Modification of MM5314N for High Voltage Nixie Tubes

[Dkuriloff]

OK

I found a circuit which can be modified to drive the 7 segment Nixie tubes (B7971) using the M5314N Clock IC but need help with mods.

Here is the article and schematic:
https://www.yousendit.com/download/S1VBT2pLU1BveE1LSkE9PQ

Here is the data sheet for the MM5314N:

http://www.alldatasheet.com/datasheet-pdf/pdf/117108/NSC/MM5315N.html?

1. Power Supply Mod:

Modify the power supply to the chip using a 6.3 - 0 - 6.3 V AC out center tap transformer, a full wave bridge rectifier, 1500µf, 35V filter cap and a 7812 regulator. This will give a regulated 12 v DC supply for Vss. The center tap is left unconnected. This is better than the 9v, unregulated supply in the schematic and may fall shy of the 11v minimum required for the IC. Can a 22 µF, 400v cap be used to filter the mains for the HV side of the circuit instead of the 15µF, 150V as in the schematic, since this is what I have on hand? Can a smaller 1 watt bleeder resistor be used say 2.2k instead of 220K in the schematic? Why was such a high value chosen in the first place?


2. Transistor Substitution Mod:

MPSA42 (NPN) will replace of the UL624. MPSA92 (PNP) will replace the SA480. Adjustment of the multiplex frequency may be necessary. For the MPSA92s, Q8- Q13, there is a pull-up resistor from the transistor bases to “ground” in the schematic. A question was raised by Alan 'A.J.' Franzman: “I've just realized another possible mistake in the schematic, but I'm not quite sure if it's wrong or not. You may need to build a temporary breadboard version of the circuit (or at least most of it) to find out. Each of transistors Q8-Q13 is shown with a 2.2M ohm resistor going to 0 V DC. I believe these may need to go to the "+150 V DC" high voltage line instead, but because the resistors are of such high value and the transistor base inputs are capacitively coupled, the circuit may work either way.”

Does anyone have an opinion and can connecting the transistor base to HV damage the IC? Should I put series diodes with the IC outputs to prevent stray voltages?

3. Mutiplex Timing adjustment on pin 23:

3000 µF is appears to be wrong, based on Figure 3 from the MM5314N.
datasheet (it's WAY off the chart!) If the author needed to use that value to get the chip to work, it was probably because of the chip not functioning correctly due to supply voltage being too low. Either .01 µF with the recommended 220K ohm resistor from 0 V to pin 23, or if I use the 100K ohm resistor as shown in the schematic, will increase the capacitor to .022 µF. It seems from the article that the values were based on needing a higher frequency to allow pulses from the IC to get through series capacitors connected to the digit enable outputs. Can anyone shed more light on this issue? Since .5 µF is no longer a common capacitor value, I will substitute .47 or higher 250 v nonpolarized mylar caps.

4. Fixing “Inter-digit Ghosting”

All of the above was devised to prevent flickering and display ghosting. I noticed that when the same IC was used for Numitron tubes that every segment of the Nixie tube needed a diode.

See http://www.tuberadios.com/numitron

Perhaps the inter-digit blanking circuit in figures 9 and 10 of the MM5314 data sheet should be used? The example given was for the 28 pin version of the clock chip MM5309 and shows the circuit connected to pins 26 and 28. Can I assume that this same circuit will work if connected to pins 23 and 1 respectively?

Any help much appreciated!

Dan

Direct email is kuriloff at NYHNI dot org.

 

[Mitchekj]

Here's what little I can say at the moment with any small degree of certainty. I may have to read up on this. Looks like a nifty little project.

Can a 22 µF, 400v cap be used to filter the mains for the HV side of the circuit instead of the 15µF, 150V as in the schematic, since this is what I have on hand? Can a smaller 1 watt bleeder resistor be used say 2.2k instead of 220K in the schematic? Why was such a high value chosen in the first place?

For the cap, that sounds good. 150V is way underrated for a mains level voltage anyway, 250V or higher is what I'd reccomend. 22uF should work out just fine. 47uF may do as well. For the bleeder resistor (you're referring to the one going from the 150V line to ground, yeah?) that is a definite no. 2.2k would dissipate about 10W over it, releasing the magic smoke. It's such a high value in order to do basically nothing while the circuit is powered and running (current through it is in the uA,) but it will drain the cap when the power is removed. It's a safety feature, so that cap doesn't hold on to potentially lethal voltage, for one.

Does anyone have an opinion and can connecting the transistor base to HV damage the IC? Should I put series diodes with the IC outputs to prevent stray voltages?

Edit: I need to read more about what's going on. This is how I'd have done it: (see attachment) But I don't know if they need the nixies connected to ground, or what the story there is.

Either .01 µF with the recommended 220K ohm resistor from 0 V to pin 23, or if I use the 100K ohm resistor as shown in the schematic, will increase the capacitor to .022 µF. It seems from the article that the values were based on needing a higher frequency to allow pulses from the IC to get through series capacitors connected to the digit enable outputs. Can anyone shed more light on this issue?

I'm not sure. I'd say go with the datasheet, and adjust as needed.

 

[Sid723]

I have been following your post and I have some observations and questions:

Observation: In the article schematic, Q8 through Q13 should be PNP transistors, but are drawn as NPN. I can see past this error, but I assume the emitter is still where it should be.

Question: If Q8 through Q13 is as I believe it to be (as stated in my observation), then the emitters will have ~ + 150 volts, while the base will have close to 0 volts. This will forward bias each transistor with no signal coming from the IC, which means that all these transistors are "ON" and conducting current. This doesn't seem right to me. It would seem more correct if the 2.2M resistors going from base to 0 volts were going from the base to the +150volt source instead. This way, the PNP transistors would be in an off state until a signal comes from the IC to turn each transistor on. Make sense?

 

[Mitchekj]

Ah, and now things make much more sense. I found a schematic very similar to what you're trying to do, though they're using low-voltage tubes instead of the Nixies. See attached.

The thing that worries me about the MPSA92 is the Vbe rating of 5V... if I've got this straight, wouldn't the Nixie version have ~138-150V across the base-emitter? That doesn't sound fun.

In any case, I'd agree w/ Sid that the pull-downs should be pull-ups.

 

[Sid723]

The thing that worries me about the MPSA92 is the Vbe rating of 5V... if I've got this straight, wouldn't the Nixie version have ~138-150V across the base-emitter? That doesn't sound fun.

I agree with Mitchekj.

Also, on Mitchekj's diagram, I am sure that the voltage input to the LM317 is about + 160 Vdc. According to the spec sheet for the LM317, the output is: Vo = 1.25 (1+ R2/R1) + Iadj * R2

According to my calculations, it comes to an output of about +58 Vdc. So, these tubes (and circuit) are very similar to Dan's circuit, just a lower voltage tube. I would go by Mitchekj's circuit for clarity.

 

[Mitchekj]

The schematic I posted has a 12Vac input... I just liked how they showed all of the tubes' connections. It helped me understand what was going on. (I still don't get why they have Vdd and Vss seemingly reversed on the datasheet.)

Looks like he's using the LM317 to adjust the brightness of those tubes (w/ a pot,) which is a pretty nifty idea.

 

[Sid723]

Oops!! My bad!

I didn't even see the 12VAC there. I just assumed it was 120 VAC just like the article's circuit. Anyway, there are many similarities in both.