than just shoot that again, I'll just display that video here for you about the open circuit voltage drop of the Nixie tubes because some people have asked, there will be a voltage drop on the Nixie tubes on the pins that you aren't actually switching on with your transistors. So all the off pins will have so much of a voltage drop on via the Nixie tube itself that you can get away with low voltage transistors. Well, hey, we've got some Nixie tubes.

Let's measure it. I'm pairing the Nixie tube from 170 volts here through the 22 okay resistor nominal that I was going to use and people wanted to know what is the open voltage on the other pins ie. when they're switched off by the driver and can you actually get away with a lower voltage driver, etc. Well, let's actually take one here and let's go around and measure: 49 volts, 24 volts.

Quite some variation. 43 Whoops. A Hundred and Twenty-two Ouch. 118, 69, 40, 40, 723 Odd.

There you go. So there is quite a significant voltage on there. So a hundred and twenty 223 volts there on a couple of pins. That means that just for this particular Miksa tube, there's obviously great variation in this.

I could go measure all eight. But yeah, and you're probably not going to get this from any datasheet. So that's the kind of open circuit voltage you need with standing voltage, the voltage rating of your driver, transistor, er, driver, you know, transistor array, or you whatever you're going to use to actually drive this thing. So yet afraid, you can't get away with relying on the voltage drop of the Nixie tube itself.

So I hope that cleared that up. And by the way, if you're wondering what the voltage is when you strap all the pins together, time all together like this, well you might have guessed. Let's have a look. Bingo! 125 odd volts.

So it basically ties it to the highest voltage I drop or lowest the voltage drop of any one of those particular segments despite don't like using the word segment one those particular digits in there. So as you saw there, we can get over a hundred and twenty volts open circuit float voltage on some of those pins. Yes, some of them were quite low that you could use low voltage output drivers like a UL N to double O three or any other sort of like Jellybean low voltage driver transistor, but hey, some of them aren't So you know we can go measure all the Nixie twos. But hey, we've got a case where the tubes I've got in hand have up to like a hundred and twenty volts and it could even be higher than that.

So obviously we need driver transistors raided to that sort of voltage. But as a few people pointed out, the classic are seven for one for one driver chip, which is now obsolete. Although yes, some people have pointed out that company in Russia still manufactures it or something like that I don't necessarily want to use that I want to use a more modern, readily available solution? Anyway, they point out that that seven for one for one is actually a fairly lowest voltage rating on those open collector output driver transistors in there. And fair enough, it's you know, 50, 60, 70 volts, something like that.

So how can these things work well if you have a look at the internal diagram for this thing. aha, look at the output pins. You can see that there's actually Zener diode clamps on the output. So that's how they're getting away with it.

So what if we actually implemented a Zener diode clamping system on a more modern jelly bean driver that we have available? Well, the classic: you Will N to double three. You can do this and we can actually use these to drive the next C tube displays. although there might be a potential issue which will just verify here. Now, if you're not familiar with the UL N Two Double-a three, it's a an array of seven high voltage driver transistors.

By high voltage I mean 50 volts maximum rating. So obviously on their own, not good enough for driving our Nixie tubes. and inside each one is an open collector driver transistor a high voltage type it. In this case, high voltage means 50 volts 50 volt maximum rating.

Not good enough round Nixie tubes on its own. But if you have a look at the internal structure of these things, then they've also got a common diode array on all seven outputs like this tied together and then the goes to the comp in. So AHA What we can do is we can then hook this up to our Zener diode clamp like that and Bingo! We can clamp all of these pins. So imagine this output here is driving our Nixie tube.

It's going to one of the open pins on our Nixie tube here. Okay, and that's at 120 volts or whatever. this pin is at 120 volts. Then we're gonna.

it's going to be. This diode here is going to be forward biased and then that is going to be clamped to a maximum. You know, let's say we used a in a 48 volt as in Iran There, you know, know anything somewhere lower than our maximum 50 volt rating of our driver transistor. Then obviously this pin here is going to be clamped at 48 volts.

plus the diode drop here. So 48.6 men, whatever. still just call it the clamp voltage and Bob's your uncle. We're protecting that transistor.

But because they're all tied together common like this, all of the outputs are basically going to be tied to, you know, the same maximum our clamp voltage there. But hey, this is a way that we can use the jellybean UL N 2 W 3 Because these things are as common as mud, and a few people have asked what I mean by a jellybean component. Well, a jellybean component is one that's super cheap, super available usually, and I almost by definition available from many different manufacturers. So you know 7 400 Series Logic 4000 Series CMOS Logic.

You know generic. You know 741 Op amps and ones like this. The UL N - Double O 3. It's actually a series that you'll in 2000 series drivers.

There's different versions as A - double, O 4 and whatnot and that they all have different pros and cons is even like a low voltage drive version than this that's specifically designed for you know, 3.3 volt logic input. But but the standard UL N, 2 W 3 or 2 W 3 Ei available from you know, countless different manufacturers for you know, since each or whatever they cost really cheap they can easily accept are 3 volts input, a 3.3 volt logic, or 5-volt logic. Not so great if you're driving hundreds of millions through the if you really want to turn them on hard, turn on this output transistor really hard then you know the input driving voltage can matter. but we're only talking about like a milliamp or two here.

Not a problem so we can easily get away with our you know, CMOS TTL art type compatible 3.3 volt and 5-volt logic input drive on this transistor. The basic difference between the different families in here is usually the bass resistor because they've got a base resistor built-in and it's actually not just one transistor is actually a Darlington pair. so it's actually are two transistors to give you extra gain there. But basically the different families just have different value drop or resistors in there.

But I mentioned a potential issue here and let's just have a quick look at it and do a quick measurement. Now let's assume that we've got our Nixie driver here. we've got our 22 K drop a resistant. got a 120 volt supply up here.

We've got one of the transistors turned on here. so one of the outputs of the UL n 2 W 3 is on. So it's basically our because we're chosen 22 K There it's around about 2 milliamps that we're going to have flowing, but all these other ones are turned off. All these transistors are switched off and we've just got basically these forward biased diodes.

Hopefully depending on the voltage output here, if it's higher than the rated voltage of the Zener then it's going to be forward voltage dip. It's lower as you saw the measurements before. Some of them are then hey, it's not going to. it's going to be reverse bias.

but some of these outputs are going to be up to as we measured before, like 120 yards or something like that. So we're going to actually get current flowing through the 22 K resistor here through. Let's say this is 120 volts here. open circuit voltage.

Then that's going to flow through here, down and be clamped by this 48 volt Zener diode. So how much leakage current do we get total out of all these other pins if we clamp this here? So I'm just going to do a simple measurement here. Don't have a 48 volt Zener to hand, but hey, I'll just use like a 30 volt Zener We'll just work it in and see what we get. so let's give it a ball.

Okay, so what I've got here is I've got a digit turned on digit zero. Whatever. it's a random one 170 volt supply up here I've got my 22 K drop a resistor I measure in the currents about 1.6 million through that 22 K resistor. and what I've done is I shorted all the other pins.

All the other spare ones on the Nixie tube here shorted those out. so I've got that going through a 30 volt Zener here. Reason: I'm shorting all of them together as sort of like a worst-case thing because these diodes because or go into a common terminal could be doing that anyway. So I'm now going to measure the there we go.

One point: 6 milliamps are flowing through the 22 k resistor. I'm going to measure the leakage current through all the other pins shorted together and through with that type 30 volt Zener clamps. So here we go. Bingo, that didn't change.

We're getting in about 0.33 milliamps at 330 micro amps, leakage current shorting all the other pins together into a 30 volt Zener So that's me. It's not a problem and the Nixie tube is still working just fine. It makes absolutely no difference to the brightness whatsoever, so looks like that solution will work a treat. Now the other issue are that I didn't really cover in the previous video about I Looked at some of the microchip, serial driver chips and some of them looked fairly ideal.

except they had totem-pole outputs. and what a totem-pole output is is an output That, instead of just having an open collector like this one ie. the collector pin is just open, it's not connected to anything else inside the chip inside. Here, these are not open collector outputs at what's called a totem pole because they're got ones top and bottom.

They look like an old Indian Indian totem pole. Anyway, something like that, It means that it's got a transistor which actively drives lobe and a transistor which actively are pulls it high as well. So it's often called a push-pull output driver totem pole. Whatever.

Now, there's an in there potential issue here and it can be a major one. and so we'll actually measure this and show why totem pole outputs aren't really suitable. We really need an open collector or open drain output like this. I've just drawn generic fits in there.

Don't worry about that, they can be MOSFETs they can be Bjts, whatever. Now, let's assume that 170 volt supply it 22 K dropper will go down Nixie tube. We've got one of these segments of course, turned on being driven low, but we've got all the other you know, nine outputs here. Actually, you know, just floating, flapping around in the breeze.

Now if we're driving it with one of these microchip drivers that has a totem pole output, it's got a height H V pen on a high voltage active pin like that, so surely you would put you would take that up to your hundred and seventy volts supply. That's naturally we had put it. but aha, will that cause a problem if this output transistor shorting on shorting all these other pins back up to the hundred and seventy volts supply? I Think we might come a gutter. So one experiment with my good Nixie tubes I Remembered that I had some that I think it was Fran was it who sent these into a very early mailbag? I've actually got our three others.

There are basically the same the twelve B type so I'll use one of these. These are looked in D soldered from boards. Obviously, they've still got that. Some of the pads left on there have they.

Oops. Anyway, we'll try one of these because you know we don't want to damage one of our precious Nixie tubes that I'm going to use for my eight display solution. Okay, so what I've got here is the Nixie tube hooked up 170 volt supply 22 K dropper resistor I got one of the segments turned on its segment zero Again, not that it matters. Okay, what I'm going to do now is actually short one of the other outputs here? Well, Nixie tube pins to the 170 volt supply and we're going to measure the current doing that.

So I've got my second current meter hooked up to the positive supply here. so that's the eye on the top of the 22 K resistor there. So right on 170 volts. So let's hook on one of the other pins and I don't think it's going to be Pleasant up night Milliamps, The current for the other one is - through the 22 K resistor.

Well, yeah, that's not very pretty. so let's have a look at the display. what happens to it when we do that? So we've got one point five milliamps at the moment. I'll turn on I'll connect one of the other pins.

and yeah, the zero still lights up, but we're drawing like eight nine milliamps something like that. Oops, So that's of Cour undesirable for the health of our Nixie tube and a reason why we can't use one of these totem pole Apple drivers. But hey, what if we hook the HV pen to the other side of the 22 K resistor like that's? alright. basically only shorting out the pin.

Well, we can try that too. I'll just change that from here to here. All right, let's try that. Hook it up to a random pin and look at that.

it's only a hundred micro 80 micro amps something like that. It's very nice as you'd expect. I'm shorting out the any of the floating Nixie pins to the positive anode up. There is no problems whatsoever, so you could potentially hook that HV pin back up to on the other side of your dropper over here.

But the problem with that is these as you saw on the microchip, Jutta see these are high number of output drivers on the one chip. They like 32 or 64 output drivers and you've got separate dropper resistors for each one of your Nixie tubes like this. so you'd have to dedicate one chip to one Nixie tube like that to be able to tie that individual pin back. I wouldn't like to tie them across multiple Nixie tubes.

you could probably get away with it, but I'd like just no. And of course some of these driver chips also had a built in our current source as well. Down in here, you could actually a bias pin, a bias voltage that you didn't need the dropper resistor up here. and that's another thing which maybe you could potentially use to get away with using a totem pole output driver, but is so it's possible.

But yeah, you've got traps like that. just be careful how you hook it up. But anyway, I don't think I'm going to be using the totem pole output solution. So there you go.

That's just a couple of extra measurements there. I Hope you enjoyed that. So what I'm going to do is I think like I do like the microchip driver solutions. they're really good but some people have not complained.

but they've you know said hey, wouldn't it be nice if you could just use a jelly bean solution that everyone can get in every country, etc etc. Okay, well yeah, all right let's go instead of a discrete transistor solution I Don't like that I think I'll actually implement the jelly bean Uln - Mm with a suitably high voltages enter on the common pin. The only issue with this is that they come in, you know, packs of seven. You get seven drivers like this.

so yeah, it doesn't even drive one Nixie tube so you know you've got to share drivers across multiple mixes and stuff like that. But yeah, that's not really an issue and also strapping the unused pins together like this to a in this case where you know a clamp voltage because we're going to. We've measured like 125 volts on here so we're definitely going to with all the pins shorted together which they do with the diode. So basically we're applying what's called a pre bias to all these pins and some designs do this actually deliberately.

One of the common reasons is that yeah, you can use lower voltage output driver transistors by applying this pre bias clamp, but in this case via our diodes and that's how some designs actually do it. They use discrete diodes as well this pre bias and they actually hook it up to a particular our supply is to prevent some of the segments some of the digits from actually are glowing due to leakage currents and stuff like that. but you know it's not really an issue here. Sometimes like this will go away depending on if you put like a filter on front, like a red filter or whatever orange filter on front of the particular display, but we're not too concerned about that.

I Mean we're really getting into the nitty-gritty details of Nixie tubes and and particular variations between tubes and manufacturers and brands and all that sort of jazz? You know it's yeah. Anyway, this is often called our pre bias as well, and that's kinda sorta what we're doing here. You.