Thermofuse Cross-section

[jonezy999]

Does anyone have a cross-section of a thermofuse?

Trying to explain to someone why throwing em on the floor to "reset" them isn't a good idea.

Cheers J.

[Hansoon]

Re: Thermofuse Cross-section

Thermofuse or thermoswitch?

Here's a Thermofuse

thermosicherungen_aufbau.jpg

Hans

[jonezy999]

Re: Thermofuse Cross-section

Thermofuse or thermoswitch?

Here's a Thermofuse

[ATTACH=CONFIG]16006[/ATTACH]

Hans

Cheers Hans, umm perhaps I meant thermoswitch, Ricoh just calls them thermostats.

I now have a new desktop deckground though

[zed255]

Re: Thermofuse Cross-section

Jonezy,

Sorry no graphic for you, but the issue as I see it is the bi-metal disk. Once a high enough temperature causes mechanical 'inversion' and it is subsequently 'reset' it will flip to the inverted state more readily during future use. Call it fatigue, loss of pre-conditioning or whatever. The disk when tripped simply push the switch contacts apart. Also, these switches are not environmentally sealed and arcing is more likely following a reset due to contact contamination.

[blackcat4866]

Re: Thermofuse Cross-section

This came from the SENSASYS website. It's not a copier thermal switch, but it does illustrate the principles involved. I hope it helps. =^..^=

BiMetal Thermostat.JPG

[jonezy999]

Re: Thermofuse Cross-section

Cheers all, much appreciated.

[DAG COPIERS & COMPUTERS]

Re: Thermofuse Cross-section

From the above diagrams it should be clear that the two components are two different things. What unites them is the variable THERMO, the heat which alters their state of existence. The thermofuse melts once and can only be replaced with another one. The thermostat or thermo/switch can break and/or remake contact depending on the temperature. But frankly speaking even me I don't understand the principle under which ''hitting'' the thermoswitch on the hard floor causes this contact to remake, although I have done it many times in the field & sometimes it does work. Another alternative is to keep it in a freezer overnight- this one can be explained!

[Shadow1]

Re: Thermofuse Cross-section

I reset them all the time with no problem. The only exception is when the machine got WAY too hot before it shut down - don't know why, but it melted the pressure roller and a bunch of other stuff - I suspect there was something wrong with the thermostat to begin with. There were lots of them back in the day that had a reset button built in, but then the mfgr realized he could sell more if they were not resettable.

[DWise]

Re: Thermofuse Cross-section

We have thrown our fair share of them as well... Seem to work nicely for us

[KenB]

Re: Thermofuse Cross-section

In the highly unlikely even that the worst should happen, and the connection NOT get broken when it should, I don't think I would want to be the tech responsible for that fix.

Some years back, we had a tech replace the 91 degree scanner lamp thermal fuse with a 192 degree one (from the fuser) on a Canon NP400 to get the customer running.

Whatever circuit it was that controlled the scan lamp stuck "on" after-hours that night.

The following morning, the customer was greeted by a blob of molten plastic parts, including the bulk of the doc feeder.

We had an emergency service meeting the following morning getting warned to NEVER EVER do that.

(No, the tech did not get fired, as he had a very good relationship with the customer, but almost anyone else probably would have.)

[Shadow1]

Re: Thermofuse Cross-section

The very few I've had problems with after a reset have tripped out early - replacing one with more than double the temp rating is definitely not smart

[NeoMatrix]

Re: Thermofuse Cross-section

:: Electrically Conductive Plastic Thermostat Housing ::

I'm waggering that not many people have see insulating plastic turn to conductive plastic over time ?
I had a call to a Ricoh machine that was tripping the earth leakage system(RCD) in the building.
I tracked the offening problem down to the fuser unit and set about finding out why.

I finally found that the insulating plastic housing on the thermostats had become electrically conductive
over time. This machine is an old machine and doesn't get used very much. But the machine is stand-bye operational 24/7.

If you believe this is not possible then have a look at the image I have attached with this post.
You can clearly see 1.444 Mega-ohm resistance displayed on the test-point multimeter.
The thermostat case is sitting on a dry sheet of paper. The multi-meter probes are inside the screw sockets. No toner or developer can get inside the screw holes.

When the plastic thermostat house is placed back in the machine both thermostat cases read a parallel resistance of around 500 Kilo-Ohms and this appears to get less as they get hotter. The drop in resistance is enough to randomly trip the RCD electrical point in the building.

I fixed the problem by replacing the entire fuser unit.

ConductiveThermostat.jpg

[blackcat4866]

Re: Thermofuse Cross-section

Neo: That is really interesting! Transfer bushings often use intentionally conductive plastic.

Years back we had a Mita DC-142 at a fire department. I was there to rebuild the fuser and was very satisfied to see that it had been two years since it had had service last, for a fuser code. While rebuilding the fuser I paid special attention to no-blow thermal fuse (paper clip). We dodged a bullet there! If the building had burned down I'm sure that the fire investigator would have found that special fuse. =^..^=

[KenB]

Re: Thermofuse Cross-section

The very few I've had problems with after a reset have tripped out early - replacing one with more than double the temp rating is definitely not smart

They were different lengths for a very definite reason.

[Lagonda]

Re: Thermofuse Cross-section

You guys reset thermoswitches!!!! Dont you realise what a can of Health and Safety worms your opening by reusing a one shot safety device? not to ignore the insurance implications if something did go wrong either.

Our techs are under strict instructions not to tamper with safety systems. If a customers machine is down, tough, its down until the correct part can be obtained to repair it. If the customers complain you just say the magic words "Workplace Health and Safety" that usually shuts them up.