Thanks for the answer.
I have read about different IV-18 builds on various sites and they use the MAX6921 driver IC.
I missed that the driver managed the current.
When I look at Adafruit design http://www.ladyada.net/make/icetube/design.html
I only see a resistor to pin 1 which is 5V.What filaments are there then?
Pins 1 and 13 are the filament.
I have found one translated datasheet for IV-18. In the datasheet it stod the following:
Filament voltage: 5V
Filament current: 85mA (typ 75-95mA)
Anode-Grid voltage: 20-30V
Grid Current: 11mA
Total segment Current: 8mA (for one digit)
Anode-Grid current (all segment and digits lit): 25-20mA
I need to drive 8 digits one at a time so the voltage need to be increased.
Maybe it was stupid to skip MAX6921 and order UDN2981AT drivers.
The UDN2981AT was cheap but now I must limit the current with resistors.
8mA total segment current, if I got it right 1mA for one segment (8 segment for 1 digit)?