I’m repairing a 3D printer with a failed controller output for the nozzle cooling fan, and I need to build a small external circuit to replicate the original behavior.

System details:

• Fan: 24 V DC, 0.4 A
• Available signals:
  • 24 V DC supply (always present)
  • 24 V “heater ON” signal (24 V when heater is ON, 0 V when heater is OFF)

Required behavior:

• The fan should turn ON when the heater signal is ON.
• When the heater signal turns OFF, the fan should remain ON for about 2 minutes, then turn OFF automatically.

Additional notes / constraints:

• Nozzle temperature reaches ~260 °C during printing.
• After printing, the nozzle cools slowly.
• If cooling is too slow, filament softens where it shouldn’t, so forced cooling must continue for ~2 minutes after the heater is shut off.
• The circuit should switch a 24 V, 0.4 A load (fan), using a transistor/MOSFET.

What I’m looking for:

• A simple, robust solution (analog or microcontroller).
• Ideas using a 555 timer, RC + comparator, or similar are fine.

 

Analog Suggestion #1, using a LMC555.

Analog Suggestion #2, using a LM393.

Digital Suggestion #3, using a micro.

 

This is a pretty simple circuit which has worked well for me in the past.





It's basically a comparator-based solution although I've modeled it here with an LM321. The capacitor drains through Rt once the ON signal goes to 0V. That particular resistor value should give you a roughly 2 minute delay but it may need to be adjusted by experimentation. The capacitor's series resistor just limits the charging current to a few milliamps (otherwise you'd have about 12W moving through it). Just keep in mind that the output BJT should be a fairly beefy power transistor.

 

The very simplest scheme would be a Normally open, closes on heat, thermostatic switch attached to some point on the nozzle assembly that reflects the temperature to be limited. That thermostat device would control the cooling fan, in addition to what ever else controls it. Since a temperature sensor might be involved anyway, that is a simple scheme that substitutes temperature for time.

 

Analog Suggestion #1, using a LMC555.
View attachment 362618
Analog Suggestion #2, using a LM393.
View attachment 362624
Digital Suggestion #3, using a micro.
View attachment 362622

I like option 1, but what is the value of R1?

 

I like option 1, but what is the value of R1?

8.2K

 

The LMC555 has a pin 4 reset threshold of 0.4-1.0V, so its timeout is operating in the region of 2.5 to 3.5 time-constants, thus the actual delay time could vary quite a bit.
Of course that may not be a problem in this application.