H158 Nixie Driver IC

[Leroy Jones]

Just ran across this H158 obscure nixie driver IC.  It has binary inputs and 1-of-10 nixie outputs.   It is from a logic family known as HLL (high level logic).   It operates at a Vcc

of 10.8 to 20 volts.    Ceramic milspec DIP package.   It has the identical pinout

as the standard 5 volt 8T01 driver IC.

Anyone ever hear of these H158 driver ICs or know anything about them?

Same for 8T01.    Just got in some 8T01 ICs to test.   They work very well but they

do have quite high "off" cathode voltages at certain times.   I see an "off" cathode

being anywhere from 50 volts to 110 volts above ground.

According to datasheet it can handle up to 68 volts.   But these were all designed to

operate the tube cathodes directly with no other parts.

100+ volts seems a bit high, but it all seems to work ok.   What is up with that high voltage?

[gregebert]

Both of these drivers use open-collector outputs, so the output voltage you measure is determined by the anode supply voltage, nixie-tube voltage drop, and the voltage-drop across the anode resistor (due to leakage of the driver IC). It's indirectly affected by the IC. 

I suspect the "high" output voltage you see is caused by lower leakage of the driver IC. You can confirm this by measuring the leakage current directly with a current meter, but it's much easier to measure the voltage across the anode resistor and calculate the current. When the driver IC has lower leakage current, the off-state (output high) voltage will be higher versus devices with higher leakage current.

One characteristic of NPN transistors that is not commonly known is that running the collector-emitter junction higher than spec does not destroy the device like it does with MOSFETs (bipolar transistors do not have vulnerable oxide layers). However, like a zener diode, you have to limit the current at higher voltage otherwise the junction will overheat and that will destroy the device.

Nice find. I vaguely recall hearing about the 8T01 many years ago, but not the H158.

[Leroy Jones]

Greg,  Thanks again for those nuggets of wisdom!   I have noticed that about NPN BJTs.   In particular, the output side of a 4N35 optocoupler is rated at 35 volts max.

However, I have been very successfully using 4N35 optocouplers in a 48 volt system for 20+ years with no issues nor failures.   I have noticed that when current is kept extremely

low or practically non-existent, that the specified maximum terminal voltage can be exceeded significantly with no problems.   I never knew exactly why.  Now I know why!  Thank you!