Thermionic cathode heater filament conundrum

Question

An old CRT I have specifies 6.3 V (DC, I believe) on the heater filament, resulting in a 0.6 A filament current (spec).

But I measure the filament resistance as 1.8 Ohm.

At V = 6.3 V that would result in a 3.5 A filament current, about 6 times too high!

I'm tempted to measure the filament current with a 6 V DC battery pack but I'm afraid to burn the filament?

Any advice is appreciated!

If you try ... do it slowly. The filament must be "just" a little "red", ... if you see it ... — Antonio51, Apr 07 '22 at 18:09
Also at 6.3V, the supply was usually AC (though DC would work and it was designed to be compatible with 3 cell Lead Acid. — , Apr 07 '22 at 19:39

score 4 · Accepted Answer · answered Apr 07 '22 at 17:53

4

1.8 ohms is the cold resistance; it will increase as the filament warms to operating temperature.

answered Apr 07 '22 at 17:53

vir

Ah. That solves it. I didn't think the effect of temperarure would be that large. Accepted. – Gert Apr 07 '22 at 19:23
It's probably >6x R increase at 1500'K – Tony Stewart EE75 Apr 09 '22 at 14:13

1 Answers1