Hello all.
I’m in and out of electronics as a hobby. I get just to the point of understanding a few things and then I have to let it go to do something else, like 3D printing… I have an upcoming project and I have a question about a circuit that I generally know will work, but I need professional help to make sure I get it right. Here’s the scenario:
I have purchased a 3D printer. The hotend heater block is heated by a 12V rail and melts the filament so it can print. I want to pull a lead off this rail to activate a transistor that will activate a separate 12V rail to power a cooling fan. The issue is, the hotend 12V rail stays powered until it reaches the desired temp (detected by a thermistor) and then shuts off to maintain the temperature. When it cools a few degrees, the thermistor reactivates the rail, it comes back on, etc. What I need to do is keep the transistor controlling the separate 12V rail powered during the off times, thus, keeping the separate 12V power to the cooling fan going. It will turn off when the hotend heater block is turned off and no longer heating. The time that the thermistor turns off 12V to the heater block varies from a few hundred microseconds to 3-4 seconds. I’m thinking I need to use a capacitor/resistor leading to the base of the transistor to keep it active for a few seconds, likely 5 seconds maximum, just to be safe.
I’m concerned about powering the fan directly from the hotend rail because of the high assumed amperage draw of the heater. I don’t want to ruin the heater or the fan in this process. 
My question is, how do I calculate the capacitor and resistor values that I need? Also, having read up on this a bit, I’m thinking I can get a transistor that activates at, say, 6V up to 12V, or something similar, so the capacitor dropping voltage would be okay, but I’m just guessing.
Also, if this isn’t the way to pull this off, please let me know what you think is a better/more efficient way.
I don’t have the printer yet, should be delivered early next week. I’m also not doing this mod for a while, but I got to thinking about it and figured I’d go ahead and ask. So, if I need accurate, real world, multimeter testing, etc. I’ll just repost my question when the time comes to build this.
Thank you ever so much in advance.