max6675, TC to SPI, best solution ----- Original Message ----- From: "Dwayne Reid" To: "Microcontroller discussion list - Public." Sent: Monday, April 22, 2013 3:17 PM Subject: Re: [EE] How to measure higher temperatures? ( was: Besttemperatur= e sensor ) > I'm still fond of thermocouples for measuring temperatures higher > than semiconductor sensors can tolerate. Circuits are simple - I > have two standard circuits that I make use of. > > One project (group of projects, actually) that I did uses a single > op-amp stage to amplify the thermocouple to a level that a PIC a/d > could make use of. Cold-junction compensation is done with a chain > of 3- 1n4148 diodes in series and fed into another a/d input. Very > simple and very effective. > > The other thermocouple amplifier that I use is actually a > thermocouple transmitter - it takes the signal from a "K" type > thermocouple and converts it to a current loop signal. Cold-junction > compensation is derived from a single 1n4148 diode right on the > board. I've built several thousands of these over the past couple of > decades - they are used in our Catalytic Heater-based Industrial Oven > control systems. The earliest version of these transmitters used a > 4-20mA loop but all are now configured for a 1-5mA loop > instead. There are just too many transmitters in a large system and > the preamp needs only a few hundred uA to operate - it didn't seem > reasonable to waste all that electrical energy as heat. > > But I digress. > > If you want to build a simple TC preamp, use an op-amp with low > Vos. I used a LT1013 running from my unregulated rail of 15V (the > LT1013 isn't R-R). The op-amp is configured as an inverting > amplifier with a gain of 200. The (+) input is grounded, the input > resistor on the (-) input is 1k0; feedback resistor is 200k with a > 100n cap across the resistor (we want a slow response). The > thermocouple is connected with (Y) lead grounded, (R) lead feeds the > 1k0 input resistor. Note that means that the TC is providing a > negative signal which is then inverted by the amplifier stage. There > is also another 100n noise suppression cap at the junction of the TC > (R) lead and the 1k0 input resistor. > > I chose the LT1013 for many reasons: its low cost if purchased from > TI (the Linear Technology version is stupidly expensive, the TI > version is darned reasonable); the input includes Gnd within its > common-mode range; it has a very low Vos; it doesn't run hot. > > The LT1013 has two identical op-amps - you can process 2 separate > thermocouples if you wish. For me - the thermocouples are used in a > catalytic heater system and I used the > extra op-amp stage as a comparitor as part of the hardware > safety-shutoff system. > > Hope this helps! > > dwayne > > > At 01:45 AM 4/22/2013, KPL wrote: >>Could this (diode junction) be (cheapest | easiest) way to measure >>temperatures up to about 200C? Looks like PT100 and similar need about >>the same amount of analog circuitry to work, but those are more >>expensive than diodes. >> >>I want to add more advanced regulator to my laminator, so I could use >>it for PCB toner transfer, but could still use it for it's original >>function as well. >>Great precision is not required, but temperatures can reach close to >>200 degrees celsium. >> >>I have no experience with any of these, as usually ds18b20 were good >>enough until now. >> >> > >> > Diode bandgap measurement does most of what you want and cost can be >> > lowish if you work at it. >> > >> > Wikipedia covers basics: >> > http://en.wikipedia.org/wiki/Silicon_bandgap_temperature_sensor >> > >> > If you sequentially apply two currents to a silicon diode the >> > delta-voltage is a function of only temperature and the two currents >> > and a constant. >> > You can notionally swap sensors using any silicon diode and get the >> > same result without recalibration - but using the same type of diode >> > is better in practice. >> > This is a substantially more accurate and generally superior method to >> > just measuring diode forward voltage drop. >> > >> > The method and the formula are not "hard" - just perhaps unexpected. >> > >> > The sensor is in theory ANY siliicon diode, swappable with out >> > calibration or recalibration, but using say 1N4148 as standard will >> > improve ease of getting good results. >> > >> > Any diode material can be used if due allowances made >> > Si is good for up to about 200C with SiC being better above 200C >> > >> > Delta_Vdiode =3D KT/q * ln ( i2 / i1 ) >> > >> > or >> > >> > T =3D Delta_Vdiode / (k * ln (i1/i2)) >> > >> > ln may be annoying but is not intractable. >> > >> > Where >> > K =3D Boltzmann's constant >> > T =3D temperature in degrees K >> > q =3D electron charge >> > i1 =3D current 1 >> > i2 =3D current 2 >> > k =3D K/Q >> > >> > > Accuracy is .5C + or - .25 C >> >> Range is - 20C to 75C >> >> Long term stability >> > >> > Achievable, yes, yes. >> > >> >> simple interface such as i2c, SPI , voltage , current , resistance >> >> ..... >> > >> > Needs two switched precision currents and suitably accurate voltage >> > measurement. >> > OR two non precisely produced currents which can be precisely measured= .. >> > >> >> simpler is better >> >> Low price >> > >> > Sensor is a silicon diode. Cost is a diode for the sensor plus a >> > suitably accurate ADC (assuming digital) for voltage and two accurate >> > currents OR a means of accurately measuring currents. >> > eg imagine that you inject I1 then I2 into a series combination of D1 >> > + R1 to ground. >> > Measure VR1 and then V(D1 + R1) >> > If R1 is stable and known then this allows measurement of 2 x current >> > and Vdiode with a single switched ADC input or two 'unswitched' ADC >> > inputs. >> > In this case current switching could be achieved with eg applicaton of >> > a dfigital high or low to a resistor network using relatively low >> > precision resistors. >> > With D and R in series the relative voltage drops of R and D can be >> > adjusted to best use ADC range. >> > >> >> no external parts >> > >> > Deep ends what you mean by "external". >> > Needs above parts. Can be integrated into whatever as required. >> > >> >> no calibration >> > >> > Yes. Neither additional or for sensor swap. >> > >> >> Can be slow ..... 30 second update .... can be as large as golf bal= l >> > >> > Easily met. >> > >> > >> > Russell >> > -- >> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> > View/change your membership options at >> > http://mailman.mit.edu/mailman/listinfo/piclist >> >> >> >>-- >>KPL >>-- >>http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >>View/change your membership options at >>http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > Dwayne Reid > Trinity Electronics Systems Ltd Edmonton, AB, CANADA > (780) 489-3199 voice (780) 487-6397 fax > www.trinity-electronics.com > Custom Electronics Design and Manufacturing > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .