Help needed for thermistor

idbruce

Hello Everyone

This thread is closely related to a couple others, but please don't be to upset with me, because this thread will pertain to a very specific experiment and it involves the Propeller BOE. Additionally, as many of you know, I am more mechinically inclined than I am electically inclined so please be patient with me.

Anyhow, as mentioned, this thread is closely related to some other theads, in the fact that I want to create an opaque photo mask for creating PCBs. As I have learned through trial and error, masks printed on a laser printer are just barely opaque enough to do a decent exposure with my exposure system.

Since I have recently purchased a LaserJet 2200D, I now have a LaserJet 6L to experiment with. Over the past several months, I have put a lot of thought into the quest for a more opaque print out from the LaserJet 6L. There are probably several different possible routes, but today I determined exactly what I want to try first. I want to alter the fuser assembly/thermistor feedback circuit. The main reason I chose this route is because many printers have a fuser setting for different types of print media, such as: paper, card stock, transparencies, photos, etc... And another reason is because the manufacturer of the transparent film that I am using, recommends the lowest possible fuser setting. However, the LaserJet 6L does not have any fuser setting that can be set either mechanically or programatically.

The fuser is basically a spring tensioned aluminum bar that houses a heating element. When toner covered paper passes between this heating element and a pinch roller, the toner is permanently fused to the paper. Besides housing the heating element, the fuser bar also contains a thermistor which provides temperature feedback to the main control board, so that the main control board can modify the temperature output of the fuser as necessary for proper operation.

Here are some basic facts about the thermistor feedback circuit.

• Voltage supplied to the thermistor 5.04 VDC
• Approximate initial resistance across the thermistor at 68 degrees fahrenheit is 400K Ohms
• As temperature to the thermistor is increases, the resistance decreases

With that in mind, my theory of operation is as follows:

As the thermistor temperature goes up, the resistance decreases within the thermistor feedback circuit, which causes the current to the fuser to decrease, thus decreasing the fuser heating element temperature.

Before modifying the thermistor feedback circuit, I would first like to monitor this circuit with the Propeller BOE during normal printer operation, in order to avoid destroying a working printer. So any suggestions pertaining to this subject would be most helpful.

After monitoring the circuit for normal operation, I then need a plan for alterations, which might be a bit above my head. Considering that the thermistor has an unknown range. and what values the main controller board expects to see, makes this experiment a tad more difficult for me. However, I do know the ultimate goal is to decrease the resistance in the thermistor feedback circuit when the fuser has reached normal operating temperature. By decreasing the resistance in the thermistor feedback circuit when the fuser has reached normal operating temperature, I should effectively be able to control the fusing (melting) of the toner to some degree (no pun intended).

Any help in this effort and experiment will be greatly and certainly appreciated.

Bruce

Phil Pilgrim (PhiPi)

idbruce wrote:

Voltage supplied to the thermistor 5.04 VDC

It's highly unlikely that a low-impedance voltage source is connected to the thermistor, else there would be no change to sense. What's more likely is that a higher voltage is being provided through a resistor, with the voltage across the thermistor being the feedback value. In order to come up with a modification, you will have to find out more about the actual circuit being used. My gut reaction, though, is that a resistor in parallel with the thermistor will lower the fuser temperature, since the feedback circuit will "think" that the fuser is hotter than it really is.

-Phil

idbruce

Phil

I may be misunderstanding you, but please refer to the attached diagram. The heating element for the fuser assembly connects to the J102 and has 120VAC coming from this junction block. The thermistor connects to J206 and has 5VDC coming from this junction block.

Bruce

idbruce

My gut reaction, though, is that a resistor in parallel with the thermistor will lower the fuser temperature, since the feedback circuit will "think" that the fuser is hotter than it really is.

Would a potentiometer connected as a voltage divider accomplish the same task?

Phil Pilgrim (PhiPi)

Here's a schematic of what I would guess that the feedback circuit looks like:



If you disconnect the thermistor and measure the voltage you will see 5V. But with the thermistor connected, the voltage will be lower. By knowing the thermistor's resistance and the voltage with it connected, you will be able to compute the value of R. From there, you can measure the voltage with the fuser at temp to compute the value of the thermistor's resistance at temp. This will give you some idea of what kind of resistance (or potentiometer) you need in parallel with the thermistor to accomplish your objective.

-Phil

idbruce

Phil

Thank you very much for your assistance.

Bruce

idbruce

Phil

Referring to your last post... I would imagine the ideal place for testing and future alteration would be at the part of the circuit that you marked feedback voltage, perhaps all the way back to the input pin of the Motorola 68030 of the Formatter board. However considering my lack of expertise in this field and the whole scenario considered, I was wondering if this would be acceptable. Let's say I make a PCB that contains (3) 2 pin headers, and we will call them J1, J2, and J3. And let's say J1 represents the main controller board connections, J2 represents test points and future parallel resistor connections, and J3 represents the thermistor connections. Similar to the attached diagram.

Would the pins of J2 be an acceptable location for measuring the voltage and resistance that you referred to?

Bruce

Bean

An added resistor in parallel with the thermister will make the fuser colder. (High value resistors will have least effect)
An added resistor in series with the thermister will make the fuser hotter. (Low value resistors will have least effect)

Bean

idbruce

Bean

Sorry it took so long, but thanks for your input.

At the current point in time, I am getting ready to start mounting a circuit board to the printer, which will extend two wires (represented by J2 in the diagram above) to the outside of the printer. Until I achieve a working setup, I will connect these two wires to a terminal strip, for testing voltage and resistance, plus with this temporary setup, I can add and remove test resistors very easily.

Instead of making a circuit board as stated above, I decided to use a chunk of proto board and left a fair amount of room to contain the entire working solution (I hope).

Anyhow, thanks for your input Bean.

Bruce

Phil Pilgrim (PhiPi)

idbruce wrote:

Would the pins of J2 be an acceptable location for measuring the voltage and resistance that you referred to?

Yes for the voltage. You won't be able to measure resistance with the power applied, however. Your first job is to measure the voltage with the power applied and the power to the fuser disconnected. You already have the resistance of the thermistor at room temp and you know that the voltage on J2 without the thermistor plugged in is +5V. From that and Ohm's law you can compute the value of R in my schematic.

Next, restore power to the fuser, and measure the voltage across R2 when the fuser comes up to temp. Again, using Ohm's law and the value you computed for R, you can compute the resistance of the thermistor at fuser temp. This will give you an idea of the resistance range appropriate to put in parallel with the thermistor to cool the fuser.

This is assuming that my schematic accurately reflects what's in your printer. You really should attempt to trace the circuit to make sure.

-Phil

idbruce

Phil

Thanks once again for your input.

This is assuming that my schematic accurately reflects what's in your printer. You really should attempt to trace the circuit to make sure.

I have faith in the knowledge that you are sharing

If the magic smoke happens to release, I have both a spare dc controller and formatter board for the printer. Just getting access to the bottom layer of the spare boards would be quite a pain to trace the circuits.

I could be wrong, but I would imagine that worst case scenario would be that the controller senses too hot or to cold a fuser temperature, and shuts down the current to the fuser, or registers an error with the thermistor and/or fuser.

Like I said, I have faith in your assumptions.

Thanks Phil

Bruce

Phil Pilgrim (PhiPi)

idbruce wrote:

I have faith in your assumptions.

That's all they are: assumptions. What's that saying? When we ASSUME, we make an A$$ out of U and ME.

-Phil

idbruce

Phil

LOL

Don't worry Phil, at least you have it in writing...

You really should attempt to trace the circuit to make sure.

Nobody can say that you did not warn me.

I would imagine, that if the magic smoke releases, there will probably be a few people laughing at me, because I did not heed the warning that you gave me.

Bruce

idbruce

Oh I just thought of another possibility, I suppose I could burn up the fuser and/or thermistor. That would be a real bummer, because I don't have spares of those parts.

idbruce

With the fuser disconnected and power applied to the printer, the voltage is 4.93V. And with the fuser connected and power applied to the printer, the voltage is 4.92V.

Which leads me to believe that the fuser does not actually heat up until there is a print command, and perhaps this is the main reason for the delay before printing actually commences. So I will slap it all back together, print a test page, and measure the voltage while it is printing.

Bruce

idbruce

It was just as I thought about the print command. As soon as the print command is issued the voltage across J2 drops to 2.0V.

idbruce

If anyone else is following along and interested, here are some known values:

• With the thermistor disconnected, the resistance across the thermistor at 68 degrees fahrenheit is 400K Ohms.
• With the fuser disconnected, thermistor disconnected, and power applied to the printer, the voltage across J2 is 5.04VDC.
• With the fuser disconnected, thermistor connected, and power applied to the printer, the voltage across J2 is 4.93VDC.
• With the fuser connected, thermistor connected, and power applied to the printer, the voltage across J2 is 4.92VDC.
• With the fuser connected, thermistor connected, power applied to the printer, and a print command issued, the voltage across J2 is 2.00VDC.

idbruce

Yes for the voltage. You won't be able to measure resistance with the power applied, however. Your first job is to measure the voltage with the power applied and the power to the fuser disconnected. You already have the resistance of the thermistor at room temp and you know that the voltage on J2 without the thermistor plugged in is +5V. From that and Ohm's law you can compute the value of R in my schematic.

Next, restore power to the fuser, and measure the voltage across R2 when the fuser comes up to temp. Again, using Ohm's law and the value you computed for R, you can compute the resistance of the thermistor at fuser temp. This will give you an idea of the resistance range appropriate to put in parallel with the thermistor to cool the fuser.

I have been studying this quote since the minute it was posted, and I must admit that either I am failing at reading comprension or I just don't know how to use Ohm's law to this extent.

I know there is 0.92 voltage drop when the thermistor is connected in the circuit and in standby mode which puts it at 4.92V, and I know there is a 2.92 voltage drop from that point when a print command is issued and the heating element is activated, resulting in 2.0V.

I also know that I am clueless in figuring this out

And to think I was an electrician. I wonder how many people have died already

idbruce

I just tried 18K, 22K, and 47K resistors across J2.

• At 18K, the toner smudges very easily and is not a good print out
• At 22K, the toner smudges very easily and is not a good print out
• At 47K, the toner smudges and it is a good print out, very close to no resistor, but a little more opaque.

Thank you very much Phil. I am defintely dialing it in.

Bruce

Phil Pilgrim (PhiPi)

Assuming the voltage dropped to 2.00V gradually, as the fuser heated up, not instantly (right?) :

4.93V / 400K = 12.325 uA.
(5.04V - 4.93V) / 12.325 uA = 8.925K = R (more probably 9.1K)
(5.04V - 2.00V) / 9.1K = 334 uA
2.00V / 334 uA = 6K = resistance of thermistor at temp.

-Phil

idbruce

Phil

I thought maybe you went to get some sleep, so I started experimenting with various resistors. It appears as though a resistance value somewhere between 44K and 47K will be perfect for paper, but I need to do my real testing with film.

IT WORKS, AND IT IS DEFINITELY DARKER

THANK YOU VERY MUCH PHIL!!!!!!!!!

idbruce

For Those That May Be Interested

For obtaining more opaque photo masks for creating PCBs, this solution will work with a LaserJet 6L.

It is noteworthy to mention that I will be installing a switch for the parallel resistor across J2 for quick disconnect and normal printer operation.

I am tickled pink

Bruce

idbruce

Hello Everyone

It is morning time and a brand new day. Time for more experimentation.

Before going to bed last night, I tried running a piece of laser transparency film through the modified printer with a resistance value of 44K across J2. It was readily apparent that this resistance was still way to high for the transparency film, because the toner was extremely well bonded to the film and more than sufficient melting of the toner had occured making the printout translucent. So from this point on, I will only be using film for my testing and I will be using a breadboard to wire up a variety of resistors in series to place across J2 to work my way down from the resistance of 44K across J2 to find the optimum resistance. In addition to the previously mentioned strategy, my future tests will also proceed with the printer density set to the maximum level of 5, and the test print will be a 3" X 4" filled black rectangle.

It is noteworthy to mention that due to preliminary testing, there is a lot of excess toner floating around the printing mechanisms, which is causing defects in the printouts. When I discover the optimum resistance across J2, it will be necessary to disassemble the printer once again to finalize the electronics and give the printer a thorough cleaning. This will be the third cleaning within several weeks and hopefully it will be the last one for quite some time AARRGGHH.

Since this truly is an experiment, I have no idea what the final result will look like, but I do know that it will be at least slightly more opaque than before this experiment commenced, and solution is long overdue. The translucency of laser printed photo masks has always hindered my PCB creation efforts and I cannot even begin to express the grief it has caused me, but hopefully after today, this will become a problem of the past.

Like I said, I know it will be a better print out than before the alterations, and for Phil's help in this matter, I will be eternally grateful.

Bruce

idbruce

To Those That May Be Interested

Without going overboard by testing every possible resistor combination in search of the optimum setting, I have finally settled with 25.2K (22K + 2.2K + 1K) Ohms across J2, because I realize I will never make an opaque image on a laser printer. With this setting, the toner will smudge, but not very easily, and when the image is coated with Acrylic Krylon Crystal Clear Satin it darkens the image and locks in the toner.

After settling into a resistance of 25.2K, I printed the exact same image with the printer in an unmodified condition, and also coated this image with Acrylic Krylon Crystal Clear Satin for direct comparison. I am happy to say that the image that was printed in a modified configuration was beyond any doubt less translucent.

With the printer in a modified state, I will now attempt to decrease toner density to see if this will improve the image, but I doubt that it will.

Bruce

Leon

Lots of people get opaque transparencies with laser printers, you just have to use the right film. No spray is needed, or modifications to the printer. When I used a laser printer, the toner never smudged.

idbruce

Leon

As I said, I got my film from a local graphic arts supply house who gave me three different typs of film, and said that people were using them to make plates. And I believe him.

And now referring back to something Phil said a while ago, about having proper toner. I would imagine my photo masks would be less translucent if I was to use a Hewlett Packard toner cartridge, instead of an aftermarket refill.

As for the comment:

Lots of people get opaque transparencies with laser printers, you just have to use the right film.

I say, "Seeing is believing."

I have never come across anyone that says they can achieve opaque photo masks from a laser printer, however, it is all over the internet that it is impossible.

And instead of worrying about me modifying my printer or spraying my photo masks, your time would be better spent practicing making PCBs, because like I said, I am going to start giving you lessons very shortly

Bruce

Leon

I used to get perfectly good PCBs with transparencies produced on a laser printer.

I've been making my own PCBs for over 40 years, I don't need much practice.

idbruce

I used to get perfectly good PCBs with transparencies produced on a laser printer.

That is quite a different issue than having an opaque mask. For instance, my test pattern is a 3" X 4" filled black rectangle. Please show me a laser print out of a 3" X 4" filled black rectangle that will not allow any light to pass through.

Bruce

idbruce

And Leon...

the toner never smudged

The smudging toner is intentional on my part. I can print out non-smudging laser prints all day long, but they will be more translucent.

Bruce

Leon

idbruce wrote: »

That is quite a different issue than having an opaque mask. For instance, my test pattern is a 3" X 4" filled black rectangle. Please show me a laser print out of a 3" X 4" filled black rectangle that will not allow any light to pass through.

Bruce

It should still produce a good PCB. It's best to use cross-hatching rather than solid areas with a laser printer.

There must be something wrong with the resist you are using if you need to use smudging transparencies. It should have much more latitude.

idbruce

Leon

You do realize that you are contradicting yourself, don't you?

Bruce