What does "pre-bias" mean in this circuit?

throbscottle

Well-Known Member

• Aug 30, 2019

• #1

https://g3ynh.info/digrdout/nixie_psu.html

If you look at the nixie driver at the bottom of the page, there is a +50v "pre-bias" connection. So it looks to me like it just protects the transistors by stopping their drains going to over 50v, and I suppose the voltage is high enough that the tube won't conduct. But is there some other reason they are calling it "pre-bias"? I'd never come across the term before.

As always, thanks in advance

 

ronsimpson

Well-Known Member

Most Helpful Member

• Aug 30, 2019

• #2

I think there is a chance that a open transistor could be pulled up to the 180V supply. (not likely but 180 volts is the supply voltage)
It is known that 50V will not light the tube. So you could place a 50V Zener across the transistor to protect it and not light the display.
Pre-bias is a way to use low voltage transistors. You pull down on the tube from 180V to 50V so the transistor has a much small voltage to switch.

 

Last edited: Aug 30, 2019

unclejed613

Well-Known Member

Most Helpful Member

• Aug 30, 2019

• #3

it limits the voltage on the jfet drains according to the text.

 

rjenkinsgb

Well-Known Member

Most Helpful Member

• Aug 31, 2019

• #4

unclejed613 said:

it limits the voltage on the jfet drains according to the text.

Click to expand...

Exactly. I'd call it a clamp voltage rather than bias voltage.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Aug 31, 2019

• #5

rjenkinsgb said:

Exactly. I'd call it a clamp voltage rather than bias voltage.

Click to expand...

I would agree, it's simply utilising the way nixie tubes work, in that you don't need to remove voltage entirely to turn them off - although I suppose you could consider that 'pre-biasing' as the tubes are permanently 'biased' partly ON.

I would suggest from the Nixie's point of view they are 'pre-biased' and from the FET's point of view they are 'clamped'

 

throbscottle

Well-Known Member

• Aug 31, 2019

• #6

Interesting. I hadn't thought about the terminology from the Nixie's point of view. I was actually looking for info about FET's when I found this exact circuit and text in a TI app note, searching for "pre-bias" got me the page I linked to, which was easier to reference.
Thanks for all the responses

 

dr pepper

Well-Known Member

Most Helpful Member

• Aug 31, 2019

• #7

I dont know how relavent this is, however I recall a while back building a dekatron display, and I had issues with the glow occuring on the central disc in the tube as well as the 'proper' electrodes, and applying a bias sorted this out.
In your case though its probably just to protect the trannys.
I did see a webpage that stated that you could drive a nixie direct from a standard 5v Ls ttl chip.

 

throbscottle

Well-Known Member

• Sep 1, 2019

• #8

That's interesting. I suppose when the gate's output is high the tube is in a non-conducting state so protects the chip from the high voltage. And when it's low the tube's cathode is connected to 0v anyway.