Homebrew PCBs

On Sun, 22 Feb 2004 03:34:18 -0000, joshdewinter <joshdewinter@...> 
wrote:

> Hi folks.
>   Well, after much experimenting with a SECOND laminator like the
> first, I have determined that my printer's toner must just melt at
> too high a temperature.  (I have an HP Color LaserJet 4500N, and an
> HP LaserJet 5si, both of which produce the same results). I have, as
> such, ripped the fuser out of an old HP LaserJet IIIp.
>   I fired it up and watched it, thinking the little thermostats
> touching the hard fuser roller that are in-line with the AC power
> would turn it off when it got hot enough...bad idea there.  One of
> the plastic parts supporting the roller that encircles the lamp
> melted.  Luckily, I caught it before it deformed it too badly.
> Those thermostats must only be for extremely high temperature
> protection, but not enough to prevent damage.  So, note to self,
> temperature regulation circuitry required.  Fortunately, it looks
> like HP's own built in thermostat will work just fine.
>   For everyone who has been experimenting with fusers, how hot do
> you let yours get for optimal melting?
>
> -Josh
>
>

Hi Josh,

there is a thermistor contacting the fuser roller.
I simply have used a transistor, a potentiometer, and this resistor to
operatre a solid state relais. additionally you need a stable power supply.

It works very well, on my digital thermometer only the last digit varies 
(+- 0.5\ufffdC)
but it could even be more stable (the last digit can change always).
This is far more exact than you need for the job.


I use it at 160\ufffdC, maybe it would need not so many passes at higher 
temperature (now 10 very slow).

I had a schematic drawn of the thermostat but i can't find it now.
It is simple, the emitter of the transistor and the thermistor in the 
printer are connected to
ground, the thermistor other pin is connected to the base.
 from V+ (10 or 5 V or anything stable) a 500k potentiometer is connected 
to the base also.
(with 1k in series to keep the smoke in if you turn it too low).
i think a smaller value pot. would do also (~100k}

the colloetor of the transistor simply drives the solid state relais (it 
switches on at about 3V.)
the other input of the SS relais is connected to V+ (obviously)

You just set the temperature with the potentiometer and watch it working.


I will redesign this some day, when i build the case for the power supply 
and the motor.
then i wil use a seperate Triac or FET and make the relais myself.
(te relais is some 70 or 80A and i would like to keep it for more 
demanding purposes)


If you plan on using a contact relais (mechanical) you NEED to add 
hysteresis.
this circuit would burn the contacts away in short time.
it is not hard to add hysteresis (say 2 degree swing or so) then it will 
only
switch ocassionaly (so it was done in the copier i took the fuser out)..


I fear the laser printer fuser has a very thin walled roller.
therefore you need good regulation to keep the temperature stable.
the copier fuser i have i very thick walled, a pcb doesn't cool it locally.


the "emergency overtemperature fuse" is only for emergencys, it should 
really not be used
for normal operation.

ST