Electrical – Current through heating element lower than resistance suggests

currentheatimpedancereactanceresistance

The fuser lamp (the heating element) in my laser printer broke from one of the ends. I measured the resistance of the, almost full length, wire which was 6 ohms. I tried to fix it by connecting the wire to the printer but the wire kept breaking after a couple seconds at the connection (it was exposed to air + mechanical stresses caused by my tinkering).

So, I decided to make my own heating element from nichrome, 7 ohms to be safe. Now comes the strange part: this 7 ohm element causes the fuse of the printer to burn almost instantly. Confused, I measured the current through and the voltage over the original broken element and they were 5A and 230V, I assume 50Hz AC, even though 6 ohms and 230V should give 40A (which, I know, is way too high with the 6.3A fuse).

I came up with two explanations:
1) I didn't measure the reactance of the original element (it's a coil).
2) The original element's resistance goes up a lot when it heats up (I don't know the material).
But neither of them seems to be significant enough to raise the impedance high enough.

So, my question is: What could cause the low current?

Best Answer

The lamp in the fuser is most probably a halogen lamp. The resistance of such a lamp in a cold condition is low. However the moment the lamps starts to glow the resistance goes up very steeply. The 5A of the lamp corresponds to 1150 W. The resistance of the fuser lamp in a hot condition is therefore 46 ohms. If you want to make your own fuser heater then you would need a 46 ohm resistance in the hot condition. But I do not consider this a good idea even if it works. First of all the mass of your heater is much more so it will take much more time to warm up. Secondly I doubt whether your own solution can be made safe enough for the environment you are working with.

In theory an impedance changes due to AC. However with the frequency involved you can ignore that. It is simply the fuser lamp resistance that has such a steep rise in resistance. This lamp has also a high current during start up like all incandecent lamps. It is also the reason that lamps mostly fail during switching on.