RTD Signal Conditioning Circuit for 0-5V Output and LED Indicator (50°C-100°C)

hey every body ... . how are ?
i need help with designing a circuit ... would you help me please ?
tanks )

Temperature is to be measured in the range of T1 (50C) to T2 (100C). The sensor is a resistance (RTD ) which varies linearly from R1 (141Ω) to R2 (171Ω ) for this temperature range.

a. Design analog signal conditioning circuit which provides a voltage varying linearly from 0V to 5V for this temperature range
b. Design a circuit to light a LED whenever the temperature is within T(50C) and T2(100C) otherwise the LED is OFF.

If you are just building one - its going to be a lot less time and effort + expense to buy one ready to go from watlow or red lion controls - I have retrofitted these into equipment which has the older bimetalic controller electro mechanical things and you cant design something better for the same money. They are also software programmable for any type of RTD and have upper and lower limit alarms and all sorts of other features - if you need something custom really then look at oven industrieshttp://www.ovenind.com - again they have products off the shelf for less than it takes to pick up a soldering iron. if you just need a display with upper and lower alarm limits there are many places who make these types of products - have a look at love controlshttp://www.dwyer-inst.com/market/love/

I cant think of a really good reason to design this as a one off unless you are designing a product of some type that will be made in high volumes - so... thats my advise and it might be worth what it cost you

ok thank you for your help ... but i'm a student and i have to make this circuit using some simple components ?

thanks any way

That sounds as a other study project................
Really if you can not handle this at school go and look out for a other study.

There is a difference between a monitor and a controller. If you want control you have completely left out those details, completely. If all you want is monitor then any decent opamp book will provide a wealth of ideas. It won't hand you a design whereby you learn absolutely nothing, but good ideas as a start. This assumes you love designing electronics as much as the rest of us do.

I dont know the device your talking about but normally you would sample the voltage across the resistor via adc input on a micro then calculate temperature accordingly You may have to use other components for example an op amp of some description to resolve Best to start with hardware first It is strange way of doing this You would normally use a temperture sensing device such is manufactured by texas instruments and then communicate with the device via one wire protocol using micro of your choice

An example of this would be a ds1820 but i think in your case your going to have to use an opamp configuration to supply the trip to micro or as someone else has pointed out Just plain ordinary window comparitor schematic with low voktage reference and high voltage reference

Yes, it would be much better for you and your learning curve to give this a shot on your own, and then, when you have specific difficulties, come back and ask (but not here -- as a _new_ question so you get a fresh set of eyes).

This would be nice and easy if he uses proton basic simply use the pot command which sample the value across the resistor or he could use adc or he could use interrupts to determine temp or he could even use the all famous bargraph ic I forget the name now LM. ..... AHA UH YES THE LM3915

I didn't read the question carefully, sorry. If you need to linearize the temperature, there are probably op-amp-circuits on the net to do just that.

If the RTD were tied between an op-amp configured as a constant current source and to ground, then the voltage at the op-amp/RTD would be about as linear as the RTD resistance.

The op-amp circuit I proposed earlier would then work to light the LED.

You could use the microprocessor, but this isn't that straightforward. If the micro has an ADC (analog to digital converter) then you can write software to linearize the RTD, or that is approximate linearization with digital steps. Most simple micros don't have ADCs. If they do the resolution is poor, which means big steps.

If you need to show a linear output then you also need a DAC to convert the internal value to analog, with steps based on the DAC. This isn't exactly a linear output. DACs are pretty rare in simple micros and they don't have very good resolution.

If you find a micro that will work, your problems are just starting. Next you need to learn how to program it, which means you need to learn how to program your micro, both learning programming in general and learning the particular instruction set. I can't imagine anyone unfamiliar with micros could do this in a few weeks. Then you have to figure out how to configure the DAC and ADC for the particular micro and write and debug the program, another process that could take awhile, especially for a novice. (I remember a take off on Faust with a programmer selling his soul to the Devil for a few hundred lines of debugged code.)Then there's the hardware you need to program the device, assuming the computer you have has the proper interface (some aren't USB or free) and most software runs of only a few operating systems.

I use micros daily and think they are great, but I don't they they make any sense at all for this project, unless you are already vary familiar with them. Then why would you be asking the question?

These comments are all correct that the thermistor is non-linea in it's characteristics. Too bad you have to use a thermistor. The way it's done in silicon is with a diode, which actually converts, by nature of the diode characteristics, in a Log/Ln function which can be converted back into a reasonable linear approximation with an ant-log amplifier, which i s simplistically an opamp a diode and resistor (simplistically).

On the other hand having designed analog fan speed controls, there is a war to make the curve much more linear. !st start looking through your thermistor carts, ones that are available and find the one that has the best linear characteristics as possible . Then though a combination of either series/parallel or both add external resistors to further linearize it. I have doe this and got as good as within +/-3 % and over a temperature range from 0 C t0 125 C.. that is unless you MUST stick with what you were given. Technically the actual value of the thermistor is in most cases, not that important.

Having said that there may be a way, would have to investigate, to take an opamp with the same thermistor and attached to the same heat source such that it's gain characteristics null out the non linearity, just a thought, something to look at.

Here is another way, without a micro, or a lookup table. you calibrate a many points along the curve as you wish say eight. You then have threshold detectors which turn on/off essentially giving you 8-bit resolution on the temperature, That can be the converted to an analog signal with a D/A. I know this works, used it before. Normally you would'n need separate threshold detectors but again the signal isn't linear as a function of temperature.

can you use a 7805 regulator but put the thermistor from gnd terminal if the ic to +out and a trimmer 200R from gnd to circuit negative. that would get rid of needing op amp.
stage 2 you can use the fact that the led needs a certain voltage to light. make a voltage reference for the led ground and it will do the work for you. use series diodes. for example; led(1.7v) plus 5x in4001 in series (3v) means led will light at 4.7 volt (3+1.7) but dont forget you still need a current limiting resistor, so factor that in.
i remember doing this at college, i did the whole thing with diodes and resistors just to be clever. worked fine, everyone else used 741 op amps. i never got extra points for being smart

The 7805 is a clever solution but won't the output be at least the regulators set point, that is won't the minimum output start at over 5v for a 5v regulator? You would need a negative supply for an output of 0v to 5v and the total supply range would have to be greater than the max output by the regulator plus the head space.

An RTD is not a thermistor. A graph on the net indicated that RTDs are pretty linear in the project's temperature range.

It mentions ,"control," but gives absolutely no details on the control such that you would have a servo loop. I did't even look at the RTD characteristics, but if this is linear this is a slam dunk, Freshman level stuff. PICs, Micros, anyone ever hear of an Obama. Do a Wiki search , " Operational Amplifier."

Over & out.

That is clever, but probably no extra points because that design wouldn't be thermally stable.

171/141 gives 1.212 ratio. 1.212*5=6 so if you have a power supply -5 0 +6 and and put rtc to negative rail, join to 171 ohm resister to + rail, then i think where they join you will have your 0-5v linear output. i think anyway, my math is rusty. i bet your tutor never specified the powerr supply it muust run from, and you want a SIIMPLE circuit, then thats as simple as it gets, combine that withe the diode/led design i mentioned earlier. no chips, no computer, basic electronics. tutor will hate you.

If I understand correctly you are suggesting a dual supply with -5v and +6v outputs. Not everyone has those. If you had a +5v supply and a +12v supply you could connect them in series, that is -12v to +5v and get what you want. The total voltage across the supplies is +17v which is well within the input voltage range for the regulator. Standard IBM-pc power supplies have +12 and -5 outputs. The pinouts for these are easily found on the net.

I'm still not sure that this is easier than a single supply, an op-amp for linearization and a dual op-amp for the LEDs. The op-amp circuits with pots will be much easier to adjust and you can get 4 op-amps in a single package. Plus learning about op-amps doesn't hurt. Your solution doesn't have broad applications.

I'd say that a linear regulator is technically a chip even though it only has 3 pins.

For a student this is a simple design - the suggestions given all will work fine - but for you, I believe you need something simple .

I gave a similair design problem to my students

here is the typical solution
Op-AMP ( 741 works fine) for the RTD with the RTD in the feedback loop ( inverting amplifier) - Use offset resistors ( non-inverting input) to Zero the output at the low temp and adjust the input resistor resistor (gain adjustment) to get 5 volt output at the high temp valve. Then use another op-amp to invert the voltage so that the output goes positive instead of negative. You can also use this amp to adjust gain ( range ) and zero the output. As for the LED - use a window comparator (LM339 I think it is) and design for the window limits. Any op-amp book should have the individutal circuits you will need. By the way you will (should) use a split power supply - ( +/- voltage )