Confusion about nixie tube 74141 outputs

[rea5245]

Hi,

I'm building a nixie clock. I've built the circuit except for the nixie tubes themselves, and I want to test it before soldering in the relatively expensive tubes.

So I feed a BCD 1 into the 74141 chip and look at the outputs on a 'scope. Some of them show a steady 1.5V while others show 60Hz noise (so they're high impedance maybe?)

What puzzles me is that the pin for driving the 1 digit is not unique. I.e. it's showing 1.5V but so are a couple of other outputs. I could understand if all the other outputs were high impedance and just the 1 digit were at 1.5V (because maybe it's tied to ground but there's some leakage current going through). But why are a couple of the other outputs showing 1.5V?

Is this just a case of "everything will sort itself out once the circuit is completed with the nixie tubes and the pins aren't floating"? Or is there a real problem here?

Thanks,
   Bob

[fourtytwo42]

The 74141 outputs are open collector meaning they need a pullup current source to make any sense, you could temporarily connect a resistor, say 10K from each output to vcc so your scope correctly displays the output state.

[james_s]

The 74141 has open collector outputs, so there will be no voltage on those pins without something external to pull them up.

[rea5245]

But if the outputs are open collector, why would I be seeing 1.5V on some of them?

[james_s]

Likely the protection diodes, leakage current, I assume you're measuring with a high impedance multimeter? You'll often get erroneous readings.

Connect a nixie tube or use a pullup resistor and it will work. You cannot accurately test an open collector part with nothing connected.

[rea5245]

OK. Thank you.

- Bob

[David Hess]

Counterintuitively, a 1.5 volt output in this case is the off state.  Inspection of the 74141 output stage reveals that when off, half of the outputs will produce a TTL high minus one Vbe as an output voltage which is indeed about 1.5 volts.  The Vbe is from the forward biased base-collector junction of the cascode output transistor which will become reverse biased and high impedance once the voltage is raised by the load of 60 volts maximum.