heated bed using diodes as heat sensors

has anyone designed a heated bed that uses silicone diodes as the temp sensor. diodes voltage breakdown varies about 2.3mV/°C using atmega internal 2.56 ref, or atemga32 internal ref for adc provides decent resolution of around 1deg C.resolution. Is this accurate enough for most heated beds?


Why do i ask, because diodes are cheap, and can handle over 100deg C.

any comments, or reasons one would not want to use a diode in series with a 10k resistor, and a node to adc?

I'm thinking about using a diode setup for a heated bed conveyor platform that i will make available soon in the reprap wiki.

Great idea, I wondered why it was not used.

Also check out the LM335, a temperature sensing IC, about $1. It has 10mv/C output and goes to 150C.

john

for use of diodes as temp sensor, the curve is pretty much linear. I'll post some specs soon enough.

the main issue is calibration of breakdown voltage needs to be done each time diode is changed out. then again is it important that the bed is actually within a temp +/-5 degrees c?

2-2.3mv per deg C
offset +/-10 //this is the calibration setting
voltageref=5v 2.56v,1v //choices of voltage ref.

resolution=voltageref/1024

breakdownv= 0.6/0.001 //breakdown voltage
forwardv=0.6/0.001 //
temp=(analogread(pin)/1024+offset-forwardv )*resolution


something like that it will change a little, but does not require a table.


benefits. it is cost effective, and easy to calibrate would work good on heater bed.

cons over 200deg c breakdown of materials and high leakage causes unreliable results, also without temp calibration temp can be off as much as 10deg c

any comments or suggestions before i make the terminology and actual code update suggestion for sprinter?

this change is to allow diodes to be used in heated beds. code will be included in temp management , no table will need to be created. 1 to 4 variables will be added to configuration.h file

Edited 1 time(s). Last edit at 01/26/2012 04:58PM by jamesdanielv.

It isn't the breakdown voltage, it is the forward voltage that is used to measure temperature.

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