Turn on a fan when chamber heater is active?
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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?
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If the fan is wired across the heater, it will turn on when the heater does.
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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.
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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.
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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).
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@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, etcA simple sticker could be enough for some people.
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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.
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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.
