Circuit to control HP LAserjet Fuser.

[AlainB]

Hi,

I used an HP Laserjet II fuser to transfer toner from paper to PCB copper plate. It is working very well. I made a driver for the motor and it is controlled by a Gcode file using TurboCNC 4,1 software.

I would have 2 questions and one wish about the curcuit controlling the temperature of the lamp.

1- There is a little chatter when the relay switch on or off (like a buzzer). Not very much but still. I read somewhere that a capacitor would correct that but I cannot find back this article. I think they were talking about a 10Mf, 25 volts. Where should I put this capacitor?

2- The heating lamp is very powerfull. About 630 watts. It can destruct the whole fuser assembly in less than 10 seconds if not well controlled. I know it because I destructed one last week. Look at the picture. My question is how can I protect the relay. It get on for one second and off for 6 to 8 second approximately. I use a 20 amp. automotive relay. My other fuser has been destroyed because the contacts points of the relay fused together. This happened just after I installed a diode on the relay. I am wondering if it was a coincidence or if having the diode installed the wrong way could have weld the contacts points? Anyways!! ...So how could I protect this new relay? I mean not putting a diode but protecting it from the power of the lamp.

3- I wish I could monitor in real time the resistance of the thermistor. The DTDP swicth purpose is to isolate the thermistor and cut power to the relay while reading the thermistor value. A constant reading would be more interesting but it is not working if I plug the Ohms meter without isolating the thermistor. Would there be a way around that?

With this circuit, the cut in/cut out differential is about 1 to 1,4K. The working temperature is when the thermistor resistance read 6 to 6,5K.

Alain

[vger]

Very cool deal, or hot in this case. One alternative you might consider, use a standard light dimmer circuit. You can get them that will handle 1000 watts. Having worked on copy machines and microfilm printers in the past I am aware that most of them used solid state controls for switching the fuser on and off. Usually a triac control connected to the fuser lamp. Might be able to use that if you still have the original boards.

Steve

[gmfoster]

If you are going to use this relay you probably need more voltage. Automobile relays work best at 13 volts or better. You need to check pull in in voltage and drop out voltage on the relay itself. Another thing You don't show what you are using for a power supply. If it is drooping it will mess your comparotor up pretty bad. At the least you need to add some capicatence to your supply right before the switch to ground.

And maybe a .01 or so at the IC supply pin, keeping leads as short as possible. And you should move the IC feed to the hot side of the switch.

Garry

[pastera]

If the relay was an automotive relay it may not have been rated to switch a 120 volt load.

Also, simply adding a diode across the relay coil will slow the release down - to the point it could easily arc and weld. If the inductive spike from the relay is too high not to use the freewheel diode, then add a resistor in series with the diode. Resistor should be sized so that supply voltage plus coil current*resistor is about 80% of the switch breakdown voltage. This will allow the relay to drop out quickly without stressing the switch.



Aaron

[vger]

You could just eliminate the mechanical relay all together.
Take an MOC3032 triac optocoupler and a suitable triac to switch the power to the fuser lamp.
here is a link to the MOC3032 datasheet:
http://www.ortodoxism.ro/datasheets/.../MOC3032-M.pdf
Take a look at the bottom of page 5 for example circuits.

Steve

[AlainB]

Hi,

Thanks!

I decided to go the solid state way. Thanks Steve to pointing me to the Triac. I removed it and the driver from the power supply and I am experimenting with them and learning a lot at the same time. Everything is working fine. The triac I have is BCR8PM and the driver is S21MD4V. I bought a MOC3020 from a suggestion on another forum and it is working as fine. From a HP tech. web page the direct replacement for the S21MD4V would be the MOC3061 family. The store say that the MOC3041 would be as good. So the MOC3032 would be a good choice too, I suppose.

I did not try the heating lamp yet but I experiment with my hair dryer using a 10K variable resistor to mimic the thermistor usable operating range and luckily did not brun anything yet even if the hair dryer has a part of inductive load (the motor) and I did not protect the triac.

This take care of my 2 questions. As for the monitoring, I have been suggested to monitor the voltage on the pin 2 of the LM358 instead of the resistance. This was a great suggestion. It can be done in real time.

I will post the definitive circuit later if all goes well.

Alain

[vger]

One other quick suggestion. Most fusers have a thermal fuse also right next to the fuser heat roller. It's wired in series with the fuser lamp and will open circuit if the temp goes too high. Might not be a bad idea to leave that in place for safety sake.

On the triacs
MOC3032 is 250V
MOC3041 is 400V
MOC3061 is 600V
S21MD4V is 600V

Since you are running on 120 V rms (175 peak) the 250 V should work fine.

Steve

[AlainB]

Yes, the thermal fuse! Of course this is the first thing that blowed up the first minute I started to play with this thing a month ago. I bypassed it! Then burned the fuser because the safety was not there anymore.

I still have one last fuse, that is until I can find another 10$ printer. But this one, I will install it with the final and well tested circuit. Until then, I will be very very carefull.

Alain