Enclosure controller

As mentioned in an earlier post I have a plan to add an Arduino Uno based controller for enclosure. Its responsibility is to monitor temperature, control ventilation fan speed and switch on the main relay. It also has a simple 1602 LCD display shield for temperature and other information. At the moment I haven't planned any use for the keys of the display shield.

So far I have been testing the design on a breadboard in order to get temperature reading and relay control working. I have plenty of 100k Ohm NTC 3950 thermistors so I used one to measure temperature using voltage divider circuit and Steinhart–Hart equation in the Arduino code. There are plenty of examples online how to wire this up.

The main relay is an automotive 40A relay with integrated fuse socket. The funny thing was it actually came with a 30A fuse. That should be enough, though. In order to energise the coil of the relay I used BC548 NPN transistor. I also added 1N4148 as a flyback diode.

Adding a PWM controlled 12V should be straightforward. I will use BC548 or BC639 but I might add a capacitor to smooth out the fan control. After I have that sorted out on the breadboard I'll solder everything together.

Heatbed thermistor

I was planning to get a silicone pad heater and machined aluminium plate but I could not find a nice plate with decent price. So I had to go with a traditional PCB with glass plate. I ordered PCB Heatbed MK2B and a bunch of NTC 3950 thermistors. I will use the rest of the thermistors with my environment control unit.

The power wires were easy to solder but I might have bought a bit too heavy wire for it. I use 4 sq mm wire between RAMPS and heatbed. That is pretty much the thickest fitting in the RAMPS screw terminals but it isn't very flexible and might cause me headaches when I try to clean up the wiring mess.

Most of the instructions use Kapton tape to attach thermistor to the heatbed. Unfortunately, I have none here so I had to improvise. I soldered the thermistor to a longer wire with heatshrink and thermal protective sleeve. Then I placed the thermistor in the PCB with some heat resistant grease as I did not have any thermal paste either. I secured the wire in place with some masking tape and then I added good amount of bathroom silicone on the wires. According to the label the silicone is good till +180 C degrees.

Not very pretty but seems to hold pretty well. I hooked up the wires to RAMPS and heated the bed up to 95 C degrees and everything worked ok. In any case this part is hidden so the appearance does not really matter.