My application is a temperature controller that uses a surface temperature sensor and Triac to adjust a heating element using a uC using PID control. Although I only need about a +/- .5 degree accuracy, I'm aiming for a temperature range of 0 to 200 degrees C which seems to be comfortably out of the range of most thermistor solutions but seems to be within the capabilities of a standard RTD. I'm struggling to implement an RTD in my design as the interfacing solutions I have seen require a lot of opamps and a precise reference voltage. Here's specifically the one I'm talking about from this microchip application note: http://ww1.microchip.com/downloads/en/AppNotes/00687c.pdf My design uses a transformerless power supply that allows me to detect zero crossing as seen in this microchip application note: http://ww1.microchip.com/downloads/en/AppNotes/91094A.pdf I feel like my range and accuracy requirements are modest, do I really need to add regulation for a reference voltage, a constant current source, + and - voltage sources for op amps in order to use an RTD??? Since I'll be taking my temperature readings in phase with the voltage fluctuation of the power supply, I wonder if regulation for a reference voltage is really necessary. Is there a smoother way to interface with an RTD given my requirements? --------------------------------------- Posted through http://www.EmbeddedRelated.com
Interfacing with RTDs
Started by ●June 1, 2011
Reply by ●June 1, 20112011-06-01
On 06/01/2011 11:47 AM, segwaypirate wrote:> My application is a temperature controller that uses a surface temperature > sensor and Triac to adjust a heating element using a uC using PID control. > Although I only need about a +/- .5 degree accuracy, I'm aiming for a > temperature range of 0 to 200 degrees C which seems to be comfortably out > of the range of most thermistor solutions but seems to be within the > capabilities of a standard RTD. > > I'm struggling to implement an RTD in my design as the interfacing > solutions I have seen require a lot of opamps and a precise reference > voltage. Here's specifically the one I'm talking about from this microchip > application note: > > http://ww1.microchip.com/downloads/en/AppNotes/00687c.pdf > > My design uses a transformerless power supply that allows me to detect zero > crossing as seen in this microchip application note: > > http://ww1.microchip.com/downloads/en/AppNotes/91094A.pdf > > I feel like my range and accuracy requirements are modest, do I really need > to add regulation for a reference voltage, a constant current source, + and > - voltage sources for op amps in order to use an RTD??? Since I'll be > taking my temperature readings in phase with the voltage fluctuation of the > power supply, I wonder if regulation for a reference voltage is really > necessary. Is there a smoother way to interface with an RTD given my > requirements?If you understand the principles behind the working of the RTD, you ought to be able to make a current source that's proportional to your ADC reference. That'll make the readings ratiometric. Then use rail to rail op-amps. That'll get you down to only needing one supply. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●June 1, 20112011-06-01
On Jun 1, 11:47=A0am, "segwaypirate" <joelbenway@n_o_s_p_a_m.gmail.com> wrote:> My application is a temperature controller that uses a surface temperatur=e> sensor and Triac to adjust a heating element using a uC using PID control=.> Although I only need about a +/- .5 degree accuracy, I'm aiming for a > temperature range of 0 to 200 degrees C which seems to be comfortably out > of the range of most thermistor solutions but seems to be within the > capabilities of a standard RTD.You might want a combination of thermistor (low temp) and thermocouple (high temp).> > http://ww1.microchip.com/downloads/en/AppNotes/00687c.pdf > > My design uses a transformerless power supply that allows me to detect ze=ro> crossing as seen in this microchip application note:What are you measuring? Frequency or voltage?> > http://ww1.microchip.com/downloads/en/AppNotes/91094A.pdf > > I feel like my range and accuracy requirements are modest, do I really ne=ed> to add regulation for a reference voltage, a constant current source, + a=nd> - voltage sources for op amps in order to use an RTD??? =A0Since I'll be > taking my temperature readings in phase with the voltage fluctuation of t=he> power supply, I wonder if regulation for a reference voltage is really > necessary. =A0No, just a stable voltage to the uC.
Reply by ●June 1, 20112011-06-01
On Jun 1, 2:47=A0pm, "segwaypirate" <joelbenway@n_o_s_p_a_m.gmail.com> wrote:> My application is a temperature controller that uses a surface temperatur=e> sensor and Triac to adjust a heating element using a uC using PID control=.> Although I only need about a +/- .5 degree accuracy, I'm aiming for a > temperature range of 0 to 200 degrees C which seems to be comfortably out > of the range of most thermistor solutions but seems to be within the > capabilities of a standard RTD. > > I'm struggling to implement an RTD in my design as the interfacing > solutions I have seen require a lot of opamps and a precise reference > voltage. =A0Here's specifically the one I'm talking about from this micro=chip> application note: > > http://ww1.microchip.com/downloads/en/AppNotes/00687c.pdf > > My design uses a transformerless power supply that allows me to detect ze=ro> crossing as seen in this microchip application note: > > http://ww1.microchip.com/downloads/en/AppNotes/91094A.pdf > > I feel like my range and accuracy requirements are modest, do I really ne=ed> to add regulation for a reference voltage, a constant current source, + a=nd> - voltage sources for op amps in order to use an RTD??? =A0Since I'll be > taking my temperature readings in phase with the voltage fluctuation of t=he> power supply, I wonder if regulation for a reference voltage is really > necessary. =A0Is there a smoother way to interface with an RTD given my > requirements? > > --------------------------------------- =A0 =A0 =A0 =A0 > Posted throughhttp://www.EmbeddedRelated.coma lot depends on whether you can calibrate the system, or do you expect it to meet accuracy first time it is power up? will the interface circuit experience temperature extremes too? the .01% error that article claims requires a lot of constraints, I suspect
Reply by ●June 1, 20112011-06-01
segwaypirate wrote:> My application is a temperature controller that uses a surface temperature > sensor and Triac to adjust a heating element using a uC using PID control. > Although I only need about a +/- .5 degree accuracy, I'm aiming for a > temperature range of 0 to 200 degrees C which seems to be comfortably out > of the range of most thermistor solutions but seems to be within the > capabilities of a standard RTD. > > I'm struggling to implement an RTD in my design as the interfacing > solutions I have seen require a lot of opamps and a precise reference > voltage. Here's specifically the one I'm talking about from this microchip > application note: > > http://ww1.microchip.com/downloads/en/AppNotes/00687c.pdf > > My design uses a transformerless power supply that allows me to detect zero > crossing as seen in this microchip application note: > > http://ww1.microchip.com/downloads/en/AppNotes/91094A.pdf > > I feel like my range and accuracy requirements are modest, do I really need > to add regulation for a reference voltage, a constant current source, + and > - voltage sources for op amps in order to use an RTD??? Since I'll be > taking my temperature readings in phase with the voltage fluctuation of the > power supply, I wonder if regulation for a reference voltage is really > necessary. Is there a smoother way to interface with an RTD given my > requirements?No op amps needed. A delta sigma ADC w/ differential ref and signal inputs (e.g. LTC2433), a stable reference resistor, and another resistor (stability not important) to lower the overall current. I would change the zener to 8 volts and add a regulator on the 5V, but you might not need it. 5V | crappy 10k | +----- A | RTD 1k nominal @ 0C (up to 1.76k @ 200C) | +----- B | 4.99k good tempco (and must be 2x the highest RTD value) | +----- C | ground ADC signal is A-B ADC ref is B-C Bob
Reply by ●June 1, 20112011-06-01
Often interfacing to an RTD is done differentially using 2 current sources, one driving the RTD and the second driving an on-board resistor. The difference is measured by means of a difference or instrumentation amp. This allows you to improve the dynamic range of the measurement. To understand, look at a TI dedicated part like the XTR105. Then try and take the same approach. For a current source try the LM334 but watch for its thermal performance and use the compensation circuit recommended. Linear makes a better and more expensive part- the number doesn't come to hand at the moment. --------------------------------------- Posted through http://www.EmbeddedRelated.com
Reply by ●June 1, 20112011-06-01
On 6/1/2011 12:46 PM, Tim Wescott wrote:> On 06/01/2011 11:47 AM, segwaypirate wrote: >> My application is a temperature controller that uses a surface >> temperature >> sensor and Triac to adjust a heating element using a uC using PID >> control. >> Although I only need about a +/- .5 degree accuracy, I'm aiming for a >> temperature range of 0 to 200 degrees C which seems to be comfortably out >> of the range of most thermistor solutions but seems to be within the >> capabilities of a standard RTD. >> >> I'm struggling to implement an RTD in my design as the interfacing >> solutions I have seen require a lot of opamps and a precise reference >> voltage. Here's specifically the one I'm talking about from this >> microchip >> application note: >> >> http://ww1.microchip.com/downloads/en/AppNotes/00687c.pdf >> >> My design uses a transformerless power supply that allows me to detect >> zero >> crossing as seen in this microchip application note: >> >> http://ww1.microchip.com/downloads/en/AppNotes/91094A.pdf >> >> I feel like my range and accuracy requirements are modest, do I really >> need >> to add regulation for a reference voltage, a constant current source, >> + and >> - voltage sources for op amps in order to use an RTD??? Since I'll be >> taking my temperature readings in phase with the voltage fluctuation >> of the >> power supply, I wonder if regulation for a reference voltage is really >> necessary. Is there a smoother way to interface with an RTD given my >> requirements? > > If you understand the principles behind the working of the RTD, you > ought to be able to make a current source that's proportional to your > ADC reference. That'll make the readings ratiometric. Then use rail to > rail op-amps. That'll get you down to only needing one supply. >There's actually a really nice RTD measurement circuit as an app note at the back of the AD7793 datasheet. -- Rob Gaddi, Highland Technology Email address is currently out of order
Reply by ●June 1, 20112011-06-01
Thanks for all the responses. To answer some questions: I'm detecting the zero-crossing only to control the mains phase with a triac I'm not measuring the voltage or frequency. Only the surface temperature sensor and its wiring will be exposed to the 200 C but the lead length will be less than 2 or 3 feet. Some calibration is ok at least for now. Bob, Thanks for your help. I know its pretty normal to use high resolution ADC's that have a differential input but in my range of 0 to 200 I can't help but think that the 10 bit ADC on board my uC can get me an acceptable result. Shouldn't I be able to use the configuration posted by Bob using two channels of that ADC taking the difference in software? If anyone is aware of an NTC surface temperature assembly that can operate at 200 C, that would obviously be the better option. --------------------------------------- Posted through http://www.EmbeddedRelated.com
Reply by ●June 1, 20112011-06-01
On 2011-06-01, segwaypirate <joelbenway@n_o_s_p_a_m.gmail.com> wrote:> Thanks for your help. I know its pretty normal to use high resolution > ADC's that have a differential input but in my range of 0 to 200 I can't > help but think that the 10 bit ADC on board my uC can get me an acceptable > result.Good luck. In my experience, a "10-bit A/D" on a uController is usually good for about 8 bits of actual, usable resolution. -- Grant Edwards grant.b.edwards Yow! In Newark the at laundromats are open 24 gmail.com hours a day!
Reply by ●June 1, 20112011-06-01
On Jun 1, 2:59=A0pm, Grant Edwards <inva...@invalid.invalid> wrote:> On 2011-06-01, segwaypirate <joelbenway@n_o_s_p_a_m.gmail.com> wrote: > > > Thanks for your help. =A0I know its pretty normal to use high resolutio=n> > ADC's that have a differential input but in my range of 0 to 200 I can'=t> > help but think that the 10 bit ADC on board my uC can get me an accepta=ble> > result. > > Good luck. =A0In my experience, a "10-bit A/D" on a uController is > usually good for about 8 bits of actual, usable resolution.Bit 9 should be OK with good PCB layouts. Bit 10 would need properly designed uC with separate power rails. As they think the world is all digital, not all uC supports separate analog power rails.
