555 timer controlled heated print bed

Posted: November 21, 2011 in Electronic
Tags: , , , ,

It has been some time since I posted something but i’m really busy since this is my final year of school. I have been busy for school but of course also making things! Currently i’m working on my new 3d printer, a prusa mendel and some other small projects like this one.

I do have a Ultimaker and thought I wanted to try out a heated bed because I have some trouble on bigger prints, warping etc. so I thought about it and took a standard 100x160mm pcb and designed a pattern in eagle to mill and make a heated plate/bed. I couldn’t find a heated bed designed these dimensions (since this is a standard pcb size?) Also, I could buy a heated bed, and maybe I will buy it for my mendel, but for now I just wanted to try it out.

The heated bed consists of just a very long line of copper on it which creates some resistance, in my case 3.4 Ohms. When a voltage is applied there is a big current through the copper which develops heat. Here’s a calculator for designing your own pcb layout. Here’s my eagle layout and the actual milled pcb:

I added a led to see if it is working, and it did!;

A nice 123 deg. C. which is enough for a heated bed, actually it even might be to much! I made a nice table and graph of the temperature over time;

As you can see the resistance decreases when the platform gets warmer and off course the current does as well, which is a nice feature.

Now normally these heated print beds are connected to the main pcb of the printer, and this is actually a really handy thing since the pcb can control the temperature of the heated bed. But I did not connect a temperature sensor (thermistor or thermocouple of whatever), and I didn’t want to control it with the printer controller pcb since this the power supply of it has not enough watts to run the printer and heated bed… Maybe I solve this problem another time.

Now I had to generate a pwm signal to make the voltage,current and temperature lower and thought why not use a 555 timer in stead of always using a microcontroller?!  I googled for generating pwm on a 555 and found out this is really simple! (check out http://www.dprg.org/tutorials/2005-11a/index.html)  The 555 is set up as a astable oscillator, so it triggers without a external signal, which makes it perfect for this application. The duty cycle on the output varies by turning the potentiometer, this way the temperature can be controlled.

So I drew the schematic in eagle and took made designed a simple one sided pcb, I used a mosfet (IRLML2502PBF), but I did not know if this could drive the actual 3.61 Amps. continue so I spared some space for another mosfet or transistor to replace the small mosfet if it didn’t work. I placed the 555 timer upside down (see the board layout). Oh and yeah make sure you supply the schematic between 4.5 and 16 volts, not more because the supply of the 555 timer ic is not regulated by the schematic, but by the supply. I used a 12V adapter to supply it.

And this weekend I milled the pcb, and it came out super nice 🙂

Baked it for about 3,5 minutes and soldered the connectors, sadly I had to do some re-soldering since I had a short in the circuit due the reflow process… took me a lot of time finding it!

Then I used a led to check and measure the signal on the scope, and as I turned the pot-meter the duty cycle changes so it works!;

Today I did some testing with the heated bed itself and it works, even with the smaller, not cooled, mosfet, nice! This way I can control the temperature (manually) to say for example 80 deg. C.;

This was a good way to learn about the 555 timer for me since I never actually used one for a real project. This is also just a started project because later on I probably want it to be 1 pcb, or control it’s temperature by the software of the printer itself…

All the design files (eagle) are on thingiverse:

I hope you like it and learned something from it!

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