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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?

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    \$\begingroup\$ Fyi, nichrome wire resistance changes as it heats up. \$\endgroup\$ – Passerby Oct 9 '16 at 17:48
  • \$\begingroup\$ @Passerby Yes, I am aware. Did I give the impression of not? \$\endgroup\$ – Hoxy Oct 9 '16 at 18:22
  • \$\begingroup\$ Yes. Hence your shock that the nichrome blows the fuse. \$\endgroup\$ – Passerby Oct 9 '16 at 18:26
  • \$\begingroup\$ @Passerby That doesn't make sense. The resistance going up with temperature would, to some extent, prevent blowing the fuse. If I didn't know about it, I would expect the fuse to blow easier. \$\endgroup\$ – Hoxy Oct 9 '16 at 18:29
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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.

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  • \$\begingroup\$ Thanks! Halogen lamps seem to use tungsten, which has a high temperature coefficient. So, it seems the second explanation from me was correct. I don't think the mass of the heater would be a problem as the printer measures the temperature when it heats up. I probably wont be making a new element as my nichrome wire is too thick for such a high resistance. \$\endgroup\$ – Hoxy Oct 9 '16 at 17:58
  • \$\begingroup\$ Yes. Your secund guess was correct. Since you are happy with my answer. Please upvote also. \$\endgroup\$ – Decapod Oct 9 '16 at 17:59
  • \$\begingroup\$ I did but as my reputation is too low, it wont show it publicly. \$\endgroup\$ – Hoxy Oct 9 '16 at 18:00
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    \$\begingroup\$ Expect ~12 times higher resistance for any bulb at its rated voltage compared to cold. \$\endgroup\$ – winny Oct 9 '16 at 18:05
  • \$\begingroup\$ It's actually the Tempco that makes the difference \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 9 '16 at 18:19
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The critical function of the fuser is to heat the roller so carbon transferred by a laser scanned organic-photo-sensitive membrane is applied to the paper . The "FUSER" is a tungsten heated pinch-roller which operates at sufficiently high temperature to burn or "fuse" the carbon to stick onto the paper.

The critical property of the fuser is heat, thus power, current and resistance at applied voltage are critical at the right desired "red hot" temperature.

  • Tungsten has a temperature coefficient of 0.0045 per 'C
  • NiChrome has a temperature coefficient of 0.0004 per 'C
    • So NiChrome is about 11 times less sensitive
  • We know light bulbs with warm white are around 3200'C and are ~7*R(25'C) which means the resistance is about 7 x when hot and peak surge current is about root(2) x7 or ten (10) times the average hot current.

  • Since your cold resistance was the same as the original fuser, the surge current would start the same but only reduced to 91% while the Tungsten heater drops to 10 % and the fuse stays in tact. (perhaps 5 ~ 10A fuse depending on voltage)

So your "makeshift Fuser" blew da fuse.

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  • \$\begingroup\$ Honorary mention for the details about the temperature coefficients. But Decapod got there first with practically the same answer and some extra information. Gave you an upvote though. \$\endgroup\$ – Hoxy Oct 9 '16 at 18:27

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