Turn on a fan when chamber heater is active?

  • I've been working on retrofitting a stratasys uprint and from the duet documentation I can't figure out a way to activate a fan if a heating element is active. I don't want thermostatic control sense the thermistor is located in the build chamber and the fans have to be active for the tempature to rise.
    This printer has two very powerful 600 watt heating elements with large radial fans blowing air past them, it's very important that air be blowing anytime the heating elements are turned on.

  • I have a 24V fan blowing on a 500W heater pulled from a stratasys printer. I simply connect the fan to the input side of the SSR that switches power to the heater so it turns on whenever the heater is powered. If you use line powered fans, connect them in parallel with the heaters on the output side of the SSR.

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    For safety there should be some sort of passive TCO that monitors the air or heater temperature to kill power to the heater if the fan(s) or SSR(s) fail. I haven't worked that out for my printer yet.

  • Moderator

    You could use the start.g macro to turn the fan on. Start.g gets run automatically when a print starts before the slicer start code is run. that would ensure that it is running for every print.

  • start.g wouldn't work if the heater is activated for say tuning or other non-printing tasks. I would simply go with mrehorstdmd's solution, or something similar with a 2nd SSR in parallell with the 1st if the fan voltge is different from what the controller outputs.

  • In the slicer profile for ABS filament I set the chamber temperature to 50C, and always set all heaters to 0C at the end of prints in the printer profile custom gcode. I also have a macro to preheat the machine for ABS printing that sets the hot-end, bed, and chamber temperatures, and a macro to just set the chamber to 50C for testing without heating the other stuff up, and a cool down macro to kill all the heaters. The macros are only needed when operating via the Panel Due.

    It's safer to run line powered fan(s) wired in parallel with the heater(s) because if the SSR fails, it will fail "on" and the fan will keep blowing across the heater. Of course, that will merely delay the problem, and hopefully, a TCO will prevent a meltdown and fire. You still have to worry about the fan failing, and if the temperature gets high enough, it will fail. In my DC fan set-up, if the SSR fails, the controller won't know and will turn the fan off once the chamber temperature hits 50C, meanwhile, the heater will keep cooking.

    Ultimately, the controller board and fan are going to be the least reliable parts of this setup, so you really need a TCO.

  • The issue I have with those solutions is that the same fans which blow air past the heating elements are also responsible for part cooling as they blow air onto the model from both sides. There is a 2nd fan that brings in cold air from the outside and blows onto the cold zone, stepper motor and the part.

    This printer is going to be used by a high school robotics club to print Nylon and it has to be super safe. I just don't want the heaters to be able to be turned on without the fan first being enabled.

  • If safety is the primary concern then the problem is no longer turning on the fan. Use PTC heaters rated for the target temperature as they are to some degree self regulating, add thermal fuses that will melt and break the supply if the heaters fail for whatever reason. Use a normally open bi-metallic thermostat and wire the fan as always on, with the thermostat turning them on when the heaters are above the rated temperature?

  • If the fan is wired across the heater, it will turn on when the heater does.

  • True, but it'll stay hot longer than necessary, so if its possible for students to accidentally touch the heater it would be better to leave the fan running until its cooled down, which would either require you trust the firmware or use a bi-metallic switch.

  • There are about 100 places you could get injured by a 3D printer. If they put their fingers between a belt and a pulley they'll get pinched. If they touch the heater block on the hot end they'll get burned. If they put a finger under the bed when it's moving it may get crushed.

    Take some basic precautions in the design- don't force them to reach over a heater to remove a print, for example, but you can't possibly make the thing 100% idiot proof.

    These are high school kids in a robotics club, indicating some minimal level of intelligence (?), not 3 year olds. If they're building robots they're going to be using all sorts of tools with myriad possibilities for injury. Give them a list of dos and don'ts and turn them loose. Let them do something stupid and see what happens. That's called learning by doing.

  • If you place a thermistor near the heater and set your fan as thermostatic. You could have your fan running at 100% at 34c or more and when the heater is "safe", the fan can be idle.

    I made this for my printer:
    When the temperature in front of the fan is more than 30c, the fan runs 100%.
    When my chamber is hot, the fan is always running.

  • @tech-raton The whole point of this is safety. You can make the fan thermostatically controlled, but what happens if the controller fails? Maybe it fails with the heater on and the fan off, or maybe everything works fine except the fan. For maximum fire safety, a passive device has to interrupt power to the heater on any over-temperature condition. That can mean the fan stopped working or the controller or SSR failed leaving the heater on.

    On top of that, the OP wants students to be unable to come into contact with the heater until it has cooled. For something like that I think the only things you can do is enclose the heater so it can't be touched or enclose the printer and put a thermostatically controlled latch on the access door (and hope that system doesn't fail).

  • @mrehorstdmd
    I agree. Maybe with a reverse relay between the fan and the controller. But adding parts, adds potential problems. A tco and a mechanical lock could be good, but even with the best safety controls, you can't protect every kind of problems, like an half closed door, a fire that kept the door closed, etc

    A simple sticker could be enough for some people.

  • Nice that you got hold of a uPrint. They are nice machines.
    I have a printer with a really hot (230C) Chamber and i always run the fan. This is the most idiot proof thing there is. When i switch the printer on, the fans come on too. When i switch the printer off, the fans come off.
    You may ask, what if i want to control the airflow?
    You don´t need to. With heated chamber machines like the Stratasys, you always have the maximum airflow and just regulate the air temperature.
    Otherwise there is no reason (that i know of) to have the printer switched on but not have the fan blowing.

  • @mrehorstdmd it has thermal fuses for that exact situation. the heater is hidden and cant be accessed unless you seriously disassembled the machine.

  • The reason I want to be able to control the fans is so that I can turn them off for the first layer or for certain materials that don't need the airflow.

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