Heated bed project part 2

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

Datasheets - I hate them!  They are written by engineers who know what they are doing, for engineerers who know how to read them.  Having spent some time this morning going through a load of them and various posts about the TI Launchpad (whose documentation was written by more engineers) I've steadily come to some understanding.

Know details

Analogue to Digital convertors work by comparing an input voltage to two reference voltages.  With the Launchpad the reference voltages can easily be set to 2.5V to 0V.  From the voltage divider circuit from the previous post, I need to calculate the resistors to provide Vout to be between 2.5V and 0V depending on the temperature of the thermistor.

The supply voltage of the system is 12V, this is the voltage that the rest of my RepRap is powered at.  It makes sense to use this to power my heated bed.

The temperature for a heated bed can run up to 200°C.  So our range of temperature should be anything between 10°C and 250°C.

Thermistor selection

This range of thermistors seem suitable.  They cover the range of temperatures and are available in a number of different resistances.  It just comes down to choosing the most appropriate one.

This is a diagram of what I know and what I need to calculate.


12v ----
 | [ ] R1 | +---------- Vout (0v - 2.5v) | [ ] R2 (Thermistor) | 0v -----

The datasheet 'Temperature Measurement Glass-Encapsulated Sensors B57550 Series Data Sheet' tells us about each of the parts. The information I need can be found by looking up the thermistor against the tables provided in the Standardized R/T Characteristics document.  We can get the resistance of the thermistor at various temperatures.

From the first document I can see for the 2K has a 'No. of R/T characteristic' contained within table 8401.  A temperature/constant can be found by examining this table (page 26).    At 15°C, and 250°C these values are 1.4714 and 0.0062302.  To calculate the resistance at these temperatures, we feed the constant in to the equation found on page 1. (Rt is the resistance at particular a temperature)

Rt = K  * R25
Rt = 1.4714 * 2K and  0.0062302 * 2K
Rt = 2.9428KΩ and 12.46Ω

Using ohms law equation Vout = (R2 / (R1+R2)) * Vin means I can now calulate the Vout range using a particular thermistor at certain temperatures with several values of R1.  For example using a value of 2K gives us the two equations;

Vout = (R2 / (R1 + R2)) * Vin
VoutMax = (29428 / (2000 + 29428)) * 12 = 7.14V
VoutMin = (12.46 / (2000 + 12.46)) * 12 = 0.074V

Plugging these in to a spreadsheet means a range of values of for R1 can be plugged in and voltage range calculated. The optimum value for R1 that I found is 12K, which gives an output voltage of 2.36V to 0.012V

Since all of this is easy to calculate, and we have various thermistors to choose from I'll put together a series of calculations for those values. Also since this effort is to be used for specific temperatures (55°C PLA and 120°C ABS), I'll put these in for checks.   Also I also have the option of using a Vin of 3.5V since I need to pull down to this to voltage for the TI Launchpad.

The spreadsheet I've just put together can be see within google docs - RepRap Heated bed thermistor calculation the second sheet contains the final formula.  I've highlighted in red those combinations that put the Vout range to be between 2.5v and 0v.  There are several potential combinations of R1 and Thermistor that I can use.

I thought about a good means to choose which combination is optimal while putting the spreadsheet together.  The combination that provides the largest change in voltage towards the highest temperate points is the one. I should just pick the combination with the highest change in voltage between 180°C and 200°C.  This gives me the following values.

Vin 3.5V
R1 1.1K
Thermistor 2K
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