Circuit to control HP LAserjet Fuser.

[AlainB]

Hi,

This is my first post here. My knowledge in electronic is pretty small.

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 circuit 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? Or is there a better way to control this chatter?

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

 

[eblc1388]

You need to do two modification to your existing circuit. See image.

The addition of a 22M(a very large value) resistor is to add hysteresis to the switching circuit so that the relay do not chatter when it is required to operate. This chattering is also the most probable cause of relay contact failure. However, the addition of the resistor will shift the tripping point to a higher temperature setting. You'll need to slightly adjust the 500K setting potentiometer to shift the setting back a bit after the addition of the resistor.

The second modification is to add a diode across the relay coil to protect the output MOSFET transistor. If you look carefully, there is a narrow silver/white strip or band marked at the end of the diode body. This is the end that has to be connected to +12V.

If you want to see the operation of the circuit during use, don't use an ohmmeter. Have the NTC connected permanently and then connect a DC voltmeter between 0V(ground) and the other end of the NTC. The voltage will rise as the temperature of the fuser increases.

Alternately, you can connect the voltmeter between pin2 and pin3 of LM358 and this will give you a reading starting high and drops when the temperature rises. When the reading reaches zero, the relay will open.

Edited: Changed value of hysteresis resistor from 2M2 to 22M.

 

[eblc1388]

If you use the fuser frequently, you can also do away the relay and use a solid state relay instead. Then every thing is static and you don't have contact to wear out.

For connection, see image.

 

[AlainB]

Thank you for your suggestions. As soon as I have a chance, I will change the circuit.

I will keep the relay for now but depending on the price of a solid state relay maybe I will buy one.

I still have the Triac that was part of the controller of the Laserjet. Its number is BCR8PM. Is there a way I can use it in my circuit?

One other question. About the Mosfet I am using. My understanding is that it already have a protective diode built in. Wy then put another one?


Thanks

Alain

 

[Roff]

Thank you for your suggestions. As soon as I have a chance, I will change the circuit.

I will keep the relay for now but depending on the price of a solid state relay maybe I will buy one.

I still have the Triac that was part of the controller of the Laserjet. Its number is BCR8PM. Is there a way I can use it in my circuit?

One other question. About the Mosfet I am using. My understanding is that it already have a protective diode built in. Wy then put another one?


Thanks

Alain

According to the datasheet, you should be OK without adding a diode across the coil. The internal avalanche diode should be able to handle the flyback current.

 

[eblc1388]

@Roff,

You are correct the part can stand the back emf by going into avalanche. However, the avalanche won't happen until the back emf across the Drain-Source exceeds at least 60V or much higher. This is good if fast switching of inductive load is required. For a relay, I would still use the diode so that the back emf don't go anywhere higher than 12V. Just my choice of being cautious.

@AlainB,

You can interface with your existing controller circuit to TRIAC BCR8PM using an opto-isolated triac driver MOC3020 ~ MOC3023. This will effectively like building a SSR with discrete parts. The MOC302x is small, cheap and easily available from many component shops. The connection is shown in the attached image.

 

[picbits]

Automotive relays are rated for low voltage and high current not high voltage and high current.

You might have problems with it if you are trying to control mains with it.

 

[Roff]

@Roff,

You are correct the part can stand the back emf by going into avalanche. However, the avalanche won't happen until the back emf across the Drain-Source exceeds at least 60V or much higher. This is good if fast switching of inductive load is required. For a relay, I would still use the diode so that the back emf don't go anywhere higher than 12V. Just my choice of being cautious.

@AlainB,

You can interface with your existing controller circuit to TRIAC BCR8PM using an opto-isolated triac driver MOC3020 ~ MOC3023. This will effectively like building a SSR with discrete parts. The MOC302x is small, cheap and easily available from many component shops. The connection is shown in the attached image.

I probably would add the diode as well.

 

[AlainB]

Hi,

Thanks to your help, I have been able to isolate the triac circuit on the old HP board and make it drive my hair dryer at 600 Watts shooting 3,5v dc on the LED side of the triac driver. The driver number on the board is S21MD4V: https://www.ortodoxism.ro/datasheets/Sharp/mXvtsww.pdf. There is 2 resistors. A (edited) 150 ohm from number 4 of the driver to T1 and Neutral 120V and a 22 Ohm, again more or less, from number 6 to T2 and one side of the lamp.

I got the MOC3020 and the resistors. Will the driver work at 120V? I am from Canada, I updated my profile to include this information. I saw on one datasheet 3021,22 and 23 for 240v. and 3011,12,13 for 120v. What about 3020?

I think I will go the Triac way. Will I still need the 22M resistor?

Thank you again everybody!

Alain

 

[eblc1388]

Thanks to your help, I have been able to isolate the triac circuit on the old HP board and make it drive my hair dryer at 600 Watts shooting 3,5v dc on the LED side of the triac driver. The driver number on the board is S21MD4:

S21MD4 is sort of the same function as MOC3020, with added zero-crossing switching feature so is even better. You can use it instead of the MOC3030 if it still works.

I hope you have used a series resistor when you put 3.5V to the LED side of the S21MD4 or else you could blow it. The LED only allows maximum 50mA going through it.

There is 2 resistors. A 84 Ohm +/- from number 4 of the driver to T1 and Neutral 120V and a 22 Ohm, again more or less, from number 6 to T2 and one side of the lamp.

You would also need these resistors even you use MOC3020 in case the S21MD4 is defective or gone bad sometime later. Just replace the MOC3020 in the exact same pin location as S21MD4 and it will work the same.

I got the MOC3020 and the resistors. Will the driver work at 120V? I am from Canada, I updated my profile to include this information. I saw on one datasheet 3021,22 and 23 for 240v. and 3011,12,13 for 120v. What about 3020?

MOC3020 works at 400V peak or 240V AC so has no problem working at 120V AC.

With the Triac, will I still need the 22M resistor?

Yes or no. The 22M resistor is there to make the ON/OFF switching transition upon reaching high/low set point temperature more instantly so there won't be chatter or buzz when it happens. Although this is the case for relay, there is no buzz in opto-triac with the fuser when temperature comes to the set points. The fuser just switches ON/OFF many times. However, I still recommend to put it in but you can leave it out if you wish.

Replace the lamp with a hair dryer and blowing the heat directly towards the NTC to check out the function of the whole circuit is a good idea to see what happens with and without this 22M resistor.

 

[AlainB]

Hi L. Chung,

I guess I have been lucky. I did not put a resistor in serie and it is still working. Unfortunately, I dont know how to calculate the needed value. With 3,5v. I tried 560 ohm. It is not working. But it works with a 464 ohm resistor.

The air dryer could not heat the thermistor enough I think. The operating temperature of the fuser should be around 180 C. I guess.

Alain

 

[eblc1388]

I dont know how to calculate the needed value. With 3,5v. I tried 560 ohm. It is not working. But it works with a 464 ohm resistor.

From the S21MD4 datasheet, on top of page2, the LED has voltage drop 1.2V @20mA typical.

Therefore, to operate the LED at 20mA at 3.5V supply voltage:

Series resistor value = ( 3.5V - 1.2V ) / 20mA = 115 ohm.

Your 464 ohm is too large in value and will not ensure a reliable operation. The acceptable resistor value range in this case is 100 ~ 120 ohm.

 

[AlainB]

Thanks!

I put a 105 ohm.

I just notice that on my drawing there was a mistake on the LED resistor value. It should be 1K, not 100K. I suppose that 1K is a very safe value. I read that typical value of a red LED is 1,7v, 30Ma. so, at 12 volts R= (12-1,7)/.03 or 343 ohm. It should be fine, Right?

In fact, I think I know how to calculate values. It is how to read datasheets that is my problem. Most of the symbols I don't know what they mean and I have never been able to find a glossary for these terms. See, on this particular datasheet, S21MD4V, once pointed out to the information, it is very easy to undrestand.

Thank you again, I really appreciate your help.

Alain

 

[eblc1388]

For modern LEDs, current can range from 5mA ~ 20mA. Unless the LED datasheet for that particular LED is available, one usually don't know for sure.

For everyday use, I would say 15mA is good value for most LEDs. Please note that different color LEDs, though operates at the same current, has different voltages across itself depending on color, ranging from 1.7V for red to ~3v for white. This affects the calculation of the series resistor value.

Usually a few tens of ohms within the calculated resistor value make no visual difference. e.g. If the calculated value is say 234 ohms, then 200, 220, 240 or 270 ohm resistor will work.

Do keep us inform of your final result of this project.