74141's and blanking

[Jonathan]

Hi all,

I'm making my first clock that has to be set by the user instead of self setting. And the leading digit needsl to be blanked, as the woman it is for doesn't want to see the zero.

Anyway, I am using the blanking feature of the 74141 for the first time. It seems that some of them don't blank as well as others and to get them all to blank I have to dial the anode voltage down or lower tje pwm duty cycle which prevents max brightness during the day. One of the 74141s blanks well and so does a Russian equivalent that i have. So, am i doing something wrong and is there anything I can do about it? I suppose I can cherry pick through my stash for chips that blank well.

Thanks!

Jonathan

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Some people are like a slinky - not really good for anything but you still can't help but smile when you shove them down the stairs.
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[David Forbes]

On 6/26/2012 2:25 PM, Jonathan wrote:
> Hi all,

>
> Anyway, I am using the blanking feature of the 74141 for the first time. It seems that some of them don't blank as well as others and to get them all to blank I have to dial the anode voltage down or lower tje pwm duty cycle which prevents max brightness during the day. One of the 74141s blanks well and so does a Russian equivalent that i have. So, am i doing something wrong and is there anything I can do about it? I suppose I can cherry pick through my stash for chips that blank well.
>
> Thanks!
>
> Jonathan

Jonathan,

The 74141 contains a ~55V Zener diode on each output. It isn't really intended
to perform blanking, despite what you might be led to believe from the truth table.

The Russian chips are said to have a higher voltage Zener diode than the
American parts, so go with those. Even so, you will need to reduce the HV to the
best compromise.

--
David Forbes, Tucson, AZ

[threeneurons]

Its the 10 hours position ? Not the 10s minutes or 10s second ? Blanking those would just look weird. If so, that digit only light 0 & 1, in 12-hour mode, or 0, 1, & 2 in 24-hour mode. Why then use the 74141 at all ? If you do, then a quick fix would be to put a resistor (2 or 3, depending on 12/24 hr mode) across the anode, to the connected cathodes. Leave all other cathodes (3-9) disconnected. Calculate it so the drop across the anode-cathode is less than 120V. Say the anode resistor is 10K, the supply is 180V, and the zener break-over is 55V. 180V-120V-55V=5V. That means calculate the bleed cuurent so 5V is dropped across the anode resistor. If its 10K, the bleed is 5V/10K=0.5mA (500uA). This "bleed" is the "off" current. 120V/500uA=240K. So stick 220Ks across anode to each used cathode. That's ANODE, NOT Supply !

If its 12-hour mode try this scheme:

http://threeneurons.files.wordpress.com/2011/08/hour_blk.gif

Found on this page:

http://threeneurons.wordpress.com/nixie-power-supply/

[Jonathan Peakall]

Well, the tens of hours position is one issue. Alas and alack, the board is already made and even mounted in the case. I could do some add on action, but not much room. However, the Russian 74141 seems to be blanking that one OK. On future prpojects I will redesign the board. Great link, loved your site.

 

The real problem is blinking the nixies while the user is setting the time. I am blinking the pair of tubes being set, ie if setting the hours the two hours nixie blinks to show the user which tubes they are setting. And reccomends as to good ways to get a 74141 to blank properly?

 

However, since I am using a RTC that is supposed to lose less than a minute a year and the clock is programmed to let you increment/decrement during operation the blink won't be used often. Especially as with the back up coin battery and super cap, it will keep the time for years even if turned off. So maybe I shouldn't worry about it too much.

 

Thanks!

 

Jonathan

[John Rehwinkel]

> Well, the tens of hours position is one issue. Alas and alack, the board is already made and even mounted in the case. I could do some add on action, but not much room.

I'd probably grab a couple of MPSA42 transistors and resistors and mount them on a DIP header, to build a limited function pin-compatible nixie driver that can support
high voltage blanking.

- John

[Adam Jacobs]

This is a great idea.

-Adam