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How can I calculate the resistance for a minimization inrush current effects, to increase the life of the filament of a VFD tube (IV22). Or is there another technical solution to prevent the filament life decreasing due to this phenomenon?

Power supply: 5 V AC Nominal voltage of filament: 1 ... 1.32 V Nominal current of filament: 0.1 A Filament resistance cold = 5.2 Ohm Filament at working temp = ~ 11.5 Ohm

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Just the series resistor required to get the proper operating current will reduce the inrush current. You only need one resistor and you can use Ohm's law to calculate the value.

Don't forget to check the power dissipation of the resistor.

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  • \$\begingroup\$ I would have liked seeing the calculations; both for myself as a sanity check and for the OP to understand how to do it. (I had to make them in order to agree with this.) I find that from 33-38 Ohms limits the current to the spec when hot or cold. I do get it that they make these device specs with an eye towards this particular use. But it's always a good idea to know how to verify and not just trust. \$\endgroup\$
    – jonk
    Commented Apr 6, 2022 at 17:21
  • \$\begingroup\$ @jonk There's no spec for maximum current that I can see. There's no time constant. So just elementary math to calculate. Rnom = (5V-1.16V)/0.1A = 38.4\$\Omega\$. Peak current at start-up would be Ipk = 5V/(38.4\$\Omega\$+5.2\$\Omega\$)~=115mA, only 15% more than nominal. Resistor power dissipation is 0.38W and peak dissipation is 0.51W. So use a 1W resistor to be safe. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Apr 6, 2022 at 17:47
  • \$\begingroup\$ Note that if regulated 1.16V was supplied to the filament the peak surge current would be 223mA or ~2.25x nominal. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Apr 6, 2022 at 17:49
  • \$\begingroup\$ I was using the limitations in voltage range across the filament. But thanks. \$\endgroup\$
    – jonk
    Commented Apr 6, 2022 at 18:20
  • \$\begingroup\$ Thnx for help! " There's no spec for maximum current that I can see." - Maxim current is appx 0.120 A. Duration of the inrush current of incandescent lamps is 100 milliseconds. What is formula for this data? Thnx \$\endgroup\$
    – Miau
    Commented Apr 6, 2022 at 22:54

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