Heated bed project part 1

Posted December 29 2011, tagged reprap, launchpad, heatedbed,

With my RepRap printing well, and some of the bugs shaken out. I've been printing a number of larger objects. With these I am starting to see pieces warp as the the piece cool. The 'go to' solution is to use a heated bed.

RepRap eletronics such as RAMPS have on board controllers for heated beds. However the GEN6 electronics I choose do not. So I need a controller circuitry to manage a stand alone heated bed.

Here are a series of posts, as I try to dust off some long forgotten electronic and microprocessor circuit design.

Heated build plate circuit design

The theory is simple.  We run a current through a heater element, and measure the temperature of a plate.  If the temp is too cold, power the heater.  If its too hot then turn the heater off.

To heat the plate, we will provide current through a copper track.  To measure the temperature we will use a thermistor.  The resistance of a thermistor will decrease as the temperature rises.

A microprocessor is used to measure the temperature, and determin if the heater needs to be on or off.

Measuring resistance

Resistance cannot be measured directly, however the voltage across it can be measured using a voltage divider.

Vin ----
       [] R1
        ----------- Vout
       []R2          
GND ---------------

Vout = (R2 / (R1 + R2)) x Vin

Vin = 12v
R1  = 2K
R2  = 1K
Vout = (1K / (2K + 1K)) x 12v
Vout = 2/3 * 12
Vout = 8v

By choosing the correct combination of resistors we can get a specific output voltage from a given input. Using a thermistor (whose voltage will vary according to temperature) as one of the resistors will give us a varying voltage that is proportional to the temperature.

Microcontroller input

The MSP430 on the TI Launchpad has on Analogue to Digital Converter (ADC), this can be employeed to read the output voltage from our divider. This reading can be converted to a temperature reading via a look up table.

Depending on the temperature calculated the heater can be controlled to heat the bed up or not.

The real work comes in calculating the various details

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