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I use the toner transfer method to create PCBs. By removing the heat fuser from a laser printer and printing on gloss paper gives me wonderful results.

The heat fuser and thermistor sensor is still connected to the printer's board as when the printer turns on, it seems to test the thermistor by tuning on the heating lamp.

I guess they get the temperature, then turn on the heater and monitor the change in temperature. Unplugging the heater before start up, causes an error. Obviously unplugging the thermistor does the same.

I would like to remove the heat fuser completely from the printer. At first I thought that putting in a resistor in place of the thermistor would be enough to fool the system. At room temp the thermistor reads 200 kΩ, and at 180°C it goes to 50 kΩ. But using a 50 kΩ in place of the thermistor resulted in an error.

To test, I soldered in a pot and resistor, so that I had 50k to 200k swing. When the lamp turns on, I tried to bring the resistance down (turning the pot manually) from 200 to 50k but no luck.

I can only think that the software monitoring the temperature is super advanced. Keep in mind that sometimes the printer is turned on while the heater is still very hot and still tests the thermistor successfully.

Any idea why this approach would not work?

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  • \$\begingroup\$ It's probably not super advanced. But probably advanced enough to avoid getting fooled by a simple \$50\:\text{k}\Omega\$ resistor. The printer software doesn't have to do rocket science for that much. I don't have a clue about how to solve the problem, though. I'd have to spend a lot of time before I may, too. But +1 for the question. It's clear enough and perhaps someone has a good thought. \$\endgroup\$
    – jonk
    May 28, 2021 at 19:47
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    \$\begingroup\$ How does dry carbon dust unfused make a useful PCB? \$\endgroup\$ May 28, 2021 at 19:58
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    \$\begingroup\$ Once printed on glossy paper, it's put face town on the copper. Then heat is applied with a bit of pressure the toner fuses to the copper. Works well for me. You can use a regular unmodified laser printer, but fusing to the paper and then trying to transfer to the copper is a pain. \$\endgroup\$ May 28, 2021 at 20:25
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    \$\begingroup\$ have you considered installing a switch in series with the fuser heater? \$\endgroup\$
    – jsotola
    May 28, 2021 at 23:39
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    \$\begingroup\$ try replacing the fuser with a smaller resistor - perhaps the heater from a glue gun \$\endgroup\$ May 29, 2021 at 1:51

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