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I have a 3D printer that has a "dumb" 12V DC fan on the extruder. It is always on and starts with printer; but unless the (12V DC) heater is turned on, it is useless and just makes noises.

I can't attach manual switch to it - if I'd forget to turn it on, heater will burn if turned on. I can't simply wire fan relay to heater - fan needs to cool heater down for few minutes after heater goes off.

I need to add a switch that will turn the fan on as soon as heater turns on, BUT this switch must be "sticky" - it must stay on onward, until 3D printer is powered off. I know how to add MOSFET switch, but can't figure out how to make control input "sticky".

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  • \$\begingroup\$ So many options! There are many ready-made solutions for this if you search on eBay for inspiration. Would you prefer to make it the "analog" way or write a few lines of uC code? \$\endgroup\$ – winny Sep 30 '17 at 8:05
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    \$\begingroup\$ Buy a thermal switch and wire it in the power feed to the fan. \$\endgroup\$ – Andy aka Sep 30 '17 at 8:06
  • \$\begingroup\$ @winny, I assume ready-made MOSFET switch with some kind of capacitor attached to keep one open when triggered will solve my problem but I can't figure out how to apply one \$\endgroup\$ – Alexander Sep 30 '17 at 8:57
  • \$\begingroup\$ @Andyaka, I use delta printer - its effector must be light; can't add any circuitry on top of it, unfortunately. Heat relay would be a rock-solid solution yet I can't use one. \$\endgroup\$ – Alexander Sep 30 '17 at 8:59
  • \$\begingroup\$ I have a delta too, I just wired the fan to a MOSFET on the controller board and turn it on/off automatically with g-code. \$\endgroup\$ – Wesley Lee Sep 30 '17 at 17:21
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The answer is: latch. You can make it in thousand ways, for example like this:

enter image description here

Or with bipolar transistors and relay:

enter image description here

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  • \$\begingroup\$ Thanks, turns out that "latch" was what I've been looking for. \$\endgroup\$ – Alexander Sep 30 '17 at 18:30
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enter image description here

Figure 1. A miniature thermostat.

A quick web search for "miniature thermostat" returns images such as the one above. It should be possible to find one that will fit under the insulation of the extruder. Wire the switch in series with the fan.

A switching point of 50°C sounds reasonable to me.

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  • \$\begingroup\$ But keep in mind that the thermostat will need to withstand heat of up to 300°C depending on the filament. \$\endgroup\$ – ratchet freak Sep 30 '17 at 10:56
  • \$\begingroup\$ Good point. It might be difficult to find one that would tolerate the high temperature but have a low switchover point. \$\endgroup\$ – Transistor Sep 30 '17 at 15:49
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I would use an SCR of sufficient current capability in series with the load on the anode side, with a gate resistor and switch to positive. The device will latch until power down or shut off by momentarily shorting the anode and cathode with a suitable switch.

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There is very likely a temperature sensor in the extruder used to regulate the temperature.

If there isn't you can add a thermistor with the required operating range (up to 300°C at least). Then set the fan to turn on once it detects the extruder is warm and only shuts down after it's been cooled.

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